[Show abstract][Hide abstract] ABSTRACT: One of the core issues for multiferroicity is the strongly coupled ferroelectric polarization and magnetization, while so far most multiferroics have antiferromagnetic order with nearly zero magnetization. Magnetic spinel compounds with ferrimagnetic order may be alternative candidates offering large magnetization when ferroelectricity can be activated simultaneously. In this work, we investigate the ferroelectricity and magnetism of spinel FeCr2S4 in which the Fe(2+) sublattice and Cr(3+) sublattice are coupled in antiparallel alignment. Well defined ferroelectric transitions below the Fe(2+) orbital ordering termperature Too = 8.5 K are demonstrated. The ferroelectric polarization has two components. One component arises mainly from the noncollinear conical spin order associated with the spin-orbit coupling, which is thus magnetic field sensitive. The other is probably attributed to the Jahn-Teller distortion induced lattice symmetry breaking, occuring below the orbital ordering of Fe(2+). Furthermore, the coupled ferroelectric polarization and magnetization in response to magnetic field are observed. The present work suggests that spinel FeCr2S4 is a multiferroic offering both ferroelectricity and ferrimagnetism with large net magnetization.
[Show abstract][Hide abstract] ABSTRACT: The dielectric and ferroelectric behaviors of a ferroelectric are substantially determined by its domain structure and domain wall dynamics at mesoscopic level. A relationship between the domain walls and high frequency mesoscopic dielectric response is highly appreciated for high frequency applications of ferroelectrics. In this work we investigate the low electric field driven motion of 90°-domain walls and the frequency-domain spectrum of dielectric permittivity in normally strained ferroelectric lattice using the phase-field simulations. It is revealed that, the high-frequency dielectric permittivity is spatially inhomogeneous and reaches the highest value on the 90°-domain walls. A tensile strain favors the parallel domains but suppresses the kinetics of the 90° domain wall motion driven by electric field, while the compressive strain results in the opposite behaviors. The physics underlying the wall motions and thus the dielectric response is associated with the long-range elastic energy. The major contribution to the dielectric response is from the polarization fluctuations on the 90°-domain walls, which are more mobile than those inside the domains. The relevance of the simulated results wth recent experiments is discussed.
[Show abstract][Hide abstract] ABSTRACT: The relationship between the bipolar resistive switching and the polarization reversal is investigated at various temperatures in the Au/BiFeO3/SrRuO3 structure. It is found that the polarization-induced barrier variation in the Au/BiFeO3 and BiFeO3/SrRuO3 junctions decreases with decreasing temperature. This explains why the resistance-switching ratio decreases with decreasing temperature below 323 K and gives evidence that the polarization modulates the resistance state of the Au/BiFeO3/SrRuO3 structure. Besides, the oxygen vacancies migration and/or the carrier trapping/detrapping mechanisms are also suggested to play a very important role in the resistive switching behavior in this structure as the temperature goes above 323 K.
[Show abstract][Hide abstract] ABSTRACT: Employing the graphene sheets (GSs), the electron scattering constants are measured in the high-angle annular dark-field (HAADF) imaging by the scanning transmission electron microscopy. Single scattering is found to be dominant until the layer number of 200, complying with a simple relation of I = Io(1 - e(-tau/lambda)). The discrete layer counting of the GSs enables precise determination of incident depths. This work results values of lambda = 48.2, 61.4, 97.9 and 115.6 nm for 80, 120, 160 and 200 keV electrons, respectively. The uncertainties with the mean free paths and the cross sections are confined to 10 percent. The dependences on the electron beam energy and the collection angle are discussed based on a multislice simulation.
Journal of Nanoscience and Nanotechnology 08/2012; 12(8):6494-8. DOI:10.1166/jnn.2012.5445 · 1.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Magnetoelectric (ME) laminate composite Terfenol-D/LiNbO3((zxtw)-129°/30°) was studied. It shows a much higher ME voltage coefficient (αE) compared with the laminate using normal cut type of LiNbO3 single crystal. The αE has a wide frequency peak (bandwidth of 40 kHz) and more than one optimized bias magnetic field under high frequencies. The mechanism was discussed in this article.
