[Show abstract][Hide abstract] ABSTRACT: Co100-xNix (x = 0, 10, 20, 30, ..., 100) alloy nanostructures with controlled composition are prepared by hydrothermal method. The effects of composition on crystal structure, morphology, growth of nanostructure, and magnetic properties are described. The crystal structure is hexagonal closed pack (hcp) at x = 0, mixture of hcp and face-centered cubic (fcc) for 10 ≤ x ≤ 30, and purely fcc for x =40. The morphology is changed from nano-dendrite (x = 0) to flower like structure composed on spherical core and nanorods (x = 50) to nanosphere (x = 100). The isotropic (spherical shape) character increases with Ni Content. Compositional analyses show the controlled composition in nanostructure. The saturation magnetization is 156 emu/g at x = 0 and decreases linearly with alloy composition (x) to 55 emu/g at x = 100. The coercivity is 285 Oe at x = 0 (dendrite) and decrease gradually to 21 Oe at x = 80, and then increases to 146 Oe at x = 100. The effective anisotropy (Keff) constant decreases gradually with alloy-composition and shows decaying exponential dependence on alloy-composition (x). Magnetic and structural properties are explained by combined consequences of composition and nanoscale effects. This study reveals that the composition of alloy plays important role in growth, morphology, crystal structure and hence, properties of nanostructure.
Full-text · Article · Jan 2016 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: The structure, magnetic phase transition and martensitic transformation of quinary Ni45Co5(Mn, In, Sn)50 Heusler alloy are investigated systematically. X-ray diffraction reveals that there are two kinds of typical structure in alloys, the martensite phase with tetragonal structure and the austenite with cubic structure. Furthermore, it is found that Mn content plays an important role in the evolution of the crystal structure of quinary alloys, making the structure of alloys transform the cubic phase to tetragonal with increasing Mn content, and the threshold of structure transformation is about the Mn content of 35%. The temperature dependence of magnetization indicates that the tetragonal phase usually exhibits a low magnetization, while the cubic corresponds to a high magnetization. Particularly, the strong coupling between magnetism and structure occurs in Ni45Co5Mn40InxSn10−x alloys, and the alloys exhibit a large magnetic entropy change of 22.5 J kg−1 K−1 in martensitic transformation, making it a good candidate for magnetic refrigeration materials.
No preview · Article · Dec 2014 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: We report that by heating samples the critical current density for magnetization reversal (Jc) in a single perpendicularly magnetized layer can be decreased from 2.6 × 107 A/cm2 to about 1 × 106 A/cm2 for a temperature increase of 143 K. The nonlinear dependence of Jc on the perpendicular anisotropy field indicates that the coherent magnetic switching model cannot fully explain the current-induced perpendicular switching. By considering the current-induced domain nucleation and expansion during switching, we conclude that Jc also depends on current-induced domain behavior. Moreover, by reversing the heat flow direction, we demonstrate that the thermal related spin transfer torques have little influence on the thermally assisted magnetic switching.
[Show abstract][Hide abstract] ABSTRACT: BaTiO3 and BaTi1-xMxO3
(M=Co, Fe) nanocrystals were prepared by hydrothermal method. X-ray
diffraction analysis indicated that all of the samples were of
single-phase with tetragonal perovskite structure. The BaTiO3
prepared exhibited weak ferromagnetism rather than diamagnetism,
probably due to the oxygen vacancies at the surface. Paramagnetism was
observed for all BaTi1-xCoxO3
samples with 0.05≤x≤0.25. The Curie-Weiss fit revealed the
paramagnetic moment per Co ion were 4.09 μB, 4.12
μB, and 4.36 μB for x=0.15, 0.20, and 0.25
respectively. Room temperature hysteresis loops of the Fe-doped
BaTiO3 samples were observed at the doping level x between
0.2 and 0.5. The saturation magnetization firstly increased with
increasing Fe content, but gradually decreased. The divergence was
observed in the temperature dependence of the field cooling (FC) and
zero-FC (ZFC) magnetization curves, indicating a spin-glass behavior
arising from micromagnetic state, i.e. the mixing of ferromagnetic, and
antiferromagnetic phases. The observed ferromagnetism may originate from
the coupling between the secondary-nearest Fe ions and the
antiferromagnetism may be due to the coupling between the nearest Fe
ions. The ferromagnetic coupling competes with the antiferromagnetic
coupling. Therefore, the ferromagnetic properties are predominant when
the Fe doping level are at a certain range.
