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Abstract

In this article we report on the synthesis and multiferroic properties of cobalt ferrite (CoFe2O4)–barium titanate (BaTiO3) biphasic composites. The initial composite nanopowder was synthesized by a combination of co-precipitation and organosol methods. A ceramic sample with (3–0) connectivity, i.e. BaTiO3 grains in a CoFe2O4 matrix was obtained by a combination of spark plasma sintering and annealing. In order to understand the correlations between morphology, electric properties, and magnetization, we present a detailed study at different preparation steps and compare it to the properties of a conventionally sintered sample with the traditional (0–3) connectivity, i.e. CoFe2O4 grains in a BaTiO3 matrix. We observe that the (3–0) sample shows improved magnetic properties in comparison to the conventionally sintered composite of the same composition. In spite of relatively large leakage current for the (3–0) sample compared to the traditional (0–3) one, it exhibits a converse magnetoelectric effect that follows the Hdc dependence of the piezomagnetic coefficient. The magnetic field-dependence of electric polarization at the surface was investigated utilizing X-ray absorption spectroscopy and its associated linear and circular dichroisms.

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... Sementara itu,lapisan tipis CoFe 2 O 4 telah dijadikan sebagai penyusun material penyusun sensor giant magnetoresistance (Djamal, 2015;Ramli, 2016), dandivais gelombang mikro (Hannour, 2014). Lapisan tipis CoFe 2 O 4 merupakan komponen penting dalam multilayers atau komposit untuk penelitian multiferroic dan aplikasinya (Etier, 2015 (Setiadi, 2013). Sementara itu,lapisan tipis CoFe 2 O 4 telah dijadikan sebagai penyusun material penyusun sensor giant magnetoresistance (Djamal, 2015;Ramli, 2016), dandivais gelombang mikro (Hannour, 2014). ...
... Sementara itu,lapisan tipis CoFe 2 O 4 telah dijadikan sebagai penyusun material penyusun sensor giant magnetoresistance (Djamal, 2015;Ramli, 2016), dandivais gelombang mikro (Hannour, 2014). Lapisan tipis CoFe 2 O 4 merupakan komponen penting dalam multilayers atau komposit untuk penelitian multiferroic dan aplikasinya (Etier, 2015 Disamping itu, nanopartikel CoFe 2 O 4 merupakan salah satu partikel yang sangat berpotensi diaplikasikan dalam bidang biomedis, diantaranya sebagai bahan pembawa target dalam pengiriman obat, cairan magnetik, dan sebagai katalis (Setiadi, 2013). Sementara itu,lapisan tipis CoFe 2 O 4 telah dijadikan sebagai penyusun material penyusun sensor giant magnetoresistance (Djamal, 2015;Ramli, 2016), dandivais gelombang mikro (Hannour, 2014). ...
... Sementara itu,lapisan tipis CoFe 2 O 4 telah dijadikan sebagai penyusun material penyusun sensor giant magnetoresistance (Djamal, 2015;Ramli, 2016), dandivais gelombang mikro (Hannour, 2014). Lapisan tipis CoFe 2 O 4 merupakan komponen penting dalam multilayers atau komposit untuk penelitian multiferroic dan aplikasinya (Etier, 2015 ...
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In this paper we report the results of studies relating to the synthesis of Cobalt Ferrite (CoFe2O4) thin films by a sputtering method. The CoFe2O4 thin film has been prepared onto silicon substrate from the sputtering targets, CoFe. Structural propertiesofthinfilms were characterized byx-ray diffraction and the morphology was characterized by scanning electron microscopy. The growth parameter are: base pressure 2,8 x 10-2 Torr, ratio of Argon:Oxygen flow rate are 100:50 sccm, deposition pressure 5.4 x10-1 Torr, growth temperature 100oC.Nanostructures of the thin film that have been analyzed are crystallite size and micro strain.We obtained the crystallite size of CoFe2O4 thin films for layer thickness of 40 and 48 nm, respectively are: 32 nm and 66 nm, while the micro strain is 8.0 x 10-4 and 10.2 x 10-4.
... Multiferroic nanocomposites show the cross-coupling phenomenon among the simultaneously tunable magnetic, electric, thermal, optical, and mechanical order parameters [2]. Accordingly, the multiferroics nanocomposites cover a wide range of applications including radiation sensors, energy harvesting, solid-state refrigeration, data storage recording, and photo-voltaic technologies [3][4][5][6][7][8][9][10][11][12][13]. Consequently, the multiferroic has a potential economical impact that is estimated to multibillion US dollar market value [10]. ...
... Consequently, the multiferroic has a potential economical impact that is estimated to multibillion US dollar market value [10]. The coupling between magnetic and electric ordering parameters produces the so-called magnetoelectric (ME) multiferroic nanocomposite [1][2][3][4][5][6][7][8][9][10][11][12][13], better to work at room temperature [4]. Therefore, in order to fabricate multiferroic nanocomposite, two different material phases should be coupled: one of them represents the magnetic phase while the second phase shows its own ferroelectric behavior [5]. ...
... The main target of this monograph was to fabricate Li 0.5 Fe 2.5 O 4 /BaTiO 3 multiferroic nanocomposite using a developed modified procedure and then selecting the optimum concentration available in the different applications [6][7][8][9][10]. Moreover and for echofriendly multiferroic, it is necessary to have lead-free composition, so that BaTiO 3 is used as one of the well-known excellent ferroelectric constituents that is lead-free compound and shows remarkable piezoelectric properties [10][11][12]. At room temperature, the BaTiO 3 is characterized by its stable perovskite tetragonal crystal structure, while at a certain temperature called a structural phase transition or Curie temperature which is commonly 393 K in most cases, it converts to symmetric cubic structure [4,7]. ...
Article
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Multiferroic nanocomposites (1−y)Li0.5Fe2.5O4/(y)BaTiO3 (y = 0.0, 0.2, 0.4, 0.6, 0.8, and1.0) were synthesized by modified citrate autocombustion. XRD, FTIR, and XANES techniques were used for average crystal, chemical, and electronic/local structural analysis. XRD assured the presence of only two distinct constituent phases as Li0.5Fe2.5O4 (LFO) and BaTiO3 (BTO) within the composite. The Fe K-edge XANES signal was collected at Elettra synchrotron facility to determine the possible oxidation states of Fe and to probe the local/electronic structural features of Fe within LFO spinel. Fe K-edge XANES fingerprint confirmed the trivalent character of Fe ions (Fe3+). The influence of the weight fraction of BTO was observed clearly in the collected dielectric spectra. The dielectric parameters έ, tanδ, and σac were acquired as a function of temperature at selected applied frequencies starting from 100 kHz reaching to 5 MHz. The dielectric measurements were acquired at wide temperature range from room temperature (396 K) up to 800 K. The lowest values of the dielectric parameters (σac and έ) were observed at a given temperature at y = 0.6. The έ and σac showed an increase with temperature for all composites while tan δ gave its maximum at around 650 K. A remarkable reduction in έ and σac with frequency was observed without relaxation. XRD, functional groups, and XANES were correlated to each other. The dielectric properties was explained in the light of the average and local structural analysis. The dielectric coupling between the two ferroelectric and ferromagnetic phases was tailored and controlled through the heterointerface interaction between the two constituent phases, besides the own properties of each phase.
... There are several publications on each component (BNT-BT 0.08 and CoFe 2 O 4 ), with nanotube [17,18] and thin film [19,20] structure, part of core-shell composites [19] and thin films heterostructures [21]. There are also, few reports on hybrid piezoelectric/ferromagnetic structures, such as: CoFe 2 O 4 /BaTiO 3 [22][23][24][25][26], Pb(Zr 0.52 Ti 0.48 )O 3 /NiFe 2 O 4 [27] and NiFe 2 O 4 /BaTiO 3 [28] core-shell composites where, the authors studied the influence of ferrite fraction variation on the magnetic properties of the multiferroic core-shell -type nanostructures. Considering that the nanocomposite materials offer a vast design-space of potential material properties depending on the properties of the constituents, we synthesized and studied both electric and magnetic properties of the novel core-shell composite design BNT-BT 0.08 /CoFe 2 O 4 . ...
... Considering that the nanocomposite materials offer a vast design-space of potential material properties depending on the properties of the constituents, we synthesized and studied both electric and magnetic properties of the novel core-shell composite design BNT-BT 0.08 /CoFe 2 O 4 . We compare here our results with those presented in the previous studies [23][24][25][26][27][28][29] ) composite core-shell was prepared from BNT-BT 0.08 and CoFe 2 O 4 precursor sols. BNT-BT 0.08 precursor sol was prepared by sol-gel technique using as starting materials sodium acetate, barium acetate, bismuth (III) acetate and titanium (IV) isopropoxide. ...
... Because the remanent magnetization is less than half of the maximum magnetization (70 kOe), it can be concluded that there are no inter-particle interactions [53]. Therefore, there are intra-particle and interfaces interactions which influence the overall magnetism of a ferroelectric/ferromagnetic system [54,[23][24][25][26][27][28][29]. According to these reports, our results suggest that in the composite core-shell BNT-BT 0.08 / CoFe 2 O 4 , the intra-particle magnetic interactions can influence the spin structure of nanoparticles and the magnetic properties of the core-shell composite. ...
Article
In this work, we report on the synthesis and characterization of BNT-BT0.08/CoFe2O4 biphasic composite with core-shell structure. This artificial core (BNT-BT0.08)/shell (CoFe2O4) heterostructure was prepared by sol-gel method and the resulting composite was characterized in term of microstructure, dielectric, piezoelectric and magnetic properties. BNT-BT0.08/CoFe2O4 sintered ceramic shows high permittivity (ε′ ≥ 30) and high dielectric losses (tan δ ≥ 10) in the low frequency range (ν ≤ 10⁴ Hz), remnant polarization (Pr) of ∼7.7 μC/cm² and, remanent magnetization (Mr) of 24 emu/g at 5 K and of 14 emu/g, at room temperature. The present study reveals that the ferroelectric, piezoelectric and magnetic properties of this new architectured composite depend on the amount of each component and, can be tailored by adjusting their synthesis conditions. BNT-BT0.08/CoFe2O4 core-shell material investigated in this work provides a novel way to exploit new applications for the multifunctional composite, such as piezoelectric sensor, magnetoelectronic sensors and data storage devices.
... Magnetoelectric multiferroics are a fascinating class of materials, in which not only magnetic and ferroelectric orders coexist, but also a cross-coupling occurs between magnetic and electric degrees of freedom. This cross-coupling, i.e., the magnetoelectric effect, allows the application of these materials for the development of weak magnetic field sensors, low-power consuming magnetic-read/electric write memory elements, or energy harvesting devices [1][2][3][4][5][6]. ...
... On the contrary, composite multiferroics, where chemically different magnetic and ferroelectric phases are ''artificially'' connected, offer a convenient way to achieve sizable magnetoelectric coupling at room temperature [3]. These twophase magnetoelectrics can be fabricated with a variety of configurations, including particulate composites (with 3-3 or 3-0 connectivity) [4], pillar-matrix (1-3 connectivity) [5], and the layer-by-layer (2-2 connectivity) type [6]. ...
Article
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We report on temperature-dependent studies of ultrasonic and dielectric properties of (x)0.5(Ba0.7Ca0.3)Ti03–0.5Ba(Ti0.8Zr0.2)O3(BCZT)/(1 − x)NiFe2O4 (BCZT/NFO) composite multiferroics and their relationship to the magnetoelectric (ME) effect in these materials. The most decisive factor in the maximization of the ME effect is the strong elastic softening of the BCZT phase at the phase transition between its ferroelectric phases with orthorhombic and tetragonal symmetry. The proximity of this phase transition to room temperature makes the system promising for practical applications of the ME effect. The magnetostrictive phase does not play any direct role in the determination of the ME temperature dependence because of its weakly temperature-dependent mechanical properties.
... From [156], with permission netic, with the non-zero total moment arising as a consequence of the different spin states of the cations composing the two antiferromagnetic sub-lattices) [163,164] and with the identification of the relative site occupation of the different magnetic cations. The element specificity of XAS has been explored to obtain element-specific magnetic hysteresis loops, for example, for the Ni and Fe cations in a NiFe 2 O 4 -BaTiO 3 ceramic [165]. ...
