Observation of electronic ferroelectric polarization in multiferroic YMn2O5

We report the observation of a magnetic polarization of the O 2p states in YMn(2)O(5) through the use of soft x-ray resonant scattering at the oxygen K edge. Remarkably, we find that the temperature dependence of the integrated intensity of this signal closely follows the macroscopic electric polarization, and hence is proportional to the ferroelectric order parameter. This is in contrast with the temperature dependence observed at the Mn L(3) edge, which reflects the Mn magnetic order parameter. First-principles calculations provide a microscopic understanding of these results and show that a spin-dependent hybridization of O 2p and Mn 3d states results in a purely electronic contribution to the ferroelectric polarization, which can exist in the absence of lattice distortions.     

Sodium pyroxene NaTiSi2O6: possible haldane spin-1 chain system

Using a density functional approach, we study the structural and magnetic properties of the pyrox-ene compound NaTiSi2O6. While all previous workers are taking that NaTiSi2O6 is a quasi-one-dimensional S=1/2 system, our theoretical results indicate that this is a Haldane S=1 chain compound below the phase transition at 210 K. A good agreement is obtained between the calculated and the measured Ti-Ti distances in the dimerized low temperature phase. We present a simple explanation of the flow of the unusual phase transition which is taking place in this compound.     

Electromagnons in multiferroic YMn2O5 and TbMn2O5

Based on temperature dependent far infrared transmission spectra of YMn2O5 and TbMn2O5 single crystals, we report the observation of electric dipole-active magnetic excitations, or electromagnons, in these multiferroics. Electromagnons are found to be directly responsible for the steplike anomaly of the static dielectric constant at the commensurate--incommensurate magnetic transition and are the origin of the colossal magneto-dielectric effect reported in these multiferroics.     

Three-dimensional magnetic correlations in multiferroic LuFe2O4

We present single crystal neutron diffraction measurements on multiferroic LuFe(2)O(4). Magnetic reflections are observed below transitions at 240 and 175 K indicating that the magnetic interactions in LuFe(2)O(4) are three-dimensional in character. The magnetic structure is refined as a ferrimagnetic spin configuration below the 240 K transition. Below 175 K a significant broadening of the magnetic peaks is observed along with the buildup of a diffuse component to the magnetic scattering.     

High-temperature structural phase transition in the LiCu2O2 multiferroic

The results of thermogravimetric, X-ray diffraction, and electrical studies of LiCu₂O₂ single crystals in the temperature range 300–1100 K are presented. A reversible first-order phase transition between the orthorhombic and tetragonal phases is found to occur in these single crystals at T = 993 K. A pronounced peak on a differential thermal analysis curve and jumps in the unit cell parameters and the electrical resistivity are detected at the phase-transition temperature. The data on the crystal structure of LiCu₂O₂ and the phase transition-induced change in the entropy determined in this work are used to conclude that the revealed phase transition is caused by the ordering-disordering of Li⁺ and Cu²⁺ cations in their structural positions.     

Charge ordering of LuFe2O4 electronic multiferroic

Charge ordering of iron ions in LuFe2O4 frustrated electronic ferroelectric with variable valence is considered. Charge degrees of freedom are described in frameworks of Ising model with Coulomb interaction between sites. Mean field approximation is used to find free energies of the most probable structures. Phase diagrams of a single triangular bilayer and a system of triangular bilayers are plotted. A partially disordered dimer structure is proposed as a 2D high temperature phase. The transition from 2D to 3D ordering associated with a drop in temperature and the formation of spontaneous dipole moments is discussed.     

First-principles studies on mechanical,electronic, magnetic and optical properties of new multiferroic members BiLaFe2O6 and Bi2FeMnO6: Originated from BiFeO3

Recently multiferroic materials have attract great interest for the applications on memorial, spintronic and magneto-electric sensor devices for their spontaneous magneto-electric coupling properties. Research and development of the various kinds of multiferroics are indispensable factor for a new generation multifunctional materials. In this research, mechanical, electronic, magnetic and nonlinear optical properties of La modified BiLaFe₂O₆ (BLFO) and Mn modified Bi₂FeMnO₆ (BFMO) were studied as new members of multiferroic BiFeO₃ (BFO) series by first-principles calculations, and compared with the pure BFO to discover the optimized properties. Our results show that BLFO and BFMO have good mechanical stability as revealed by elastic constants that satisfy the stability criteria. All these compounds exhibit anisotropic and ductile nature. The enhanced properties by La and Mn substitution, such as increased hardness, improved magnetism, decreased band gap and comparable second harmonic generation responses reveal that the new multiferroic members of BLFO and BFMO would get wider application than their BFO counterpart. Our study is expected to providing an appropriate mechanical reference data as guidance for engineering of high efficiency multifunctional devices with the BFO series.     

Molecular-spin dynamics study of electromagnons in multiferroic RMn2O5

We investigate the electromagnon in magnetoferroelectrics RMn(2)O(5) using combined molecular-spin dynamics simulations. We confirm that the origin of the electromagnon modes observed in the optical spectra is due to the exchange-striction interaction between the magnons and the phonons, and the dielectric step at the magnetic phase transition is due to the appearance of the electromagnon in the low-temperature phase in these materials. The magnetic anisotropy breaks the rotational symmetry of the magnetic structures and, as a result, the electromagnon splits into three modes in RMn(2)O(5). We find that the electromagnon frequencies are very sensitive to the magnetic wavevector along the a direction q(x). Therefore, the electromagnon frequencies of TmMn(2)O(5) (q(x) ~ 0.467) are expected to be much higher than those of other materials of the family, such as R= Tb, Y, Ho, etc (q(x) ~ 0.48). We further calculate the electromagnons in the magnetic field, and find a new mode appearing in the magnetic field. Although the modes' frequencies change significantly under magnetic field, the total static dielectric constant contributed from the electromagnons does not change much in the magnetic field, suggesting that the colossal magnetodielectric effects in these materials may not be caused by the electromagnons.     