No preview · Article · Jan 2014 · Journal of Magnetism and Magnetic Materials
[Show abstract][Hide abstract] ABSTRACT: The structure, martensitic transformation, and magnetocaloric properties of quinary Ni45Co5Mn40InxSn10−x Heusler alloys have been investigated. The substitution of In for Sn was found to decrease in c/a parameter which enhances the martensite structure symmetry toward cubic phase and stabilize the martensitic phase. The martensitic transformation temperature was found to increase almost linearly, while Curie temperature of the austenitic phase decreases with increasing the In contents in the alloys. The doping of In keeps the low magnetization of martensitic phase and high magnetization of austenite phase, maintaining the strong metamagnetic behavior and magnetocaloric effect. The alloys exhibit large magnetic entropy change in the structure phase transition, meanwhile, the hysteresis loss is reduced by 74% with increasing the content of In to 8%, suggesting an effective way to reduce thermal hysteresis.
No preview · Article · Nov 2013 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: Ferromagnetic resonance with applied electric fields has been used to investigate the magnetoelectric effect in a Fe/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructure. The out-of-plane magnetoelectric effect follows a butterfly behavior in response to electric field, consistent with the linear piezoelectric strain-mediated mechanism. In contrast, the in-plane magnetoelectric effect exhibits a loop-like dependence on electric field. In addition, a relaxation of the in-plane resonant field has been observed after the ferroelectric domains are switched. The loop-like feature and the time-dependent response suggest that the in-plane magnetoelectric coupling involves the 109° switching and relaxation of ferroelectric domains.
No preview · Article · Aug 2013 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Co20Ni80 hollow-nanostructured microspheres are prepared by template-free hydrothermal method at low temperature (150 °C). The prepared CoNi nanostructures have face-centered cubic crystal lattice. Williamson–Hall analysis shows crystallite size of 55.5 nm and microstrain of 0.00265. Elemental mapping confirms the uniform distribution of Co and Ni atoms in sample. The size of hollow spheres is in the range of 1–3 μm which increase to 10 μm by increasing the reaction time. A stepwise growth mechanism based on time-dependent experiment is briefly described. The X-ray photoelectron spectroscopy analysis suggests the only metallic chemical state of Co and Ni in sample (no surface oxidation). The specific surface area of 80.56 m2/g is obtained for 2 μm hollow sphere. The Co20Ni80 hollow-nanostructured microsphere shows 0.92 mass% hydrogen absorption and 81 % release of stored hydrogen. Hydrogen storage is attributed to crystal defects. The effects of morphology and surface area on hydrogen storage capacity and response are described. The saturation magnetization of 72.1 emu/g and coercivity of 157.21 Oe are observed for 2 μm hollow spheres. However, saturation magnetization of 77.5 emu/g and coercivity of 120 Oe are observed for 10 μm hollow spheres. The enhanced coercivity is related to surface and shape anisotropy due to hollow structure.
Full-text · Article · Jul 2013 · Journal of Nanoparticle Research
[Show abstract][Hide abstract] ABSTRACT: Tree like FeCo alloy microstructures with stem, branches, and leaves are prepared by template free hydrothermal method at relatively low temperature using high concentration of NaOH. Prepared FeCo microstructures have bcc crystal and equiatomic composition. The crystallite size of 18.5
nm and micro-strains of value 0.00285 were estimated by Williamson-Hall analysis. The tree like FeCo alloy structures have size about 4 μm and leaves have width less than 300 nm. The growth mechanism of tree-leaf like microstructures is proposed. Prepared FeCo microstructures possess comparatively,
high saturation magnetization (214 emu/g), low coercivity (45 Oe), and low remnant magnetization (4.3 emu/g). The tree-leaf like FeCo microstructures have potential applications in the fields of biomedicine and catalytic process.