... One example where the use of the natural dichroism that arises with changes in the crystal symmetry induced by strain is employed to understanding the nature of the magnetoelectric coupling in strain mediated multiferroic nanostructures is provided by Schmitz-Antoniak et al. [156,165] in [001]-oriented CoFe 2 O 4 /BaTiO 3 multiferroic nanocomposite, where they consider the structural changes on the BaTiO 3 induced by the CoFe 2 O 4 magnetostriction. This is seen as the emergence of an inplane linear dichroism at the Ti L 2,3 edge going from the case where the magnetic field is applied out of plane (resulting in an in-plane cubic symmetry) to applying the magnetic field in-plane (resulting in a uniaxial distortion of the BaTiO 3 lattice), as shown in Fig. 10.10. ...
Chapter
In this chapter we discuss the capabilities of X-ray photoemission and absorption spectroscopies for the investigation of the electronic, magnetic and electric properties of multiferroic materials and heterostructures. As complementary techniques providing element selective information on both occupied and empty states, their combination delivers a comprehensive picture of the chemical state of individual species, magnetic moments, bulk and surface band structure, and local atomic environment at the interface between dissimilar materials. By directly probing the electronic structure at the atomic level, unique insights can be learned about the mechanisms responsible for the magnetoelectric couplings in this fascinating class of materials.
... For the ferroelectric-piezoelectric phase, especially in the last decade, researchers have focused on the development of lead-free materials due to the RoHS regulation (2002/95/ EC) concerning the restriction of hazardous substances, which deals with the restriction of using certain hazardous substances in electronic devices, including lead. Examples of recently used ferroelectric-piezoelectric systems are found: K 0.5 Na 0.5 Nb 2 O 3 (KNN) [5,6], (x)Bi 0.5 Na 0.5 TiO 3 -(1-x) Bi 0.5 K 0.5 TiO 3 (BNT-BKT) [7], Sr x Ba 1-x Nb 2 O 6 (SBN) [8], and BaTiO 3 [9][10][11], the latter having been widely explored as a ferroelectric phase in magnetoelectric composites, mainly due to its chemical stability. Thus, to ensure the adequate performance of the magnetoelectric particulate composite, it is essential that the particulate composite presents high densification, control and homogeneity of grain sizes, chemical and stoichiometric control of the interfaces, and, above all, a high level of dispersion of the constituent phases in the biphasic system in the powder state and consequently in the sample conformed [1][2][3]. ...
... The results of XRD analysis in Fig. 4 revealed that the in situ synthesis of 0.8BaTiO 3 /0.2Ni (1-x) Co x Fe 2 O 4 (x= 0, 0.25, 0.5, 0.75, and 1) system was achieved successfully at 800 °C for 1 h; these temperature and holding time conditions were considerably lower than those typically reported in the literature for conventional solid-state reactions, which use around 1200 °C and 3 or 4 h [10], which indicated that the in situ sol-gel method using PAA1800 as a chelating agent can be used to quickly obtain the BaTiO 3 and Ni (1-x) Co x Fe 2 O 4 biphasic system in a single reaction step. The main diffraction peaks for the BaTiO 3 , namely (100), (111), (110), (200), (210), (211), and (220), can be assigned to the perovskite structure, cubic Pm-3m spatial group (ICSD 01-75-0214), with a maximum diffraction intensity for the (111) plane. ...
Article
Full-text available
Abstract Nanopowders of the 0.8BaTiO3/0.2Ni(1-x)CoxFe2O4 (x=0, 0.25, 0.5, 0.75, and 1) were synthesized by the in situ sol-gel method using poly(acrylic acid) (PAA1800) as a chelating agent. The synthesis developed using PAA1800 ensured the simultaneous crystallization of two phases. The results provided a reliable method for producing biphasic nanopowders in all the compositions of Co-Ni-ferrite with an average particle size of less than 50 nm with a potential application in lead-free magnetoelectric particulate composites. The XRD results indicated the formation of only BaTiO3 and Ni(1-x)CoxFe2O4 phases, verifying that there was no interdiffusion between the perovskite BaTiO3 and spinel Co-NiFe2O4 phases during the crystallization phase. A detailed analysis of the crystalline structures was conducted to quantify the phases using the Rietveld method, and thus ensure the success of the in situ sol-gel synthesis.
... The coupling coefficient for this materials is achieved by means of the mechanical contact between a piezomagnetic (or magnetostrictive) material and a piezoelectric (or electrostrictive) phases. Such an approach is attractive due to possibility of the independent components selection for the performance at room temperature, huge coupling coefficients [6][7][8] and different connectivities [7,9]. ...
... The coupling coefficient for this materials is achieved by means of the mechanical contact between a piezomagnetic (or magnetostrictive) material and a piezoelectric (or electrostrictive) phases. Such an approach is attractive due to possibility of the independent components selection for the performance at room temperature, huge coupling coefficients [6][7][8] and different connectivities [7,9]. ...
Article
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The composite material filled with nano-sized BaTiO3 and Fe3O4 was designed and studied. The aluminium phosphate ceramics was used as a matrix. The XRD analysis demonstrates only the crystalline structure of the fillers used. The thermogravimetric analysis proves the thermal stability of the composites up to 950 K. The Maxwell–Wagner relaxation was observed in the dielectric spectra of the investigated composites. The dielectric spectroscopy proves the close contact between the nanoparticles with the different ferroic ordering. The phosphate-based composites have been proved to be a prospective candidate for the multiphase multiferroic materials design and development.
... 5,6 It is one of the most widely studied material due to its superior properties, such as moderate saturation magnetization (~ 80 emu/g) at room temperature, high coercivity (5400 Oe), high cubic magnetocrystalline anisotropy (K1 ~ 10 6 erg/cm 3 , highest among all ferrites), high Curie temperature (~ 520 ) and high chemical stability. [2][3][4][5][6][7][8][9][10][11][12][13][14][15] Also, CoFe2O4 (CFO) can be used as one of the constituent phase (magnetostrictive phase) along with the piezo/ferroelectric phase enabling the designing of magneto electric/multiferroic composites for magnetoelectric device application. 15,16 These superior properties makes cobalt ferrite interesting for application in spintronic devices, magnetostrictive sensors, transducers, actuators, drug delivery, magnetic recording media, magnetic fluids, and memory devices. ...
... [2][3][4][5][6][7][8][9][10][11][12][13][14][15] Also, CoFe2O4 (CFO) can be used as one of the constituent phase (magnetostrictive phase) along with the piezo/ferroelectric phase enabling the designing of magneto electric/multiferroic composites for magnetoelectric device application. 15,16 These superior properties makes cobalt ferrite interesting for application in spintronic devices, magnetostrictive sensors, transducers, actuators, drug delivery, magnetic recording media, magnetic fluids, and memory devices. 3 However, it must be noted that the properties of CFO depends on various factors viz., synthetic route, particle size, cation distribution etc. 2,17-196-18 CFO generally crystallize into mixed spinel structure with space group having structural formula (Co1-δFeδ)A [CoδFe2δ]BO4, where round and square brackets represent A-site and B-site, respectively and δ represents degree of inversion . ...
Article
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The present study explores the effect of Dy³⁺ rare-earth ion substitution on the crystal structure, morphology, and magnetic properties of magnetostrictive Co0.7Mn0.3Fe2O4 spinel ferrite and demonstrates their potential applications in magnetomechanical sensors. The intrinsic CoFe2O4 and Dy-substituted Co0.7Mn0.3Fe2−xDyxO4 (x = 0.0–0.1) were prepared by the standard solid-state chemical reaction method. X-ray diffraction studies along with the Rietveld refinement confirm that all the samples exhibit single-phase cubic spinel structure with space group Fd3¯m. Raman and Mössbauer data analyses reveal that the cation redistribution with Mn and Dy cosubstitution in CoFe2O4 and confirm the presence of a mixed spinel structure. Electron microscopy analysis indicates the significant effect of Mn, Dy cosubstitution on the microstructure of CoFe2O4. All the samples exhibit the magnetic hysteresis (M-H) loops at 5 K and 300 K. Saturation magnetization (Ms) and the cubic anisotropy constant (K1) values increase with Mn substitution, while with Dy substitution, Ms reduces due to the decrease of magnetic interactions with Dy substitution. However, the coercive field decreases with Mn and increases with Dy substitution. Higher values of magnetostriction coefficients (λ11 = −95 ppm and λ12 = 52 ppm) and the strain derivative (dλ/dH=0.075ppm/Oeat600Oe) observed make Co0.7Mn0.3Fe1.95Dy0.05O4 a suitable candidate for designing torque/stress sensors and a magnetostrictive phase for making magnetoelectric composite. Chemical composition optimization yields higher values Ms (89 emu/g, i.e., 3.73 μB) at lower coercivity (Hc = 241 Oe) for Co0.7Mn0.3Fe2O4 and higher values of λ11, λ12, and dλ/dH at a lower magnetic field (below 800 Oe) for Co0.7Mn0.3Fe1.95Dy0.05O4. The results suggest and demonstrate that Co0.7Mn0.3Fe2O4 and Co0.7Mn0.3Fe1.95Dy0.05O4 are the potential candidates for designing magnetomechanical sensor applications.
... However, all the reports in the literature on the CFO-BTO composites cite much low ME coefficient than that expected compared to the theoretical predictions. This has been ascribed to the type of connectivity, interface bonding, the strong influence of elastic coupling, the effect of sintering temperature and microstructure of the composites [31][32][33][34][35][36]. In the reported studies, the CFO-BTO particulate composites are sintered in the temperature range of 1000-1300°C for different durations from 1 to 12 h. ...
Article
The magnetic properties of sintered particulate magnetoelectric CoFe2O4−BaTiO3 composites have been studied. The particulate composites are sintered in the temperature range 1000–1300 °C, for a short duration of 10 min. The magnetic transition temperature (Tc) of CoFe2O4 is found to decrease with increasing the sintering temperature and sintering time, as well as on increasing the BaTiO3 content in the composites. Similarly, a reduced saturation magnetization at room temperature, compared to that expected for the CoFe2O4 content, is also observed. Powder X-ray diffraction studies showed the presence of the impurity phase related to the hexagonal ferrite BaFe12O19 in all the composites. Ba2Fe2Ti4O13 is observed as a second impurity phase in the BaTiO3−rich composites. The present studies suggest that the composition of the piezomagnetic (CoFe2O4) phase in the CoFe2O4−BaTiO3 magnetoelectric composite is affected during the sintering process.
... However, using example values for cobalt-ferrite gives a change in the spin dependent dipole density of 1×10 −6 e/Å for a 90 • rotation of the surface spins of the 4x4 Co slab. [24] This estimate assumes an average bond length of 0.14 nm and ignores any distortion of the molecule on the surface. ...
Preprint
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The mechanism by which paramagnetic or diamagnetic molecules are able to alter the anisotropy of transition metals remains elusive. Here, we present a molecule-metal bilayer whose low temperature coercivity of up to 1.6 T and energy product of over 350 kJ/$m^3$ rival those of rare earth permanent magnets at low temperatures. Since this result is obtained using a non-magnetic molecule, $C_{60}$, such large coercivities cannot be explained by conventional exchange bias models. Instead, we propose a new form of surface anisotropy, dubbed $\pi$-anisotropy, based on the spin-dependent $\pi$-d hybridisation at metallo-molecular interfaces and the resultant spin-dependent interfacial dipole. We give evidence that this effect is currently limited to low temperatures only because of the rotational degree of freedom of the $C_{60}$ molecule and anticipate that further research may reveal metal-molecule composites which could exhibit this behaviour at higher temperatures.
... Whereas, with the increase of frequency, the pile up effect is reduced because electrons keep on switching their direction of motion and hence the polarization decreased. Another factor contributes to the permittivity at lower frequencies is the space charge polarization due to inhomogeneous dielectric structure and other imperfections, such as the existence of impurities, porosity, and grain structure [21]. When a small amount of free charges stay in the center of the boundary surface to neutralize each other, a space charge layer will be formed and thus the space field can be changed, which is equivalent to increasing the dielectric properties. ...