[Show abstract][Hide abstract] ABSTRACT: The structures of
N20 − n
(n = 6−18),
the clusters of boron nitride, are investigated by the density functional theory
calculations. The structures of the obtained low-lying isomers can be described by the
following six prototypes: single ring, double ring, three-ring, graphitic-like sheet,
fullerene and others. B10N10 is demonstrated to be the most stable
cluster against the nonstoichiometric ones. Nonzero magnetic moments, 1.999, 1.998, 2.000,
3.999 and 1.999μ
B respectively, are found in five
6, 7, 11, 12, 13) clusters. Further analysis indicates that the magnetic moment of the
B6N14 cluster is mainly originated from the N atoms, while those
of others are from the B atoms. The magnetic moment are finally attributed to the
interesting issues of the 2p electrons due to the breaking of local
symmetries, the change of coordination number, charge distribution and orbital
The European Physical Journal D 07/2011; 63(2):201-207. DOI:10.1140/epjd/e2011-10559-8 · 1.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate the effect of Al-doping on the multiferroicity of DyMn1−xAlxO3. It is indicated that a slight doping (0.005 ≤ x ≤ 0.01) can significantly enhance the electric polarization which will be suppressed as x>0.03. The possible mechanism for the polarization enhancement is the rearrangement of the Mn spiral spin order upon the Al-doping. Our work demonstrates that the ferroelectricity of DyMnO3 is sensitive to the B-site nonmagnetic doping, allowing an unusual enhancement of the electric polarization.
[Show abstract][Hide abstract] ABSTRACT: A series of polycrystalline pyrochlore rare-earth titanate Ho2−xCrxTi2O7 are synthesized in order to enhance the ferroelectricity of pyrochlore Ho2Ti2O7. A giant enhancement of polarization P from 0.54 μC/m2 at x = 0 up to ∼ 660 μC/m2 at x = 0.4 is obtained, accompanied with an increment of ferroelectric transition point Tc from Tc ∼ 60 K up to Tc ∼ 140 K. A magnetic anomaly at Tc ∼ 140 K together with the polarization response to magnetic field is identified, implying the multiferroic effect in Ho2−xCrxTi2O7.
[Show abstract][Hide abstract] ABSTRACT: Hydrogen molecules adsorption and storage in Sc coated Si@Al(12) cluster were investigated using density functional theory methods. Scandium atoms can bind strongly to the surfaces of Si@Al(12) due to the charge transfer between Sc and Si@Al(12), and do not suffer from clustering on the substrate. Si@Al(12) cluster coated with three and four Sc atoms can adsorb 16 and 18 H(2) molecules with a binding energy of 0.28-0.63 eV/H(2), corresponding to hydrogen storage capacity of 6.0 and 6.3 wt %, respectively. The stable Si@Al(12) can be applied as one of candidates for hydrogen storage materials at ambient conditions.
The Journal of Chemical Physics 06/2010; 132(22):224308. DOI:10.1063/1.3439689 · 2.95 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A ferromagnetic shape memory alloy was utilized to construct a Ni <sub>43</sub> Mn <sub>41</sub> Co <sub>5</sub> Sn <sub>11</sub>/ Pb ( Zr , Ti ) O <sub>3</sub> laminate. Large direct and converse magnetoelectric (ME) effects were observed at resonance frequencies around the martensitic transition temperature of Ni <sub>43</sub> Mn <sub>41</sub> Co <sub>5</sub> Sn <sub>11</sub> alloy, and the temperature affects the ME properties remarkably. The ME coefficients respond almost linearly to the applied ac electric or magnetic field. The mechanism of both direct and converse ME effects in the laminate is discussed.
[Show abstract][Hide abstract] ABSTRACT: We studied the structure and electronic properties of Al7OH clusters using density functional theory with generalized gradient approximation. OH prefers to the ontop site of Al atoms, and the hollow form is not stable. The Al7 moieties can keep their structures in ontop form Al7OH cluster, while it dramatically changes for bridge form ones. The binding strength between Al7 and OH are much larger than that of Al7I and Al7H. The bonding characteristic of Al7-OH is mainly ionic according to the population analysis. Large binding energy, HOMO-LUMO gap, and high ionization potential imply that Al7OH cluster is physically high and chemically stable.