[Show abstract][Hide abstract] ABSTRACT: Spin wave resonance with an applied electric field has been used to
investigate the magnetoelectric (ME) effect in a
multiferroic heterostructure. The surface spin wave resonance peaks
disappear along with the destruction of the initial spin orientation by
the switch of ferroelectric domains. Meanwhile, the degree of symmetry
for the uniform ferromagnetic resonance mode can also be modulated by
the E field. These results are interpreted in terms of the
voltage-driven spin reorientation.
[Show abstract][Hide abstract] ABSTRACT: 3-D flower like CoNi alloys nanostructures composed of nanorods have been synthesized by template free hydrothermal method at relatively low temperature (120 °C). The detailed characterizations confirm the formation of good crystalline fcc CoNi alloy, average crystallite size of 18.8 ± 1.0 nm, lattice parameter of 3.531 ± 0.01 Å, and the nearly equiatomic composition (Co50Ni50). Highly uniform flower like nano structures are built up with nanorod of diameter about 100 nm and length in range of 200–400 nm. The nanorods (building blocks of flower) have single crystalline nature with  preferred growth direction. The concentration of NaOH plays a vital role in formation of alloys and high concentration promotes the formation of CoNi alloy at low temperature. The concentration of NaOH also affects the morphology remarkably by changing the growth/reaction rate of CoNi nanostructures and results in hollow spheres to nanoplate flower of CoNi alloys. Based on the evolution of the morphology of the products, a step wise growth mechanism is rationally proposed for flower like nanostructures by considering the effects of kinetic parameters on growth. Magnetic measurements show Co50Ni50 flower like nanostructure have high saturation magnetization, coercivity, remanent magnetization, and high effective anisotropy constant of value 101.3 emu g−1, 210.5 Oe, 16.2 emu g−1, and 4.457 × 104 J m−3 respectively. The enhancements of coercivity and effective anisotropy constant are attributed to nanoscale effects such as shape/surface anisotropy.
[Show abstract][Hide abstract] ABSTRACT: The structural, dielectric, ferroelectric, and magnetic properties of bulk Fe-doped Ba0.5Sr0.5TiO3 (Ba0.5Sr0.5Ti1−xFexO3; BSTF) solids prepared by standard solid-state reaction were investigated. X-ray diffraction (XRD) patterns confirmed the tetragonal structure of BSTF samples. Rietveld refinements of XRD data revealed that the doping ions lead to unit cell expansion in three directions. Ferroelectric and ferromagnetic orders of the samples were observed simultaneously at room temperature. According to experimental and simulated results, two possible exchange coupling mechanisms exist in the BSTF systems, namely, nearest-site antiferromagnetic coupling and next-nearest-site ferromagnetic coupling. The two coupling mechanisms coexist in BSTF systems, leading to extraordinary ferromagnetic behavior.