Article
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Magnetoelectric composite ceramics Fe3O4/PbZr0.52Ti0.48O3 with different molar ratios (Fe3O4/PbZr0.52Ti0.48O3 = 1:1, 1:2 and 1:8) were prepared by combining hydrothermal method and sol–gel method, effects of molar ratio on the structure, dielectric and multiferroic properties were investigated. The results indicate that the synthesized composites show bi-phase structure, ruling out the presence of any obvious impurity phases. The grains can be divided into two types, the larger grain is attributed to PbZr0.52Ti0.48O3 while the smaller one can be considered to be Fe3O4. With the molar ratio increasing, the grain shape of PbZr0.52Ti0.48O3 changes from stripe to bulk-like while the shape and size of Fe3O4 is near the same. Both the dielectric constant and loss decrease with frequency, the specimen with the molar ratio of 1:8 shows the largest dielectric constant, while the sample has the lowest dielectric loss when the molar ratio is 1:2. With the increase of molar ratio, the height of the relaxation peak decreases and the peak position shifts to higher temperature. When the molar ratio is 1:8, the relaxation peak disappears due to less interface polarization. The remnant polarization increases with increasing the molar ratio, the maximum value is 1.12 μC/cm², obtained at 1 kHz when the molar ratio is 1:8. Anomalous magnetic properties are observed, in which the magnetization increases first and then decreases with molar ratio, the largest saturation magnetization is ~ 30 emu/g when the ratio is 1:2 due to the strong interface interaction. The sample with the ratio of 1:1 shows the largest magnetic loss because of the highest content of magnetic phase. The magnetization shows a monotonic variation behavior with the molar ratio, indicating the strong interface interaction between the two phases. The maximal ME coupling coefficient is about 5.12 mV/(cm.Oe) for the sample 1:2 due to the stronger magnetic and ferroelectric properties.
... The relatively low values of α E31 and α E33 are ascribed to the presence of pores and voids that make the mechanical connectivity poor between the grains of the piezomagnetic-piezoelectric phases [43]. For conclusive evidence of the presence of the spontaneous polarization, the room temperature polarizationelectric field (P-E) loop has been performed for x ¼ 0.2, 0.3, and 0.5 nanocomposites [44] at a f of 43 Hz as shown in inset of Fig. 2(a). The similar kind of polarizationelectric loop has been measured to confirm the ferroelectric behavior of materials [8,9,11,18]. ...
Article
Nanocomposites of xNiFe2O4 – (1-x)ZnO (x = 0.2, 0.3 and 0.5) have been prepared through chemical “pyrophoric reaction process”. Structural characterizations confirm the formation of both NiFe2O4 and ZnO phases in nanocomposites. The room temperature magnetoelectric coupling, dielectric and temperature dependence of electrical properties of the prepared nanocomposites have been investigated. The observed magnetoelectric coupling may be owed to the field dependent magnetostriction property of the piezomagnetic phase of the nanocomposites. The magnetoelectric voltage is measured in both transverse and longitudinal directions. Electrical transport studies are carried out using an ac impedance spectroscopy and dc resistivity techniques. Metal-semiconductor transition of the nanocomposites has been explained on the light of delocalized and localized charge carrier. Detailed analysis of ac conductivity with frequency at varying temperature suggests the thermally activated electronic transport conduction mechanism for large and small polaronic hopping in the nanocomposites. The semiconducting band gap of the nanocomposites has also been estimated using recorded absorbance spectra. Light-dependent current-voltage characteristics are non-Ohmic in nature and exhibit significant electrical memory effect with resistive switching. On light illumination, there is a significant decrement in current with quenching of hysteresis loop suggesting high recombination of the photo-generated carriers.
... Similar superparamagnetic behavior has been observed for other magnetic materials such as CoFe 2 O 4 , MnFe 2 O 4 , and Fe 3 O 4 , prepared using the coprecipitation method. [25][26][27][28] In addition, the magnetization of all the samples saturated at approximately 1500 Oe, except for the N3Z7FO/BTO sample. The saturation magnetization, M s , values were determined by extrapolating the M-H curve to zero applied field, and the results are listed in Table II. ...
Article
Ni1−xZnxFe2O4/BaTiO3 (x = 0.3, 0.4, 0.5, 0.6, and 0.7) magnetoelectric composite ceramics have been prepared by combining the coprecipitation and sol–gel methods, and their structural and multiferroic properties studied and compared. The results indicate that the synthesized composites present biphase and composite structure, with no evident impurities observed. The lattice of the Ni1−xZnxFe2O4 crystal structure is distorted owing to the incorporation of Zn²⁺ ions. The samples present irregular microstructure and abnormal grain growth, which can be attributed to the heterogeneous distribution of the ferroelectric and magnetic phases during preparation. The chemical composition of the larger grains is Ni1−xZnxFe2O4, while that of the smaller grains is proven to be BaTiO3. The dielectric constant of the ceramics first increases then decreases as the Zn²⁺ ion content is increased, which is related to the irregular microstructure of the ceramics. Both the frequency dependence of the dielectric loss and the temperature dependence of the dielectric constant present two relaxation peaks for all samples. The dielectric loss peaks are attributed to the slow polarization process, such as turning-direction and space-charge polarization, while the dielectric constant peaks can be ascribed to the ferroelectric phase transition of BaTiO3 and relaxation polarization of the composites. The abnormal magnetization behaviors can be induced by the A–B superexchange interaction caused by the addition of nonmagnetic Zn²⁺ ions.
... The main parameters of the loops are listed in Table 2. The coercive field of the studied samples is in the range of 2300-2600 Oe, which is higher in comparison to the previously reported data [28] and may be attributed to a strong demagnetizing field. However, it is similar to the measurements of the same 30 nm CoFe 2 O 4 particles [29]. ...
Article
Bulk BaTiO3-xCoFe2O4, x = 0.1 – 0.6 magnetoelectric composites were prepared using the phosphate bonded ceramics approach. XRD analysis proved the purity of both phases. The dielectric properties are governed by a series of composition-dependent Maxwell-Wagner relaxations and conductivity at lower frequencies and a phase transition-related anomaly at higher frequencies. A dielectric constant as high as 616 – 9387i is observed at 500 K for BaTiO3-0.6CoFe2O4. The magnetic hysteresis demonstrates a high Ms/Mr ratio of 0.46, which is related to the around 30 nm size of the CoFe2O4 particles. The measured direct magnetoelectric coupling coefficient of 1.1 mV Oe⁻¹ cm⁻¹ is higher than that of the conventionally sintered ceramics and compatible with that of core-shell structures.
... Several synthetic pathways have been used for the preparation and production of cobalt ferrite. Among others, co-precipitation [21,22], polyol [23], microwave-assisted [24] and solid state synthesis [25] have already demonstrated great potential for the production of these materials. However, one of the most versatile methods for producing ferrite is the sol-gel method. ...
Article
Spinel cobalt ferrites (CoFe2O4) with varying inversion degrees were prepared via a citrate-nitrate sol-gel method. The gels, prepared with different cation stoichiometries, were dried at 110 °C, further treated at 850 °C and subjected to structural and magnetic characterization. X-ray diffractograms of the samples confirm the successful synthesis of the spinel. The synthesis conditions displayed a strong influence over the positioning of the cations in the crystal structure of these ferrimagnets. Mössbauer spectra yield inversion degrees of approximately 1.0 and 0.0 for the samples prepared with excess cobalt and excess iron, respectively. These cationic arrangements are also confirmed via Raman spectroscopy, which shows a larger splitting of the A1g signal in the sample with the highest inversion degree. The magnetic character of the samples at room temperature does not agree with what should be expect from the net magnetic moments per unit cell. However, this phenomenon was explained by the different exchange integrals at play in each of the systems, as shown by the magnetic critical temperature of each arrangement, which differed by approximately 100 °C. The proposed pathway has proven to be an accurate method for completely switching the cationic arrangement of CoFe2O4, further enabling the application of this material in advanced magnetoresistive and spintronic devices.
... Enhancements of ME coefficient have long been actively studied in both fundamental and engineering fields, many attempts have been studied [7][8][9]. Recently, researchers have engineered the ME coefficient by studying different connectivity schemes (geometries) [10,11] and orientations [12][13][14] of multiferroic composites, which has been a new approach to develop high ME coefficient composites. ...
Article
In this study, orientation dependences of magnetoelectric coefficient α E33 along arbitrary directions were investigated in 3 m Pb(Zr 0.6 Ti 0.4 )O 3 (PZT60/40) - m3m CoFe 2 O 4 (CFO) composites with 2-2, 1-3 pm, and 1-3 m/p connectivities using coordinate transformation method. Orientation dependences of parameters of elastic, dielectric, piezoelectric, permeability, and piezomagnetic properties for PZT60/40 and CFO were investigated independently to finally obtain anisotropy characteristics of the magnetoelectric coefficient of composites. Orientation dependence of α E33 in composites is found mainly determined by that of d 31 and e 31 of PZT60/40 and q 31 of CFO for all three investigated connectivities. The greatly enhanced α E33 ' of composites along specific orientations of piezoelectric/magnetostrictive phases were presented, which lies in: PZT60/40 || [100] - CFO || [001] combination for 2-2 laminate (f = 0.43, α E33 = 81 mV cm ⁻¹ Oe), PZT60/40 || [001] - CFO || [001] combination for both 1-3 p/m (f = 0.6, α E33 = 125 mV cm ⁻¹ Oe) and 1-3 m/p fiber composites (f = 0.6, α E33 = 121 mV cm ⁻¹ Oe). Volume fraction f max is found unchanged when various orientations of piezoelectric or magnetic phases were applied. The results suggest that the magnetoelectric coefficient of multiferroic composites with various connectivities could be significantly enhanced by adopting specific orientations.
... In recent years multiferroic composites have been a lot of interest in class of multi-functional materials which exhibit strong magnetoelectric (ME) [1][2][3][4]. The ME materials can couple with stimulating either by electric or magnetic fields [5][6][7][8][9][10]. ...
Article
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We have investigated structure, magnetic and dielectric of La 0.5 Sr 0.5 Fe 0.9 Mn 0.05 Ti 0.05 O 3 (LSFMTO) system using ultrasonic mixing method. LSFMTO system is driven by 40 kHz frequency and calcinate process up to 750°C for 30 minutes. Then sintering process also carried out by systematic various of temperature 900 °C, 1000 °C, and 1100 °C respectively, as same time previously. Very interestingly, the LSFMTO system exhibits ferroelectric (La,Sr)(Fe,Mn,Ti)O 3 and ferromagnetic SrFe 12 0 19 properties and minor phase La 2 O 3 by XRD interpretation analyzation. We have also carried out the measurement of LSFMTO system using a systematic applied the external magnetic field from 0 to 1 T and the electric polarization from 0 to 50 kV/m . The hysteresis curve of LSFMTO system shows the electricty coercive (Ec) increases while the saturation polarization (Pc) and the remanent polarization are tendency constant at the magnetic strength of 0.12 T. We believed that our finding of LSFMTO system is potensial to multiferroic material as magnetic sensor application.
... The average grain size of the pure BT and composites are shown in Table 2. The grain size of BT agrees well with the earlier reported values [23]. We see from Table 2, the average grain size of the composites gradually decreases with increasing LF in composites. ...
Article
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In this paper, we report the synthesis of a multiferroic composite BaTiO 3 (BT) doped with Li 0.5 Fe 2.5 O 4 (LF) having chemical formulae (1-x) BaTiO 3 + (x) Li 0.5 Fe 2.5 O 4 (x = 0, 0.05, 0.1, 0.15) using conventional solid state reaction method. We have focused on the structural and multiferroic properties of LF doped BT. We have characterized the grown composites by XRD, FESEM with EDAX, Piezoelectric coefficient, dielectric studies, A.C conductivity studies, magnetic properties and magneto-electric (ME) voltage coefficient. The XRD studies reveal the tetragonal nature of all composites. We have calculated the lattice parameters and the crystallite size from XRD pattern. The XRD patterns of all composites exhibit diphasic nature. The dielectric constant of all composites decreases steeply with an increase in the frequency and attains a nearly constant value in the high-frequency region. The variation of conductivity with the frequency reveals that at high-frequency region, the maximum conductivity decreases with an increase in the % of LF. The variation of conductivity with temperature reveals that the Curie temperature of the composite decreases as and when the % of LF is increased in BT. We have also studied the temperature dependence of the magnetization and the variation of the magneto-electric voltage coefficient (ME) with a magnetic field. The transition temperature and Neel temperature of all composites increased with increase in LF. The Magneto-electric (ME) coefficient of composites decreased with increase LF content.
... [32][33][34][35][36] Interestingly, the performance of such composites, gauged by the ME coupling coefficient α, is found to be highly sensitive to microstructure. [37][38][39][40] On the other hand, modeling predicts complex domain evolution in composites. [41][42][43] PFM based investigation (direct ME effect) is expected to reveal a strong modulation of the local piezoresponse (i.e., polarization) as a function of the externally applied magnetic field 20-28 giving an indirect insight into the spatial distribution of the local magnetoelectro-mechanical interactions. ...