International Journal of Modern Physics B 04/2010; 24(10):1253-1259. DOI:10.1142/S0217979210053690 · 0.94 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The lowest-energy geometries and electronic-structure properties have been
obtained for AlnHn (n=1-10) clusters within the
density-functional theory using the generalized gradient approximation for
the exchange correlation potential. The resulting geometries show that the
hydrogen atoms tend to occupy outside positions and no hollow positions are
found. The subunit Aln of AlnHn (n=1-5) have little
distortion, in comparison with corresponding pure Aln cluster, whereas
the subunit Aln have large distortion from n=6. The stability has been
investigated by analyzing the binding energy per atom and the second
difference in energy, indicating that Al8H8 exhibit higher
stability than others. The bonding property has been analyzed by calculating
the Mulliken charges and Al–H distances. The calculated energy gap between
the highest occupied molecular orbital and the lowest unoccupied molecular
orbital (HOMO-LUMO), the vertical ionization potential, and the vertical
electron affinity also confirm that Al8H8 is a stable cluster. The
density of states (DOS) shows that AlnHn exhibit changes from
molecular-like (Al1H1) to band-like structure (Al10H10)
as n increases.
The European Physical Journal D 04/2010; 57(2):197-205. DOI:10.1140/epjd/e2010-00022-y · 1.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Extensive studies of structural and magnetic properties of pure Ni
(n=2–21) clusters are carried out
using density functional theory with generalized gradient approximation. A new structural growth pattern is
reported and the icosahedron sequence is not the global minima structures for size n < 16. The evolution
pattern of Ni5–Ni10 clusters is based on square pyramid. For cluster between n = 9 and n = 14,
structures of them are trilayered stacking pattern. Especially for Ni13, it seriously deviates from
icosahedral pattern which was frequently assumed in previous studies. Ni10, Ni13 and Ni14 are
pieces of a double tetrahedron. For cluster size larger than 16, icosahedron based configurations are found to
be the dominative structure pattern with exception of Ni19. But trilayered structure is a strong
competitor due to small difference in energy between this pattern and icosahedral family. The comparison of
magnetic behavior between the calculations and experimental results are conducted. The average coordination
mainly dominates on the magnetic behavior in the size range we considered.
The European Physical Journal D 01/2010; 61(2):389-396. DOI:10.1140/epjd/e2010-10129-8 · 1.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Free-standing graphene sheets (GS) with several to over a hundred layers are prepared by splitting the expandable graphite. The raw material is firstly transferred to a heating chamber for the thermal flashing at 1500 οC in a hydrogen chamber. The expanded powder (1 mg) is then dispersed in 10 ml of the PmPV/DCE suspension for high-power sonication and further centrifugation. After drop-cast onto a holey Formvar film, the GSs are suspended on the grid. The electron scattering is carried out in a Tecnai F20 TEM/STEM with a field emission gun. A high-angle annular dark field (HAADF) detector is fitted to collect the scattered electrons with a tunable collecting angle. The quantitative STEM image simulation is carried out using amultislice algorithm .
[Show abstract][Hide abstract] ABSTRACT: Upon the development of the quantitative scanning transmission electron microscopy (QSTEM), individually counting the gold atom has been achieved. The correlation of the signals to the atom quantity (N), i.e. calibration, is its crucial work. One has to seek the ultra-fine mass standards, where the size-selected gold clusters rank the best recently. However, practically this is difficult for light elements, since STEM intensity is proportional to Zα, where Z is atomic number of the elements and α is in the range of 1.5-1.9 depending on the detector collection angle, sample thickness, and the Debye-Waller factor of the atomic species. As a result, the light elements would have a much weaker image contrast than that of heavy elements. This could be even more difficult for the carbon particles supported by the Formvar films. Graphene layers are featured by large scale uniformity and discrete layer steps, which shed light on precision calibration of the carbon atomic scales. To establish direct correlation between the STEM intensity with the number of layers in graphene sheets, we apply an independent layer counting method by utilizing dark lines in bright field TEM images at the edge of graphene sheets . We search over a large area of the sample for the freely suspending graphene sheets (for example, the area indicated by the arrows in Fig. 1a). Fig. 1b displays a typical example of such an edge, where the layer number can be counted in the high-resolution bright field image and the STEM intensity is also obtained. The calibration in the few-layer range gives Fig. 1c, where a linear relationship is apparent. The calibration performed over an extended thickness presents a precise exponential growth. The behaviour can be described by the incoherent scattering picture of single scattering. This leads to accurate measurement of the electron mean free path (The error is reduced to 5 percent). The calibration relation works from individual up- - to millions of carbon atoms.