Full-text · Article · Jan 2013 · Journal of Magnetism and Magnetic Materials
[Show abstract][Hide abstract] ABSTRACT: Flower like FeCo alloy nanostructures assembled with nano triangular prisms have been synthesized via template free hydrothermal method at relatively low temperature (120 °C). The concentration of NaOH plays vital role in the formation of alloy phase and morphology. The excessive amount of NaOH is determined to be responsible for formation of alloy phase during hydrothermal reaction. X-ray diffraction analysis confirms the formation of good crystalline FeCo alloy with average crystallite size of 24 nm. Energy dispersive X-ray spectroscopy and inductive coupled plasma atomic emission spectroscopy reveal the nearly equiatomic composition (Fe0.5Co0.5) of prepared flower like nanostructures. Scanning electron microscope and transmission electron microscope observations show that flower like FeCo alloy structures are composed of six nano triangular prisms. The diameter of flower like structure is in the range of 0.5–1.0 μm, and the base and height of nano triangular prism are in the range of 100–200 nm. Electron diffraction pattern shows the single crystalline nature of nano triangular prism. A step wise growth mechanism of flower like structure is proposed. Magnetic investigations show high saturation magnetization of 209 emu/g, low coercivity of 30 Oe, and remanent magnetization of 3.01 emu/g at 300 K. This nearly super-paramagnetic behavior of FeCo alloy nanostructures may be due to the crystallite size proximity to its super-paramagnetic critical size.
No preview · Article · Nov 2012 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: Cobalt ferrite nanoparticles are prepared by varying the concentration of reducing agent NaBH4 by hydrothermal method. Transmission electron microscope observations show that particle size increases from 11.47 to 28.05 nm by increasing the concentration of NaBH4 from 30 to 70 mM. Williamson–Hall analysis on X-ray diffraction patterns show that strain increases in nanoparticles with increase in quantity of NaBH4. Energy dispersive X-ray analysis and Inductive coupled plasma atomic emission spectroscopy indicate large amount of oxygen deficiency which increase with amount of NaBH4. The magnetic hysteresis loops measured at room temperature clearly illustrate the influence of concentration of NaBH4 on magnetic properties of CoFe2O4. Saturation magnetization, coercivity, and anisotropy constant increases with the quantity of NaBH4 and they reach the maximum values of 93 emu/g, 2210 Oe, and 5.28 × 105 J/m3, respectively, with addition of 70 mM NaBH4. Estimation of micromagnetic parameters (exchange length, critical single-domain volume, etc.) suggests that prepared nanoparticles of CoFe2O4 are in single domain and size lies between superparamagnetic to critical single domain. The enhancement of magnetization is attributed to oxygen deficiency. This technique is useful in tuning the size and magnetic properties of cobalt ferrite nanoparticles.
No preview · Article · Oct 2012 · Journal of Nanoparticle Research
[Show abstract][Hide abstract] ABSTRACT: The magnetization reversal processes of magnetic nanorings (Co50Fe50) with different geometric shapes are investigated. In addition to the expected onion and vortex magnetization states, other metastable states are observed in the magnetization processes. We anatomize the formation and transition of magnetic states, and the propagation and annihilation of domain walls in the reversal process through the dynamic picture. Phase diagrams for the magnetization switching behavior depending on the geometric parameters are presented. The simulation shows that the vortex state is stabilized in thick and narrow rings. The switching field from vortex to onion states turns out to increase with thickness and decrease with width and diameter.
[Show abstract][Hide abstract] ABSTRACT: A quantitative method to detect ferromagnetic resonance using magnetic tunnel junction structure has been developed. Experimental results reveal three distinct regions for single elliptical permalloy film of micrometer lateral size. Above the spin wave instability threshold, the experimental results show a linear response of the longitudinal magnetization component to the microwave field amplitude over a large range rather than a lock-up phenomenon appeared in macroscopic permalloy films and then a phase limiting behavior. The linear behavior can be described by the theoretical model describing subsidiary resonance.
No preview · Article · Dec 2011 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: We have demonstrated that spin wave resonance in a permalloy microstrip can be detected by an electrical method based on magnetic tunnel junction structures. The detection method promises high spatial resolution and sensitivity. Both even and odd spin wave resonance modes can be clearly observed in a permalloy microstrip. The spin wave induced voltage is proportional to the input microwave power at each resonance mode. Data analysis using the model of quantized dipole-exchange spin wave resonance suggests the edge pinning of spin wave sensitively depends on the order of the spin wave mode, as well as on the excitation frequency for modes of the higher order.