Article
The term big-data in the context of materials science not only stands for the volume, but also for the heterogeneous nature of the characterization data-sets. This is a common problem in combinatorial searches in materials science, as well as chemistry. However, these data-sets may well be 'small' in terms of limited step-size of the measurement variables. Due to this limitation, application of higher-order statistics is not effective, and the choice of a suitable unsupervised learning method is restricted to those utilizing lower-order statistics. As an interesting case study, we present here variable magnetic-field Piezoresponse Force Microscopy (PFM) study of composite multiferroics, where due to experimental limitations the magnetic field dependence of piezoresponse is registered with a coarse step-size. An efficient extraction of this dependence, which corresponds to the local magnetoelectric effect, forms the central problem of this work. We evaluate the performance of Principal Component Analysis (PCA) as a simple unsupervised learning technique, by pre-labeling possible patterns in the data using Density Based Clustering (DBSCAN). Based on this combinational analysis, we highlight how PCA using non-central second-moment can be useful in such cases for extracting information about the local material response and the corresponding spatial distribution.
... The literature survey reveals that there ismuch research works carried out by several connectivity schemes on BTO and spinel ferrite such as 0-3 connectivity schemes [1,[20][21][22][23][24][25][26], 2-2 connectivity schemes [11,27,28] and 3-1 connectivity schemes [11,29,30]. These studies were basically focused on ME coupling, crystal structure and dielectric measurements. ...
Article
Nanocomposite of BaTiO3 (BTO) and CoFe2O4 (CFO) exhibits magnetism which can be explained by assuming the surface interaction between BTO and CFO. The crystal phase of the nanocomposite has been studied by the X-ray diffraction (XRD) technique which has been supported by the Raman spectra analysis. Surface morphology has been studied using SEM (scanning electron microscopy) and TEM (Transmission electron microscopy). The ball milling has transformed diamagnetic microcrystalline of BTO to nanocrystal ferromagnetic BTO which could be due to oxygen vacancies at the surface of BTO. Thus there has been ferromagnetic exchange interaction between nanocrystalline BTO and CFO at their interfaces which ultimately affect the spin arrangement at the surface of CFO. This interaction leads to high saturation magnetization. A hump in the M–H loop at around ±4000 Oe has been observed for the BTO-CFO composites. It changes its shape with the increase in concentration of CFO. It is probably due to increase of superexchange interaction between CFO and BTO at the interface.
... In comparison to YbBT phase, the hysteresis loops of composites (C3, C6 and C9) are not saturated. The non saturated and rounded edges in the P-E loops are due to the leakage currents in the composites [58]. The leakage current generated in the multiferroic composites is due to the presence of ferrite phase, which is distributed uniformly in such particulate composites. ...
Article
The quest for multiferroic materials, where the phenomena of ferroelectricity and ferromagnetism are intimately coupled, is of great technological and fundamental importance. In this work, multiferroic particulate composites, (1-x) Ba0.95Yb0.05TiO3 -xCoYb0.1Fe1.9O4 (x = 0.3, 0.6 and 0.9), were prepared by the solid-state reaction method. In all the composites, lattice parameters of CoYb0.1Fe1.9O4 phase decrease with the decrease in weight percentage of Ba0.95Yb0.05TiO3 phase. This effect resulted in an increase in the density and magnetoelectric effect of the composites. The dielectric loss and conductivity of composites was minimized, and a drop in Curie temperature was observed by doping ytterbium in the individual phases. The Austin-Mott model predicts that the conductivity can be attributed to the small range polaron hopping. The Mott conductivity dominates at low temperatures and disturbs the domain structure resulting in unsaturated P-E loops. The Law of approach was adopted to calculate the magnetocrystalline anisotropy, as all the composites were multidomain structured. The incorporation of ytterbium increased the magnetostriction in CoFe2O4 phase which results in an increase in magnetocapacitance of all the composites.
... Wang et al. report 0.3 emu/g saturation for MENPs (1.8 kA/m assuming the particle has a density close to CFO of 6.02 g/cc 101 ) for an immobilized single-layer array 102 and Etier et al. report 20 emu/g (120.4 kA/m assuming the same) for both a loose powder and fixed powder. Etier et al. note that hysteresis is not present in the loose powder, because the particles can freely rotate 103 . This is as seen clinically in vivo, which our model represents. ...
Article
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Magnetoelectric materials hold untapped potential to revolutionize biomedical technologies. Sensing of biophysical processes in the brain is a particularly attractive application, with the prospect of using magnetoelectric nanoparticles (MENPs) as injectable agents for rapid brain-wide modulation and recording. Recent studies have demonstrated wireless brain stimulation in vivo using MENPs synthesized from cobalt ferrite (CFO) cores coated with piezoelectric barium titanate (BTO) shells. CFO–BTO core–shell MENPs have a relatively high magnetoelectric coefficient and have been proposed for direct magnetic particle imaging (MPI) of brain electrophysiology. However, the feasibility of acquiring such readouts has not been demonstrated or methodically quantified. Here we present the results of implementing a strain-based finite element magnetoelectric model of CFO–BTO core–shell MENPs and apply the model to quantify magnetization in response to neural electric fields. We use the model to determine optimal MENPs-mediated electrophysiological readouts both at the single neuron level and for MENPs diffusing in bulk neural tissue for in vivo scenarios. Our results lay the groundwork for MENP recording of electrophysiological signals and provide a broad analytical infrastructure to validate MENPs for biomedical applications.
... In many cases these methods follow wet chemical routes [5]. Barium titanate, BaTiO 3 (BTO), is often used as a ferroelectric because of its excellent piezoelectric properties and lead free chemical composition [7]. The dielectric and ferroelectric properties of BaTiO 3 have been extensively studied since the 1940s because of their wide potential technological applications [8]. ...
Article
In this work, we have studied the microscopic, ferroelectric, magnetic and microstructural characteristics of cobalt ferrite/barium titanate core–shell magnetoelectric nanocomposites. Magnetoelectric nanocomposites with 22 nm mean particle size were successfully obtained and observed by High Resolution Transmission Electron Microscopy and Scanning Transmission Electron Microscopy. Ferroelectric and magnetic properties were studied and confirmed by Scanning Probe Microscopy. Ferroelectric nanoregions were observed at the nanocomposites shell and magnetic nanoregions were highlighted in the nanocomposites core. Thus, magnetoelectric nanocomposites suitable for use in multifunctional biological/biomedical/bioengineering applications, as magnetic field nanosensors and other multifunctional applications were obtained.
... However, most of the reports in the literature on BT based composites cite much lower ME coupling coefficient than that has been achieved from theoretical predictions. This small value may be attributed to the microstructure of the composite, interface bonding, type of connectivity and influence of elastic coupling as reported in previous literature [21][22][23][24]. In this work, by combining the excellent ferroelectric characteristics of modified BT and ferrimagnetic characteristics of MFO, we have synthesized (1-x)Ba 0.83 Ca 0.10 Sr 0.07 TiO 3 -(x)MnFe 2 O 4 (x = 0.0, 0.05, 0.10, 0.20 and 0.30) and systematically investigated their structural, dielectric, ferroelectric, piezoelectric and magnetic properties. ...
Article
We report the successful synthesis of Ba0.83Ca0.10Sr0.07TiO3–MnFe2O4 multiferroic composites showing significant improvement in electromechanical and magnetoelectric properties. All the composite samples have formed a diphasic perovskite-ferrite composite without the presence of any impurity or intermediate phase. The bare as well as composite samples have shown classical dielectric behavior even at higher ferrite substituted samples. The electrical characteristics of composite samples have shown slight deterioration, which is mainly attributed to non-ferroelectric MnFe2O4. However, the composites still exhibit high enough piezoelectric behavior and the modification in the electromechanical response of composites is mainly caused by a change in applied stress with MnFe2O4 addition. The M-H loops of composites have demonstrated a ferrimagnetic behavior with a substantial increase in saturation magnetization on increasing the ferrite concentration. Further, the composites have shown better coupling between the ferroelectric and ferrimagnetic phases, which has resulted in an improved magnetoelectric characteristic. The role of oxygen vacancies on ferroelectric and magnetic properties of prepared composites has been systematically studied.
... From Fig. 6a, it becomes clear that the P-E loop obtained for the YbPZT shows typical saturation, whereas for all the composites the loops are not completely saturated and CNdFO doesn't have any kind of saturation there. The less saturation observed in the P-E loops of the composites is because of the presence of the conductive ferrite part (CNdFO) in the composites which results in the leakage currents from the mixed valences of Fe 2+ -Fe 3+ ions in it [41]. The ferroelectric parameters obtained from the P-E loops summarized in Table 4 confirm that the ferroelectric phase YbPZT has a maximum value of saturation polarization (Ps = 5.64 µC/ cm 2 ) and the coercivity (Ec = 12.88 kV/cm), whereas for all the composites both Ps and Ec decrease as compared to the YbPZT and increase from C2 to C7. ...
Article
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In this report, the multiferroic composites consisting of Yb-doped PbZrTiO3–Nd-doped CoFe2O4 of varying concentrations were synthesized by solid-state reaction method. The XRD studies of all the prepared samples confirm their crystallographic phases. The Williamson–Hall (W–H) approach was employed to obtain the average crystallite size and the strain in the composites. The SEM analysis ascertained the non-uniform distribution with agglomerated grains in the composites. The ferroelectric nature of the composites was confirmed from the P-E loops traced out. The M-H hysteresis loops obtained confirmed the ferromagnetic nature of the composites. The magneto-dielectric studies of the composites manifest the strong coupling between the ferrite and ferroelectric phases. The magneto-capacitance MC (%) revealed improvement at the higher ferrite content of the composites. The simultaneous occurrence of both the magnetic and ferroelectric (M-H and P-E) hysteresis loops and strong magneto-electric coupling signifies the multifunctionality and applicability of the prepared composite multiferroics in modern technology.
... Also, the addition of non-ferroelectric CFO phase restricts the switching of domains with the applied electric field hence decrease in polarization parameters. For x = 0.5 the ferroelectric parameter increases for NBT-CFO composite which can be explained by the dependence of leakage current on the ferroelectric behavior of composites and is similar to the behavior explained by Etiar et al. [41]. As already discussed, that the leakage current in these composites mainly arise due to the charge hopping mechanism between the ions of the similar elements throughout the samples, thus enhances leakage current and ferroelectric properties deteriorate. ...
Article
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Multiferroic is the future of the next generation memory devices; because of its advantages over the single-phase ferroelectric and ferromagnetic memory systems. In conventional ferroelectric memory system fatigue effect lowers the storage capacity. Other side high magnetic energy is essential for writing the data in ferromagnetic-based memory system. Considering these facets and taking environmental issue in the account we developed lead-free Na0.5Bi0.5TiO3 (NBT)–CoFe2O4 (CFO) multiferroic system via two wet chemical methods. To check the feasibility of the system for device application various physical measurements were carried out. The structural analyses have been done by XRD, FTIR and FESEM which confirm the coexistence of bi-phase in these composites. The dense microstructure was observed for composite sample with highest CFO concentration. In comparison to pure ferrite phase, the composite exhibit low leakage current density, high dielectric constant with low dielectric losses. Interestingly, the CFO affected the phase transition temperature of the NBT as evidenced by the dielectric behaviour as a function of temperature. Also, multiferroic studies were influenced by the content of CFO/NBT phases in composite system. Furthermore, the highest value of ME coupling (~ 64 μV/Oe-cm) was observed for the 50CFO/50NBT composite system.
... In addition, high energy ball milling is a simple, economic and effective process to achieve homogeneous composites where the interphases between both phases are important for its applications [20]. Various methods, such as Sol-Gel [39], conventional double sintering [40], and coprecipitation [41], have been employed. In this study, high-energy ball milling is used as a mixing process in the preparation of the composites, allowing the formation of a homogeneous composite microstructure, which could promote the magnetoelectric and magnetodielectric coupling [42,43]. ...
Article
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In this work, a magnetodielectric coupling observed in barium titanate–cobalt ferrite composites synthesized using high-energy ball milling assisted via a thermal treatment is discussed. Vibrating sample magnetometry and dielectric spectroscopy showed that multiferroic composites possess both ferromagnetic and dielectric behaviors inherited from the parent ferromagnetic cobalt ferrite and ferroelectric barium titanate phases. The magnetocapacitance (up to 35%) recorded for x = 0.3, (1-x)BaTiO3–xCoFe2O4, can be attributed to the spin-dependent filtering mechanism. The composite with the aforementioned composition exhibited a homogeneous matrix–particle composite microstructure, which was achieved via high-energy ball milling during the mixing stage.