Vacuum Electron Sources Conference and Nanocarbon (IVESC), 2010 8th International; 01/2010
[Show abstract][Hide abstract] ABSTRACT: Plasmon coupling of dimers of Ag nanoparticles is studied by the EELS as shown in Fig 1a . The spectroscopic images in the above Fig. 1a are extracted from 2.8eV, 3.4eV and 3.6eV respectively, where one can see the preferable exciting points of the features around the Ag nanoparticles. With the aid of DDA calculation(Fig. 1b), the three features are identified as the in-phase plasmon coupling, non-coupling and anti-phase coupling modes. The in-phase mode is able to conduct the light transportation due to its dipolar nature, which redshifts from the 3.4eV at large spacings to 2.7eV near touching (Fig. 1c). Such behaviour is also observed in the chain of 3 and 4 nanoparticles. The experiment provides clear evidence on the plasmon coupling of Ag nanoparticles. Extracting the dopant states in the Co-doped ZnO nanoparticles . In order to assign the dopant-introduced states in the low-loss EELS, we employ both the core-loss EELS and the valence EELS on the same Co-doped ZnO nanoparticles by spatially mapping the dopant concentration distribution via the core-loss EELS (Fig 2a 2b 2c) and simultaneously by identifying the dopant features in the accumulated valence EELS via the measured dopant richness (Fig 2d). Three Coincidentally-growing Co dopant states are successfully determined in Fig. 2e. The experimental inputs feed the first-principles calculation, which generates full electronic structure of the doped ZnO nanoparticles. The experiment supports the carrier mediated room-temperature magnetism, which is a main confusion with current dilute magnetic semiconductors.
[Show abstract][Hide abstract] ABSTRACT: Rare-earth titanate cubic pyrochlore Ho <sub>2</sub> Ti <sub>2</sub> O <sub>7</sub> ceramics are prepared by the standard solid state reaction. The magnetic, dielectric, and ferroelectric behaviors are measured in order to investigate the multiferroic behaviors of Ho <sub>2</sub> Ti <sub>2</sub> O <sub>7</sub> . Upon the gradual spin ordering sequence with decreasing temperature, a ferroelectric transition with a small polarization is identified. We discuss the origin of ferroelectricity and its possible correlation with the spin order at low temperature.
[Show abstract][Hide abstract] ABSTRACT: Bilayer PVDF/Metglas laminates with remarkably anisotropic ME coupling effect have been fabricated. By a careful evaluation of these anisotropic behaviors both experimentally and theoretically, we demonstrate that an ultra-sensitive detection of weak dc magnetic field and its spatial orientation is practically applicable using the bilayer PVDF/Metglas laminate. This magnetic field probe has quite high sensitivity. The reason for such a high magnitude and orientation-dependent magnetoelectric coupling is explained.
Sensors and Actuators A Physical 06/2009; 153(1):64-68. DOI:10.1016/j.sna.2009.04.033 · 1.90 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Cobalt nanostructured films were successfully prepared by depositing Co clusters onto a Si(100) substrate in our home-made
cluster beam deposition apparatus. During preparation, the energy of the cluster beam was accelerated by applying an external
electric field. The effect of the cluster beam energy on the microstructure, phase transformation and magnetic behavior was
investigated for the cluster-assembled film in the case of both low energy cluster deposition and energetic cluster deposition.
The scanning electron microscopy observation confirms that the films are assembled by the regular spherical nanoclusters.
Compared with the low-energy Co cluster-assembled film, the magnetization of the energetic Co cluster-assembled film was improved
significantly. The present work describes an effective way to prepare the magnetic nanostructured films.
The European Physical Journal D 04/2009; 52(1):163-166. DOI:10.1140/epjd/e2008-00253-5 · 1.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The effect of the orientation of 0.52 Pb(Fe1/2Nb1/2)O3-0.48 PbTiO3 single crystal (PFN-PT) on the magnetoelectric (ME) properties in tri-layer laminate composites consisting of TbxDy1-xFe2 alloy and PFN-PT has been studied. The different alphaE as a function of applied magnetic field was observed in the laminate composites with different PFN-PT orientations. The origin of alphaE dependent on the orientation of PFN-PT is discussed based on an analysis of crystal physics. It was found that the shear strains also play an important role in the ME effect besides the normal strains.
Journal of Physics D Applied Physics 01/2009; 42(1). DOI:10.1088/0022-3727/42/1/015005 · 2.72 Impact Factor