No preview · Article · Nov 2011 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Nanoscale Cu1−xMnxO powder is prepared by using the combustion synthesis technique with two different fuels. The structural properties of the powder are determined using Rietveld refinement of X-ray diffraction data, high-resolution transmission electron microscopy, and Fourier transform infrared spectroscopy, while its magnetic properties are analyzed by means of hysteresis loop and temperature dependence of magnetization. The results show that (1) the Cu1−xMnxO nanocrystal is of monoclinic CuO structure, with grain size of 10–30nm varying with the type of fuel, the nitrate/fuel ratio (N/F), and the Mn concentration, the doping of Mn has a little influence on the lattice parameters; (2) when the Mn concentration is higher than 7%, a small amount of impurity phase of CuMn2O4 appears and annihilates the potential cation vacancies; (3) all of the samples with x≥5% exhibit low-temperature ferromagnetism with the Curie temperature of ∼90K, which increases slightly by raising the Mn concentration; (4) the paramagnetic moment per Mn ion is around 2–4bohr magneton above the Curie temperature, which decreases with increasing Mn concentration, implying that the nearest Mn ions are antiferromagnetically coupled and the ferromagnetic order could originate from the super-exchange of next nearest Mn ions along the [101̄] direction.
No preview · Article · Sep 2011 · Physica B Condensed Matter
[Show abstract][Hide abstract] ABSTRACT: We report here the evolution of zinc based high purity phases with novel morphologies such as Zn3N2 hollow structures, ZnO nanowires and nanopowders, as well as metallic Zn layered hexagonal microparticles at progressively increased reaction temperature of 600 °C, 700 °C, 800 °C under NH3 gas atmosphere using Zn powder precursor and keeping all other experimental parameters unchanged. Growth mechanism for Zn3N2 obtained by nitridation, ZnO by oxidation and Zn microparticles via thermal evaporation & condensation process are discussed briefly. The as-synthesized products were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM). Photoluminescence (PL) studies have revealed very interesting and infrequently observed emission bands at 378 and 661 nm for Zn3N2, 359 and 396 nm for ZnO as well as 389 nm for Zn polyhedral microparticles.Graphical abstract.Highlights► Highly pure Zn based phases like Zn3N2, ZnO and Zn microparticles prepared under NH3 gas environment. ► Vapor–solid and self-catalytic processes based growth mechanisms are proposed for the products. ► PL spectra of the products exhibited very interesting and infrequently observed emission bands. ► Optical studies revealed the promise of the structures for applications in light emitting devices.
No preview · Article · Jun 2011 · Applied Surface Science
[Show abstract][Hide abstract] ABSTRACT: We studied the real-time evolution of magnetic dynamic and static properties of 20 nm CoFeB thin film during annealing at 380 °C. The ferromagnetic resonance linewidth quickly reduces by 30% within 300 s annealing, and monotonically increases upon longer annealing. The magnetic static coercivity shows similar trend. The underlying physical relation between linewidth and anisotropy can be connected by the two-magnon scattering theory. By doping of Nb into CoFeB films, the damping was maintained at a low value within 2000 s annealing. This method to tailor the dynamic properties of CoFeB may benefit the development of magnetics and spintronics based microwave devices.
Full-text · Article · Jan 2011 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Unlike previous studies that emphasize the important role of thermodynamics or surface energy on the structure stabilization of ZnS nanocrystals, we successfully controlled the crystalline structure of ZnS nanocrystals simply by tuning sulfur precursor addition rate under exactly the same other conditions. We observed the structure of as prepared ZnS nanocrystals was evolved from wurtzite into zinc blende with increasing the addition rate of sulfur precursor. The method may extend to engineer other nanomaterials with desired physicochemical properties by controlling crystalline structure. On the other hand, it also makes a new approach to understand the crucial factors that determine the growth mechanism and the crystal structure of nanomaterials in theory.
No preview · Article · Dec 2010 · Journal of Nanoscience and Nanotechnology