... Due to a wide range of choices available for suitably performing magnetostrictive and piezoelectric phases, the composite approach to form a ME material has been well exploited. However, the performance of such composites, which is gauged by the ME coefficient α, is often found to be weak and highly variable even within a given material system [6][7][8][9][10]. Of the various possible phase connectivity schemes for ME composites, the multilayered configuration (2-2) is the simplest, and also most frequently studied due to the high ME coefficient values and the option of mechanical resonators [11]. ...
Article
Local observation of the stress mediated magnetoelectric (ME) effect in composites has gained a great deal of interest over the last decades. However, there is an apparent lack of rigorous methods for a quantitative characterization of the ME effect at the local scale, especially in polycrystalline microstructures. In the present work, we address this issue by locally probing surface magnetic state of barium titante - hexagonal barium ferrite (BaTiO<sub>3</sub> - BaFe<sub>12</sub>O<sub>19</sub>) ceramic composites using Magnetic Force Microscopy (MFM). The effect of the piezoelectrically induced local stress on the magnetostrictive component (BaFe<sub>12</sub>O<sub>19</sub>, BaM) was observed in the form of the evolution of the magnetic domains. The local piezoelectric stress was induced by applying a voltage to the neighboring BaTiO<sub>3</sub> grains, using a conductive atomic force microscopy tip. The resulting stochastic evolution of magnetic domains was studied in the context of the induced magnetomechanical anisotropy. In order to overcome the ambiguity in the domain changes observed by MFM, certain generalizations about the observed MFM contrast are put forward, followed by application of an algorithm for extracting the average micromagnetic changes. An aggregate change in domain wall thickness of 50 nm was extracted, giving a lower limit on the corresponding induced magnetomechanical anisotropy energy. Furthermore, we demonstrate that this induced magnetomechanical energy is approximately equal to the K<sub>1</sub> magnetocrystalline anisotropy constant of BaM, and compare it with a modeled value of applied elastic energy density. The comparison allowed us to judge about the quality of the interfaces in the composite system, by roughly gauging the energy conversion ratio.
... Beyond this maxima, a decrease in a is observed, and a slight change is observed at high magnetic fields. The increase in a is ascribed to the improved elastic interaction between BDT and ZCFO phases due to dominance of easy axis towards magnetostriction [65]. The maximum value of a is observed near the saturation field (B 1.5 kOe), at which maximum strain is transferred from the magnetic ZCFO phase to ferroelectric BDT phase [38], supported by the M-H hysteresis loops. ...
Article
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To manipulate electric properties by magnetic field in functional multiferroic materials has driven an ever-increasing demand for four state memories. In multiferroic materials, the various physical properties depend upon the interface area between the two ferroic phases. Among functional properties, the magnetoelectric effect arises due to the strain mediated coupling and depends upon the interface area between the two ferroic phases. Keeping this in view, we fabricated (1 − x) Ba0.99Dy0.02Ti0.99O3 − x CoFe1.9Zn0.1O4 (x = 0.02, 0.04 and 0.06) particulate multiferroic composites. In composites, the magnetoelectric effect is found to be dependent on mass fraction of CoFe1.9Zn0.1O4 magnetic phase and crystallite size of composites. The Dy³⁺ ions have proved themselves as potential candidates to enhance dielectric constant and the Mott theory established hopping conduction mechanism. A typical ferroelectric and ferrimagnetic nature of Ba0.99Dy0.02Ti0.99O3 and CoFe1.9Zn0.1O4 phases is ascertained by tracing P–E and M–H loops respectively. The magneto-crystalline anisotropy for multi-domains is evaluated by using the Law of Approach to saturation. From magnetoelectric coupling studies, high magnetoelectric output is obtained in composites with higher ferrite content. In conclusion, the particulate composites of Ba0.99Dy0.02Ti0.99O3 and CoFe1.9Zn0.1O4 phases coexist with tunable magnetoelectric properties, thereby making them as a multifunctional materials.
... However, using example values for cobalt-ferrite gives a change in the spin dependent dipole density of approximately 1 × 10 −6 e/Å for a 90 • rotation of the surface spins of the 4x4 Co slab. 25 Despite the very high adsorption energy of the C 60 molecule on the Co surface, DFT simulations of the transition state (TS) predict a maximum energy barrier to rotation from HP to HH of 0.25 eV, Fig. 3(a). This demonstrates that the energy required to rotate the C 60 molecule on the surface is signficicantly lower than the adsoprtion energy and is likely to be further reduced in real systems due to surface defects. ...
Article
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High coercivity magnets are an important resource for renewable energy, electric vehicles, and memory technologies. Most hard magnetic materials incorporate rare earths such as neodymium and samarium, but concerns about the environmental impact and supply stability of these materials are prompting research into alternatives. Here, we present a hybrid bilayer of cobalt and the nanocarbon molecule C60 which exhibits significantly enhanced coercivity with minimal reduction in magnetization. We demonstrate how this anisotropy enhancing effect cannot be described by existing models of molecule-metal magnetic interfaces. We outline a form of anisotropy, arising from asymmetric magnetoelectric coupling in the metal-molecule interface. Because this phenomenon arises from π−d hybrid orbitals, we propose calling this effect π-anisotropy. While the critical temperature of this effect is currently limited by the rotational degree of freedom of the chosen molecule, C60, we describe how surface functionalization would allow for the design of room-temperature, carbon-based hard magnetic films.
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Ferroelectric–ferrite composites of BaTiO3–CoFe2O4 (BT–CFO) is synthesized via solid state reaction method. Powder XRD confirms the phase purity as well as composite formation with tetragonal phase of the BaTiO3. The FTIR and SEM–EDS studies also confirm the formation of BT–CFO composites. The P–E loop measurement confirmed the ferroelectric nature of the sample. The maximum energy storage density and efficiency achieved for BT–5CFO (5% CoFe2O4) composite was 8.33 mJ/cm³ and an efficiency of 59.7% respectively. The coupling between the ferroelectric and ferromagnetic phases was observed in the variation of P–E loop with magnetic field. A decrease in the maximum polarization was found with increase magnetic field. The P versus M analysis confirmed the highest magneto-electric (ME) coefficient of 13.33 mV/cm/Oe for BT–5CFO composite. These results are related to the volumetric and piezoelectric strain, suppression of ferroelectric ordering reveal the magnetoelectric nature in BaTiO3–CoFe2O4 composites.
Article
Here, we report a series of multiferroic composites with general formula 0.8[(1–x)BiFeO3 + xCrFe2O4] + 0.2Cr2O3 (x = 0.0, 0.33, 0.66 and 1.0), prepared using sol–gel based auto-combustion method. Structural analysis helped us to verify the three phases in the composites and confirmed the dominance of spinel phase with introducing the CrFe2O4 contents. Field emission scanning electron microscopy confirmed the distinct, sharp, spherical shaped grains, and the reduction in the grain size with the evolution of spinel phase. Energy dispersive X-ray spectroscopy verified the presence of key elements in the required stoichiometric ratios. Increment in the maximum and remanent polarization with the addition of spinel phase contents was revealed by the ferroelectric analysis. Recoverable energy was observed to be decreased at first, with the increase in CrFe2O4 concentration, but it increased abruptly when the rhombohedral phase was completely replaced by the spinel phase. A systematic analysis of percentage efficiency has been presented and correlated with the crystalline structural phase transformation in the composite series. The successful incorporation of third phase (Cr2O3) has helped to improve the composite phase stability and ferroelectric pursuits. Magnetic analysis showed an increase in magnetization with the increasing concentration of spinel phase, confirming its ferrimagnetic nature and establishing the multifunctional utilization of prepared tri-phase composites. The studies of magneto-polarizability unwrapped the aptitude of these composites for their usage in multi-state devices and illustrated the aptitude of synthesized composite for their applications in energy storage devices.
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Magnetoelectric (ME) (100-x) Na0.5Bi0.5TiO3 (NBT)–xCoFe2O4 (CFO) (x = 30 and 35) particulate composites were prepared from the pre-sintered individual constituents by adopting mixed oxide route. The composites consisting of CFO grains embedded in a twice-grain-sized NBT matrix were investigated for the enhancement of the ME coefficient. CFO grains are not interlinked which implied that composites are within percolation limit and thereby 0–3 particulates are formed without impurity phase at the interface is evidenced by XRD, SEM micrographs and elemental mapping. Raman modes due to the influence of CFO are visible around 213 cm⁻¹. A non-saturated PE loop indicates leaky behavior and the MH loop depicts the magnetic behavior of samples. ME coefficient is 18.9 mV/cm Oe for the 65NBT-35CFO composite and it is higher than that reported in the NBT–CFO composites. Theoretical modeling of the ME coefficient signifies that the observed experimental value is much smaller than the theoretical value.
Article
Multiscale optimal designs that maximize the magnetoelectric (ME) effect are presented for typical ferroelectric and ferromagnetic material combinations, based on a study of novel microstructures, in an effort to achieve ME properties superior to those of the conventional structure. The asymptotic homogenization theory was employed for scale bridging and numerical solutions were obtained using the finite element method. The longitudinal homogenized ME constant was adopted as the objective function. The phase composition, arrangement, and polarization direction in the microstructure were set as design variables. For all material combinations, multiscale optimization was achieved for novel microstructures beyond existing laminated structures, with the optimum microstructure depending on the material combination. The findings of the study are expected to be useful in the development of high-performance multiferroic composite materials.
Article
In this paper, we report on synthesis magnetoelectric (ME) sandwich type of BaTiO3-P(VDF-TrFE)/CoFe2O4-P(VDF-TrFE)/BaTiO3-P(VDF-TrFE) flexible composite. Nanoparticles of CoFe2O4 (35–40 nm) and BaTiO3 (110–130 nm) were chemically synthesized and then mixed severally with P(VDF-TrFE) copolymer. The sandwich type sample was fabricated using simple cast molding and blending techniques. Structural studies confirmed the phases and the formation of the sandwich magnetoelectric composite. The film showed good ferroelectric and ferromagnetic properties. The enhancement of the magnetoelectric effect for the composite is due to the good isolating circumstances of the piezomagnetic cobalt ferrite phase as well as the proper contact and bonding between the magnetostrictive and electrostrictive phases. The fabricated film could be a potential candidate for different magnetoelectric applications and sensors technology.
Article
Magnetoelectric multiferroic fluids composed of BaTiO3@CoFe2O4 composite nanoparticles dispersed into highly insulating nonpolar oleic acid/silicone oil mixture have been elaborated. The effects of particle volume fraction, magnetic field as well as electric field on the ferroelectric and magnetic properties as well as the magnetoelectric coupling effect have been systematically studied and discussed in this paper. The magnetic characterization shows approximative superparamagnetism, both the remanent magnetization(Mr) and the coercive field(Hc) increase with the increase of volume fraction and the applied electric field. Similarly, superparaelectric state has been observed of the multiferroic fluids, of which both the remanent polarization(Pr) and coercive field(Ec) are near zero, while they enhance with increasing applied magnetic field and volume fraction. Large converse and direct magnetelectric coupling coefficients are estimated to be H = 8.16×10-4 Oe.cm/V and E = 1.58 ×104 V/cm.Oe, respectively. Further analysis indicates that the composite particles can be aligned under external magnetic/electric field so that their magnetic/electric moments can be parallel to external field which in turn results in the changes of the magnetization/polarization directions. These results imply that besides these magnetoelectric fluids consisted of core shell structured nanoparticles, conventional multiferroic fluids based on composite particles may have an opportunity to gain electrical control of magnetization and vice versa, implying potentional applications.
Article
Polycrystalline composite samples of (1-x) (Ba0.8Ca0.2TiO3)–x(Co0.6Zn0.4Fe2O4) (with x = 0.00, 0.01, 0.02, 0.03, 0.04, and 1.00) hereby designated as BZT-CZFO were synthesized via wet chemical method. Room temperature X-ray diffraction confirmed the bi-phase nature of the composites. This was further confirmed by Raman scattering recorded at room temperature. No intermediate or secondary phases were observed for all the compositions. Increasing ferrite concentration decreases the ferroelectric properties of the compounds and the leakage current measurement value was found to obey Ohmic conduction mechanism for all the samples. X-ray photoelectron spectroscopy measurement was carried out to investigate the chemical state of the constituent cations present and it was observed that Ti/Co/Ba retained their characteristic oxidation states, while mixed state was observed for Fe-ions. With increasing ferrite concentration saturation magnetization (Ms) were enhanced and a maximum value achieved for x = 0.04 concentration sample, but remained lower than pure CZFO sample.
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Multiferroic particulate composites have been fabricated by taking the morphotropic phase boundary composition of ferroelectric phase Bi(Ni1/2Ti1/2)O3-PbTiO3 and magnetic phase (Ni,Zn)Fe2O4. The ferroelectric phase has coexisting monoclinic and tetragonal perovskite structures with space group Pm and P4mm, respectively whereas the magnetic phase has spinel cubic structure with space group Fd3m. Rietveld structural analysis for the each components of composite reveals that the tetragonality (c/a) of the ferroelectric phase continuously increases with increasing the concentration of magnetic phase suggesting partial ionic diffusion between ferroelectric and magnetic phases, after composite formation. Composition dependent M\"ossbauer spectra of (1-x)Bi(Ni1/2Ti1/2)O3-PbTiO3/x(Ni,Zn)Fe2O4 reveals the superparamagnetic like behavior for the ferroelectric rich composition with x=0.2. The magnetic ordering increases for the composition with x=0.4 and 0.6 which completely transform into ferrimagnetic for the composition with x=0.9 for the magnetic phase rich compositions. Unlike the ferroelectric or magnetic components which do not exhibit the magnetoelectric response separately, large value of magnetoelectric coefficient (30 mV/Oe-cm) in (1-x)Bi(Ni1/2Ti1/2)O3-PbTiO3/xNi0.6Zn0.4Fe2O4 composite makes it promise for multifunctional applications.
Article
The study deals with design, structural and magnetic characterization, electric and magnetoelectric transport and broad-band dielectric spectroscopy of the all-perovskite core-shell composite nanoceramics of 0-3 connectivity, in which ∼50 nm cores of ferromagnetic La0.65Sr0.35MnO3 (LSMO) metal are embedded in ferroelectric BaTiO3 (BTO) shells of mean 13 nm thickness. The BTO spacers lead to significant barriers for itinerant carriers, at low temperatures being transmitted by tunneling, but additional conduction paths are temperature activated by hopping across them. The dielectric, terahertz and infrared spectroscopy data of [email protected] are modelled by effective medium approaches, which enables to explain the broad dielectric and AC conductivity dispersion including the giant low-frequency permittivity values. Temperature dependent terahertz transmission spectroscopy clearly indicates the ferroelectric transition in the nano-BTO shells in the same temperature range as in the bulk BTO.
Article
In this work, three phase magnetoelectric nanocomposites are successfully prepared by solvent casting from α-PVDF. The extrinsic piezoelectric effect is insured by 20 vol% of NaNbO 3 ferroelectric nanoparticles prepared by the hydrothermal process. Nickel is chosen as the magnetic phase. In order to maintain insulation, electrical percolation is avoided by using nickel contents lower than the percolation threshold. Two different morphologies are employed: spherical nanoparticles (NiNPs) and nanochains (NNCs) with an aspect ratio of 80. NNCs are obtained by a polyol process under a magnetic field. Thin films of nanocomposites are processed by hot pressing. The real part of the dielectric permittivity (ε') of composites is governed by Maxwell-Wagner-Sillars polarization at the PVDF/Ni interfaces. An important increase of ε' at low frequency is observed for PVDF/NaNbO 3 /NNCs composites; it favors the magnetoelectric coupling. Piezoelectric properties of composites are dependent upon mechanical modulus so that the most higher d 33 is obtained for PVDF / NaNbO 3 / NiNPs. Magnetoelectric coupling coefficient (α ME ) is determined from magnetoelectric intensity. PVDF / NaNbO 3 / NiNPs composites exhibit enhanced magnetoelectric coupling. The a ME coupling coefficient reaches 4.2V/(mOe). The magnetic properties are significantly improved by the NiNPs.
Article
In multiferroic composites, the strain-mediated magnetoelectric coupling is strongly dependent on the characteristics of interface between two (ferroelectric/ferromagnetic) mechanically coupled phases. In this paper, we have studied the magnetic ordering in 0.25BaTiO3-0.75CoFe2O4 (BTO-CFO) nanoparticles and the results are compared with pure CoFe2O4 (CFO) nanoparticles. The cubic spinel structure of CFO and tetragonal BTO is analyzed with X-ray diffraction pattern. The magnetic hysteresis measured at room temperature for both multiferroic BTO-CFO and pure CFO are quite different, that indicate magnetic ordering, which in BTO-CFO composite is changeable. Comparatively higher coercivity in pure CFO than BTO-CFO nanoparticles is observed at room temperature which suggested freezing out of thermal excitations and the onset of stable ferromagnetic ordering. The value of blocking temperature (corresponding to superparamagnetic state), TB is 385 K is observed in pure CFO and 355 K of BTO-CFO. However, the magnetic curve of zero field cooling magnetization is reasonably different than that observed in pure CFO, which is further studied with frequency dependent ac magnetic susceptibility measurement. It results into spin-canting and spin-glass states formation. The spin-glass transition temperature is calculated with both dynamical scaling and Vogel-Fulcher methods. The ferroelectric hysteresis is observed at room temperature is reasonably varied with applied magnetic field of 2, 4, 5, 6 kOe and the variation in the values of polarization and coercive field might be consistent with the observed magnetic hysteresis.
Article
Multiferroics materials have been used widely in various fields such as ceramic supercapacitors, active and multifunctional devices due to their high relative permittivity with magnetic and ferroelectric behavior even at room temperature. The composites of cobalt ferrite, barium titanate have been synthesized through co precipitation technique and sintered at very high temperature1273K. The existence of titanate single phase and cobalt ferrite structural analysis carried out through ‘X’ Ray diffraction analysis. From the AC analysis the role of phase, dielectric constant, loss tangent and conductivity was also studied. AC conductivity variation was very low at lower frequencies and increases with frequency. Magnetization measurements were carried out at room temperature shows soft ferromagnetic property is 2.8089 emu/g.
Article
A multiscale optimization is presented for multiferroic composite materials to enhance magnetoelectric (ME) effect. The challenge of this study is to discover innovative microstructures beyond the conventional laminated structure which is the best of existing materials. The asymptotic homogenization theory was employed for scale bridging between the macrostructure and the microstructure. The homogenized ME coefficient of macrostructure was set to an objective function. The phase configuration and polarization directions in the microstructure were utilized as design variables. The computation yielded to an optimized microstructure, and found its macro homogenized ME coefficient is 21% larger than the conventional structure. The optimized microstructure consists of four rectangle regions and they are periodically repeated. Ferromagnetic and ferroelectric phases are alternately lined in each region and their polarization directions are determined by the special stated Euler angles. The computation successfully established that the optimal microstructure exists beyond the conventional layered structure. The findings open new avenues for enhancing physical properties of functional materials, and are expected to expand to digital fabrication by 3D printing.
Article
Optical properties of cobalt ferrite (CoFe2O4)/barium titanate (BaTiO3) nanoparticles are modeled and simulated utilizing density functional theory (DFT) and finite element analysis (FEA) intended for various particle sizes. The simulated absorption maxima of electronic excitations is red-shifted from 259.51 nm to 315.27 nm employing quantum mechanical approach and from 260 nm to 280 nm using finite element analysis, corresponding to enlarging particle sizes from 8 nm to 50 nm. The measured absorption maxima corresponded well to the simulated results and red-shifted to longer wavelengths from 302.02 nm to 321.28 nm accompanied by an increment in particle sizes from 30 nm to 50 nm. The FEA simulated, DFT simulated and experimentally extracted optical band gap energies were as well obtained and compared. Additionally, the ferromagnetic behavior of the CoFe2O4/BaTiO3 nanoparticles was investigated.
Article
Four magnetoelectric (ME) composite systems: having composition [80 wt% BaFe 12 O 19 (BaM) – 20 wt% Na 0.5 Bi 0.5 TiO 3 (NBT)] considering the variation of grain size of both the phases are synthesized by solid state method. X-ray diffraction, FESEM and TEM studies are employed to confirm the pure phase formation, grains size and connectivity of both the phases in composite systems. The exchange-spring mechanism (ESM) has been examined with the help of MH loops and magneto transport studies. From analysis it has been found that spin polarized electron tunnelling at ferromagnetic (FM)-insulator (I) interfaces has significant effect on ESM.
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The composite multiferroic materials manufactured by combining ferroelectric phase with the ferromagnetic phase offer promising applications for functional devices. The main aim of this work is on the detailed investigation of dielectric, magnetic and magnetodielectric properties of xCoMn0.1Fe1.9O4-(1 − x) Pb0.93Yb0.07Zr0.52Ti0.48O3 (x = 0.02, 0.05 and 0.08) multiferroic composites. We also present First Order Reversal Curve analysis of the multiferroic composites. The composite multiferroics were synthesized by solid state reaction method while the individual phases, Pb0.93Yb0.07Zr0.52Ti0.48O3 and CoMn0.1Fe1.9O4, were prepared by sol gel auto-combustion technique. The XRD studies confirm the phase formation of the composites with no observation of any additional phases in the systems. By studying the dielectric and magnetic properties, it was demonstrated that the increase in the ferrite content of the composites has been found to remarkably improve the dielectric and magnetic properties of the composites. The observed anomalies in the dielectric constant with the temperature results from the ferroelectric phase transition of YbPZT, and the transition temperature enhances with increasing the ferrite content in the composites. The P-E hysteresis loops of the ferroelectric phase are well saturated; however, the composites have less-saturated loops in comparison to the YbPZT phase. Both the CMnFO and the composites displayed the well saturated magnetic hysteresis loops. Also the First Order Reversal Curve (FORC) analysis of the composite multiferroics further enlightens the information about the domain state of magnetisation and the interactions in the system. All the FORC diagrams exhibited a single contour implying that all the composites have unique magnetic phase (CMnFO). The ME coupling measured as a function of applied magnetic field demonstrates enhancement with increased ferrite content. This improved ME-effect demonstrates the strong coupling between the piezoelectric and magnetostrictive phases, which eventually will expand their scope in future generation material.
Article
Hybrid multiferroic systems composed of complex oxides have drawn sustained attention for decades due to their intriguing physical effects and potential technological exploitations. Designing innovative nanostructures of multiferroic nanocomposites to overcome inherent shortcomings for conventional layered (e.g. clamping effect) and vertically aligned (e.g. leaky) films are instrumental to realize genuine multiferroic behaviors. Here the self-assembled architecture with three-dimensional (3D) framework of heterostructures is fabricated, wherein well-ordered multiferroic ε-Fe2O3 (ε-FO) nanoboats are embedded in a ferroelectric perovskite PbTiO3 (PTO) matrix. The biphasic system combines strong interfacial magnetoelectric couplings of vertically aligned heterostructure and addressed leakage issue via preferential epitaxy of a high-resistance transition layer of PTO. Additionally, the two phases maintain full lattice coherence along vertical interfaces allowing for efficient interfacial strain coupling. Ferroelectricity and piezoelectric switching of the 3D nanostructured PTO:ε-FO film have been corroborated macroscopically by polarization hysteresis (Ps ∼ 45 μC cm⁻²) and locally by piezoresponse force microscopy, and strong magnetoelectric coupling has been manifested as a sizable modification of piezoelectric switching characteristics via applying a DC magnetic field, all conducting at room temperature. The novel 3D multiferroic-ferroelectric heterostructure offers great potential for nanoengineering of multiferroic composites, thus opens an avenue towards superior microelectronics and spintronics.
Article
Di-phase ferroelectric-ferrite composites with the same composition close to the percolation limit (0.66BaTiO 3 -0.33CoFe 2 O 4 )and with different phase arrangements (randomly mixed phases and tri-layer structures)have been consolidated by using spark plasma sintering and their properties were comparatively analyzed. The intrinsic effective dielectric constant has been estimated by using finite element method and a value almost ten times higher was predicted for the randomly mixed composite, as result of different field distributions inside the ferrite and ferroelectric phases. At room temperature, experimental permittivity contains an intrinsic behavior overlapped onto a very strong extrinsic component determined by the interfaces and by local charge inhomogeneity. The extrinsic dielectric behavior has been discussed in terms of thermally activated relaxation mechanisms developed in both types of investigated structures, as revealed by the temperature-dependence impedance spectroscopy. The magnetic properties are derived as “sum property” from the ferrimagnetic character of CoFe 2 O 4 and show the lowest magnetization for the randomly mixed composites and a weak magnetic anisotropy when the magnetic field was applied in-plane or out-of-plane in the layered structure. Due to the different levels of interface doping specific to the different types of interfaces, the magnetic Curie temperature of the two composites ranges from 720 K in the randomly mixed structure to 746 K for the layered one. The ferroelectric P(E)loops and a modest ME coefficient of 14 mVOe ⁻¹ cm ⁻¹ at 100 Hz were determined only for the layered structure, due to the high leakage and to the small grain size (below 200 nm)in the ferroelectric BaTiO 3 component.
Article
This paper reports the formation of BaTiO3/CoFe2O4 (BT‐CFO) filled low‐density‐polyethylene (LDPE) composite via the process of melt mixing and characterization for their dielectric and electromagnetic shielding behaviors. The X‐Ray diffraction (XRD) result of synthesized CoFe2O4 shows the cubic phase structure formation with a crystallite size of 28.6 nm. The BT nanopowder and synthesized nanopowder of Cobalt ferrite (CFO) were mixed together in an equal amount (50:50), and sintered at 800°C for 1 hour. A series of samples of BT‐CFO was prepared by taking a different amount of BT‐CFO (ie, 4, 8, 12, 16, and 20 wt%) with LDPE. The dielectric measurements of BT‐CFO/LDPE composites were carried out at different temperatures using LCR meter at a frequency range from 40 Hz to 5 MHz. The dielectric behaviour of samples exhibited a change in dielectric constant with an increase in temperature and frequency. The electromagnetic shielding measurements were examined over X‐band with a frequency range from 8.2GHz to 12.4GHz of the prepared BT‐CFO/LDPE composites. The EMI shielding effectiveness of prepared BT‐CFO/LDPE composites exhibited a maximum value of 17.9 dB.
Article
This paper analyzes the ultrarrapid microwave sintering process versus the conventional sintering process of the nanocrystalline composite (1-x)PMN-PT/(x)CoFe2O4, synthetized by the polymeric precursor method based on the novel one pot methodology aiming to achieve particulate composites ensuring an exceptionally homogeneous two-phase distribution. Main attention was paid to the sintering method effect on the microstructural attribute and physical properties. The activation energy for the initial stage of conventional sintering was calculated as a reference for microwave sintering. The microstructure characterizations revealed that microwave sintering without PbO loss control obtained a particulate composite with high homogeneous fine microstructures and great distribution of the Fe2CoO4 phase in the PMN-PT matrix with high electrical resistivity. Furthermore, the results showed that the sintering method has a substantial effect on their grain size and consequently on magnetoelectric properties.
Conference Paper
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Nanopowders of cobalt iron oxide (CoFe2O4) were successfully fabricated by the co-precipitation method followed by a technique to prevent particle agglomeration. Particle sizes were in the range of 24 to 44 nm. The size of cobalt iron oxide particles decreases with increasing the concentration of the precipitation agent. The crystal structure was confirmed by X-ray diffraction (XRD), the chemical composition by energy dispersive spectroscopy (EDS), and phase changes by thermogravimetric differential thermal analysis (TGA-TDA). The particle morphology was analyzed by scanning electron microscopy (SEM). Magnetic properties were investigated by SQUID magnetometry and Mössbauer spectroscopy. Being nearly monodisperse and non-agglomerated the prepared cobalt iron oxide powders are the base for synthesizing magnetoelectric composites embedded in a ferroelectric BaTiO3 matrix.
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NiFe2O4@BaTiO3 multiferroic composite particles were produced by a simple hydrothermal method in two steps: preparing NiFe2O4 nanoparticles and then synthesizing core-shell nanocomposites. Multiferroic composite ceramics were sintered from these powders. X-ray diffraction, Raman scattering and energy dispersive x-ray analyses indicated that the core-shell composites with a NiFe2O4 core and BaTiO3 shell were formed in the hydrothermal environment. Different types of sharp interfaces were self-assembled owing to the minimization of direct elastic energy. The saturation magnetization of the composites linearly increased with the NiFe2O4 content while the dielectric constant decreased. A dielectric peak appeared at around 460 °C because of the oxygen vacancies in the BaTiO3 ceramics. It resulted in an enhancement of magnetic permeability in the composites, indicating magnetoelectric coupling that was also observed by direct magnetoelectric measurements.
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Nanosized cobalt ferrites having the general formula CoFe2−xAlxO4 (for x=0.00, 0.25, 0.50) have been synthesized by the sol–gel route. The effect of Al3+ ions on structural, Curie temperature, DC electrical resistivity and dielecltric properties are presented in this paper. From the analysis of powder X-ray diffraction patterns, the nanocrystallite size was calculated by the most intense peak (311) using Scherrer formula. The crystallite size decreases with increase in aluminium concentartion. The lattice parameter ‘a’ also decreased with increase in aluminum concentration. The magnetic and electrical properties have been studied as a function of temperature. The Curie temperature was determined from AC magnetic susceptibility measurement. It is observed that Curie temperature decreases and DC electrical resistivity increases with increasing concentration of Al3+ ions. The observed variations in DC electrical resistivity have been explained by Verwey's hopping mechanism. The activation energy was found to increase with increasing Al3+ ions. The variations of dielectric constant for all the samples have been studied as a function of frequency in the range 500Hz to 1MHz at room temperature. The room temperature dielectric constant decreases with increase of trivalent Al3+ ions. The observed variation in dielectric constant has been explained on the basis of space charge polarization.
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Cobalt ferrite/barium titanate nanoparticles with a core/shell structure were synthesized by combining co-precipitation and organosol methods. The average particle size was about 110 nm with an average shell thickness of about 40 nm. Dielectric and magnetic properties of the particles were studied using impedance and Moessbauer spectroscopy, respectively. The particles are promising for fabrication of multiferroic ceramics with the core-shell structure.
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Magnetic and magnetostrictive properties of magnetic-annealed polycrystalline CoFe2O4 were investigated. The magnetic hysteresis loops showed obvious uniaxiality with an induced easy direction parallel to the annealing field. Magnetic force microscopy study revealed that the domains were fixed by magnetic annealing. The uniaxial behavior was also observed in the magnetostrictive measurement, which showed a significantly enhanced magnetostriction of − 273 PPM when the external field was applied perpendicular to the annealing field direction. A physical mechanism for the effect of magnetic annealing on polycrystalline CoFe2O4 is developed, in which the induced uniaxiality is ascribed to the realignment of easy axes in polycrystals. The uniaxial behavior of magnetism and enhanced magnetostriction could be well explained by this model.
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Multiferroic composites were prepared by covering CoFe2O4 nanoparticles with a shell of BaTiO3 using a sol–gel technique. Scanning probe microscopy confirmed the formation of a core–shell structure with a magnetic core and a piezoelectric shell. The converse magnetoelectric effect was studied at different temperatures and bias fields. The magnetoelectric coefficient peaks at approximately 270 K and reaches the value αC≈(2.2 ± 0.1)10 − 11 s m − 1, which surpasses those reported previously for similar structures. A change of the sign of the magnetoelectric coefficient observed for an increasing magnetic bias field is related to the non-monotonic field dependence of magnetostriction in polycrystalline CoFe2O4.
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X-ray absorption spectroscopy and photoemission electron microscopy are techniques commonly used to determine the magnetic properties of thin films, crystals, and heterostructures. Recently, these methods have been used in the study of magnetoelectrics and multiferroics. The analysis of such materials has been compromised by the presence of multiple order parameters and the lack of information on how to separate these coupled properties. In this work, we shed light on the manifestation of dichroism from ferroelectric polarization and atomic structure using photoemission electron microscopy and x-ray absorption spectroscopy. Linear dichroism arising from the ferroelectric order in the PbZr0:2Ti0:8O3 thin films was studied as a function of incident x-ray polarization and geometry to unambiguously determine the angular dependence of the ferroelectric contribution to the dichroism. These measurements allow us to examine the contribution of surface charges and ferroelectric polarization as potential mechanisms for linear dichroism. The x-ray linear dichroism from ferroelectric order revealed an angular dependence based on the angle between the ferroelectric polarization direction and the x-ray polarization axis, allowing a formula for linear dichroism in ferroelectric samples to be defined.
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The L2,3 x-ray-absorption edges of 3d0 compounds are calculated with use of an atomic description of the 2p63d0 to 2p53d1 excitation, with the inclusion of the crystal field. For reasons of clarity, we confine ourselves to d0 compounds in octahedral symmetry, but the same approach is applicable to all other dN compounds in any point-group symmetry. The experimental spectra of FeTiO3, Sc2O3, ScF3, CaF2, and the potassium halides are well reproduced by the present calculations, including the previously misinterpreted small leading peaks. The splitting between the two main peaks in both the L3 and L2 edge are related, though not equal, to the crystal-field splitting. Comparison to experiment showed that the broadening of the main multiplet lines is different. This can be related to Coster-Kronig Auger processes for the L2 edge and to a solid-state broadening which is a combination of vibrational (phononic) and dispersional broadenings. With the full treatment of the atomic multiplets, the atomic effects can be separated from solid-state effects, which offers a better description of the latter. This includes vibrational broadenings, the covalent screening of the intra-atomic Coulomb and exchange interactions, via the position of small leading peaks, and surface effects. The same general framework can be used to discuss crystal-field effects in both lower symmetries, with the possibility of polarization-dependent spectra (e.g., TiO2), and partly filled d bands.
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A commercial superconducting quantum interference device (SQUID) setup (MPMS 5S from Quantum Design), equipped with a magnetic ac susceptibility option, is modified for measurements of the linear magnetoelectric (ME) effect, i.e., of the magnetic moment induced by an applied external electric field in a ME sample. Test measurements on a Cr(2)O(3) (111) single crystal are in excellent agreement with previously reported data of its ME susceptibility. The main advantages of the proposed setup are the improved precision due to the high sensitivity of the SQUID magnetometer in combination with the lock-in technique and a relatively simple experimental realization.
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Ferrites are a class of cohesive new materials required for many specialised applications. Cobalt ferrite (CoFe2O4) has been identified as a substitute for carbon and serves as a non consumable anode for an eco-friendly and energy efficient production of aluminium. Pellets of cobalt ferrite have been prepared by powder metallurgical process and their electrical properties have been investigated from ambient temperature to 1273 K. The structural and morphological features have been studied by X-ray diffraction and scanning electron microscopy. The relationships between such properties, chemical composition and sintering temperatures are thoroughly discussed.
Article
A series of highly dense barium titanate (BaTiO3) ceramics with the average grain size (GS) from 0.29 to 8.61 μm are successfully prepared by two‐step sintering, and the GS effect on piezoelectric coefficient (d 33) is systematically discussed in this work. It is found that when GS above 1 μm, d 33 can be enhanced with decreasing GS, reaching a maximum value of 519 pC/N around 1 μm due to the high activity of domain wall mobility. Subsequently, d 33 rapidly drops with a further decrease in GS owing to the reduced domain density. The results suggest that it is possible to prepare high‐performance BaTiO3 ceramics by controlling the GS and domain configuration properly, which brings great revitalization to the BaTiO3‐based piezoceramics.
Article
Dense nanocrystalline barium strontium titanate Ba0.6Sr0.4TiO3 (BST) ceramics with an average grain size around 40 nm and very small dispersion were obtained by spark plasma sintering at 950°C and 1050°C starting from nonagglomerated nanopowders (~20 nm). The powders were synthesized by a modified “Organosol” process. X-ray diffraction (XRD) and dielectric measurements in the temperature range 173–313 K were used to investigate the evolution of crystal structure and the ferroelectric to paraelectric phase transformation behavior for the sintered BST ceramics with different grain sizes. The Curie temperature TC decreases, whereas the phase transition becomes diffuse for the particle size decreasing from about 190 to 40 nm with matching XRD and permittivity data. Even the ceramics with an average grain size as small as 40 nm show the transition into the ferroelectric state. The dielectric permittivity ε shows relatively good thermal stability over a wide temperature range. The dielectric losses are smaller than 2%–4% in the frequency range of 100 Hz–1 MHz and temperature interval 160–320 K. A decrease in the dielectric permittivity in nanocrystalline ceramics was observed compared to submicrometer-sized ceramics.
Article
For high spatial resolution, deep body region measurement of biomagnetic signals in the pico- and femtotesla regime, dense vector-field sensor arrays are required. Current state-of-the-art sensors, like SQUIDs, are too bulky and not applicable.In this paper for the first time MEMS sensors based on magnetoelectric (ME) composites with vacuum encapsulation using wafer-level packaging technology are presented. Silicon device and cap wafers with 150 mm diameter were fabricated using micromachining processes and bonded afterwards for hermetic sealing. The device wafer contains rectangular cantilever beams with a stack composed of SiO2/Pt/AlN/FeCoSiB, Au metal-bond frames and conduction lines. The lateral dimensions of the cantilever were 200 μm × 900 μm with an overall thickness of 7.8 μm. The cap wafer comprised the vacuum cavities, alignment marks and Au/Sn metal-bond frames. For avoiding degradation of the temperature sensitive amorphous magnetic material [(Fe90Co10)78Si12B10] a special developed low-temperature, hermetic sealed transient-liquid phase bonding process was used to package the sensor devices. Characterization of a sensor showed a giant ME coefficient of 2390 (V/m)/(A/m) and a corresponding sensitivity of 3800 V/T in resonance at 7.1 kHz. A minimum resolution of 30 pT and noise levels as low as 27 pT/Hz1/2 have been reached in resonant operation.
Article
Barium titanate-20wt% cobalt ferrite (BaTiO{sub 3}-20wt%CoFe{sub 2}O{sub 4}) nanocomposites were sintered from nanocrystalline BaTiO{sub 3} and CoFe{sub 2}O{sub 4} powders using spark plasma sintering (SPS) and pressureless sintering (PS) techniques. Using SPS, dense polycrystalline composites were obtained at a sintering temperature as low as 860 C and a time of 5 min whereas PS required a higher sintering temperature (1150 C) and time (120 min) to obtain similarly dense composites. Microstructural analysis of the composites showed that both the techniques retained nanocrystalline grain sizes after sintering. High resolution X-ray diffraction measurements revealed that the BaTiO{sub 3}-20wt%CoFe{sub 2}O{sub 4} composites sintered by the SPS technique did not exhibit formation of any new phase(s) due to reaction between the BaTiO{sub 3} and CoFe{sub 2}O{sub 4} phases during sintering. However, the PS technique resulted in the formation of additional phases (other than the BaTiO{sub 3} and CoFe{sub 2}O{sub 4} phases) in the composites. While the composites synthesized by SPS were of superior phase-purity, evidence of Fe diffusion from the spinel to the perovskite phase was found from X-ray diffraction and permittivity measurements.
Article
In this study, X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray magnetic circular dichroism (XMCD) and element- and site-specific magnetic hysteresis (ESMH) are used to elucidate the effect of geometry (0-3- and 2-2-type) on the magnetic properties of CoFe2O4–PbTiO3 (CFO–PTO) multiferroic composites by comparison with those of the reference CFO and PTO powders. Magnetic Co ions in CFO have been confirmed to be located at both the tetrahedral (A)- and octahedral (B)-sites. CFO retains its mixed-spinel structure as verified by the EXAFS, XMCD and ESMH measurements. ESMH measurements further demonstrate that the magnetic moments of Co2+ and Fe3+/Fe2+ cations at both the A- and B-sites in the composites are smaller than those of the CFO powder. The reduction of the magnetic moments in the 2-2-type composite was larger than that in the 0-3-type composite. The reduction of the magnetic moments in the composites was attributable to the formation of anti-phase boundaries owing to the compressive strain in CFO, which is the largest strain in the 2-2-type composite. Based on the Ti L3,2-edge XMCD measurements of the CFO–PTO composites, no induced magnetic moment was observed at the Ti sites in the PTO matrix, excluding the possibility that the Ti ions in the PTO matrix affect the magnetic properties of these CFO–PTO composites.
Article
Both saturation and hysteresis appear in Braun tube oscillograms made at various temperatures with a condenser whose dielectric consists of Rochelle salt slabs cut perpendicular to the a-axis. The dielectric constant for such slabs may reach a value of 18,000. Curves are also given, showing the variation in mechanical and electrical saturation with temperature. These correspond in only a general way to the piezoelectric constant's variation with temperature. Certain marked peculiarities are noted in the resulting mechanical deformation when Rochelle salt is excited with alternating potentials. Clear Rochelle salt half-crystals have been produced up to forty-five centimeters in length.
Article
In this paper we present the development of a composite magnetostrictive material for automotive applications. The material is based on cobalt ferrite, CoO⋅Fe2O3, and contains a small fraction of metallic matrix phase that serves both as a liquid-phase sintering aid during processing and enhances the mechanical properties over those of a simple sintered ferrite ceramic. In addition the metal matrix makes it possible to braze the material, making the assembly of a sensor relatively simple. The material exhibits good sensitivity and should have high corrosion resistance, while at the same time it is low in cost.
Article
Antiphase boundaries (APBs) were observed in Fe3O4 single crystal films grown on MgO. The APBs are an intrinsic consequence of the nucleation and growth mechanism in films. Across an APB, the intrasublattice superexchange coupling is greatly strengthened, while the intersublattice superexchange coupling is weakened, reversing the dominant interaction from that found in the bulk. Thus the APB separates oppositely magnetized regions, consistent with Lorentz microscopy measurements. The APBs induce very large saturation fields and nearly random magnetization distribution in zero field.
Article
The relationship between ferroelectric domain growth and the size of crystallites in ultrafine BaTiO3 powders has been investigated. It was observed that the critical size for the c → t-BaTiO3 phase transformation was about 30 nm and that BaTiO3 crystallites probably transformed from single-domain to multidomain when the particle size was larger than 100 nm. X-ray diffraction patterns of BaTiO3/PVDF composites before and after poling indicated that the degree of poling efficiency for composites with fine BaTiO3 powders is much smaller than for coarser powders larger than 100 nm. The present work indicates that the presence of ferroelectric domains in BaTiO3 powder depends on the constraining force introduced by hard agglomeration of the crystallites. The particles obtained by calcining at a higher temperature (larger than 100 nm and partially sintered), composed of several crystallites, may behave similar to grains in fine-grained BaTiO3 ceramics where ferroelectric domains are generated due to mechanical clamping. On the other hand, particles of small size composed of single crystallite and without clamping force, may deform similar to a lattice cell and avoid formation of twins.
Article
Nanostructured CoFe2O4 particles were prepared by a sonochemical approach, first by preparation of the amorphous precursor powders, followed by heat treatment at relatively very low temperatures. The precursor was prepared by sonochemical decomposition of solutions of volatile organic precursors, Fe(CO)5 and Co(NO)(CO)3, in Decalin at 273 K, under an oxygen pressure of 100−150 kPa. The amorphous nature of these particles was confirmed by various techniques, such as scanning and transmission electron microscopy (SEM and TEM), electron microdiffraction, and X-ray diffractograms. Magnetic measurements, Mössbauer, and electron paramagnetic resonance (EPR) spectral studies indicated that the as-prepared amorphous particles were superparamagnetic. The Mössbauer parameters and the significantly low (45 emu/g) observed saturation of magnetization of the annealed sample, compared to that of the bulk sample (72 emu/g), reflected its nanocrystalline nature.
Article
Connectivity is a critical parameter in composites designed for use as piezoelectric transducers or as pyroelectric detectors. There are ten important connectivity patterns in diphasic solids, ranging from a 0-0 unconnected checkerboard pattern to a 3-3 pattern in which both phases are three dimensionally self-connected. Processing methods for manufacturing some of these patterns are described. Series and parallel models for composite piezoelectrics and pyroelectrics lead to several interesting results, such as a diphasic pyroelectric in which neither phase is pyroelectric. The models are also helpful in interpreting the structure-property relations in single-phase materials where the crystal structures mimic certain connectivity patterns.
Article
The structural and magnetic properties of nanocrystalline manganese, cobalt, and nickel spinel ferrites dispersed in a highly porous SiO2 aerogel matrix were studied. X-ray diffraction and high-resolution transmission electron microscopy indicate that single crystalline ferrite nanoparticles are well dispersed in the amorphous matrix. The cation distribution between the octahedral and tetrahedral sites of the spinel structure was investigated by X-ray absorption spectroscopy. The analysis of both the X-ray absorption near edge structure and the extended X-ray absorption fine structure indicates that the degree of inversion of the spinel structure increases in the series Mn, Co, and Ni spinel, in accordance with the values commonly found in the corresponding bulk spinels. In particular, fitting of the EXAFS data indicates that the degree of inversion in nanosized ferrites is 0.20 for MnFe2O4, 0.68 for CoFe2O4, and 1.00 for NiFe2O4. Magnetic characterization further supports these findings.
Article
The ‘‘organosol’’ precipitation method is proposed to produce nanosized particles of barium titanate (BaTiO3) at temperatures as low as room temperature. The advantages of this method are a high yield, a simple but precise control of the size of the particles, low process temperature, short reaction time, as well as low cost of reagents. The particles were systematically characterized by powder X-ray diffraction (XRD), Raman scattering, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), thermogravimetric thermal analysis (TGA/DSC), infrared spectroscopy (IR), and impedance analysis. The as-prepared BaTiO3 nanocrystals exhibit a granular shape of around 15 nm in diameter. Oleic acid retards crystallization and thus allows generation of a uniformly small grain size and excellent dispersibility in organic solvents. The surface energy of the particles is modified and crystallization in cubes also arises. The mechanism of powder formation is discussed. The method offers an alternate low-cost route to perovskite nanopowders easily dispersed in organic media.
Article
The observation of strong magnetoelectric (ME) coupling is reported in zinc-substituted layered composites of ferrites and lead zirconate titanate (PZT). Multilayer samples contained cobalt zinc ferrite Co1-xZnxFe2O4 (CZFO) (x=0–0.6) or nickel zinc ferrite Ni1-xZnxFe2O4 (NZFO) (x=0–0.5) and were prepared by laminating and sintering ferrite and PZT thick films obtained by tape casting. The ME voltage coefficient αE was measured for transverse and longitudinal field orientations for frequencies 10–1000 Hz. A substantial enhancement in αE is observed with the substitution of Zn. The largest increase, by about 500%, is observed in CZFO-PZT and the smallest increase of 60% is measured for NZFO-PZT. As the Zn concentration is increased, αE increases and shows a maximum for x=0.2–0.4, depending on the ferrite. The data is analyzed based on a theoretical model for a ferrite-PZT bilayer, taking into consideration less than ideal coupling at the interface. The interface coupling parameter k is quite small for CZFO-PZT; it increases from 0 to 0.6 as Zn concentration is increased from 0% to 40%. Composites of NZFO-PZT, however, have a near perfect interface coupling. The Zn-assisted enhancement in the ME coefficient is discussed in terms of joule magnetostriction, initial permeability, and magnetomechanical coupling for the ferrites.
Article
In this letter, we investigate a multimodal system for simultaneous energy harvesting from stray magnetic and mechanical energies by combining magnetoelectric and piezoelectric effects. The system consists of a cantilever beam with tip mass and a magnetoelectric laminate attached in the center of the beam. At 2 Oe magnetic field and mechanical vibration amplitude of 50mg, both at frequency of 20 Hz, the system was found to generate open circuit output voltage of 8 VP.P.. An equivalent circuit model is proposed that predicts a summation effect for both mechanical and magnetic energies.
Article
Dense nanocrystalline BaTiO3 ceramics with the average grain size of 20 nm obtained by spark plasma sintering were investigated. The dielectric data show a ferroelectric to paraelectric phase transition with a maximum permittivity of ≈ 930 at 115 °C and at 1 kHz. The polarization-reversal characteristics and the local ferroelectric switching behavior were measured; the typical piezoelectric hysteresis loops were recorded. The present results provide experimental evidence, indicating that if a critical grain size exists for ferroelectricity it is less than 20 nm for polycrystalline BaTiO3 ceramics.
Article
Marked changes in the magnetostriction of CoFe2O4 in the substitution of Mn for Co has been observed. The magnetostriction parallel to the applied field direction decreases with increasing Mn content, whereas the magnetostriction perpendicular to the field showed a marked increase for 10% of the Mn substitution. The observed magnetostrictive properties are correlated with the magnetic properties. The results suggest the possibility of tuning the magnetostrictive properties of cobalt ferrite for various applications by proper substitution.
Article
The results are presented of the theoretical and experimental studies of ferroelectromagnets—crystals with magnetic and ferroelectric ordering. Considerable attention is paid to reviewing the results of the phenomenological analysis of the influence of the magnetoelectric interaction on the thermodynamic properties of ferroelectromagnets, their reaction to constant and variable electric and magnetic fields, the spectrum of spin waves and ferroelectric oscillations and the methods of exciting them. A table is presented of the expected magnetoelectric effects. The existing experimental studies of magnetoelectric interactions are described and the possible applications of ferroelectromagnets are discussed. A table is given of the known ferroelectromagnetic compounds.