Optimizing conditions of fabrication and the properties of BaNb2O6-SrNb2O6 binary ceramics

Conditions for fabricating (Ba_0.5Sr_0.5)Nb₂O₆ ceramics are optimized. It is shown that materials synthesized at T = 1250°C and sintered at T = 1375–21400°C exhibit the best ceramic characteristics. The (Ba_0.5Sr_0.5)Nb₂O₆ solid solution is found to be a ferroelectric with a diffuse phase transition. The possibility of fabricating Ba_0.5Sr_0.5Nb₂O₆ thin films from ceramic targets prepared in the resulting optimum regimes is shown.     

Resonant soft X-ray scattering studies of multiferroic YMn2O5

We performed soft X-ray resonant scattering at the MnL _2,3- and OK edges of YMn₂O₅. While the resonant intensity at the Mn L _2,3 edges reflects the magnetic order parameter, the resonant scattering at the O K edge is found to be directly related to the macroscopic ferroelectric polarization. The latter observation reveals the important role of the spin-dependent Mn-O hybridization for the multiferroicity of YMn₂O₅. We present details about how to obtain correct energy dependent lineshapes and discuss the origin of the resonant intensity at the O K edge.     

Order parameters and phase diagram of multiferroic RMn2O5

The generic magnetic phase diagram of multiferroic RMn2O5 (with R=Y, Ho, Tb, Er, Tm), which allows different sequences of ordered magnetic structures for different R's and different control parameters, is described using order parameters which explicitly incorporate the magnetic symmetry. A phenomenological magnetoelectric coupling is used to explain why some of these magnetic phases are also ferroelectric. Several new experiments, which can test this theory, are proposed.     

Structural anomalies and multiferroic behavior in magnetically frustrated TbMn2O5

We have studied the magnetostructural phase diagram of multiferroic TbMn2O5 as a function of temperature and magnetic field by neutron diffraction. Dielectric and magnetic anomalies are found to be associated with steps in the magnetic propagation vector, including a rare example of a commensurate-incommensurate transition on cooling below 24 K, and in the structural parameters. The geometrically frustrated magnetic structure is stabilized by "canted antiferroelectric" displacements of the Mn3+ ions, an example of the magnetic Jahn-Teller effect. The Tb moments order ferromagnetically at low temperatures in an applied field, while the Mn magnetic structure is largely unchanged.     

Magnetic ground state and transition of a quantum multiferroic LiCu2O2

Based on resonant soft x-ray magnetic scattering, we report that LiCu2O2 exhibits a large interchain coupling which suppresses quantum fluctuations along spin chains, and a quasi-2D short-range magnetic order prevails at temperatures above the magnetic transition. These observations unravel the fact that the ground state of LiCu2O2 possesses long-range 2D-like incommensurate magnetic order rather than being a gapped spin liquid as expected from the nature of quantum spin-1/2 chains. In addition, the spin coupling along the c axis is found to be essential for inducing electric polarization.     

Observation of relaxor ferroelectricity and multiferroic behaviour in nanoparticles of the ferromagnetic semiconductor La(2)NiMnO(6)

We report a diffuse phase transition (extending over a finite temperature range of ～50?K) in sol-gel derived nanoparticles (～25?nm) of the ferromagnetic double perovskite La(2)NiMnO(6). The macroscopic polarization (P-E hysteresis loop), validity of the Vogel-Fulcher relation and high dielectric permittivity (～9?×?10(2)) confirm relaxor ferroelectric phenomena in these magnetic nanoparticles. Compared to the corresponding bulk sample, appreciably large enhancement of the magnetocapacitive effect (MC ～?30%) is observed even under low magnetic field (0.5?T) around the broad relaxor dielectric peak temperature (～220?K), which is close to the ferromagnetic transition temperature (θ(f)?～?196?K). All of these features establish the multiferroic character of the La(2)NiMnO(6) nanoparticles. The inhomogeneities arising from chemical and valence mixing in the present La(2)NiMnO(6) nanoparticles and the inter-site, Ni/Mn-site disorder along with surface disorder of the individual nanoparticles resulting in local polar regions are attributed to the observed dielectric behaviour of the nanoparticles. The wave vector dependent spin-pair correlation is considered to be the plausible cause of the colossal magnetocapacitive response near the transition temperature. High permittivity and large magnetocapacitive properties make these ferromagnetic La(2)NiMnO(6) nanoparticles technologically important.     

Effects of Co-substitutes on multiferroic properties of Bi5FeTi3O15 ceramics

The samples Bi₅FeTi₃O₁₅ (BFTO) and Bi₅Fe_0.5Co_0.5Ti₃O₁₅ (BFCT) were prepared by incorporating BiFeO₃ (BFO) and BiFe_0.5Co_0.5O₃ (BFCO) into the host Bi₄Ti₃O₁₂ (BTO) using the solid state reaction technique. The Raman shift data indicate that the as-prepared materials are of the four-layer Aurivillius phase with an orthorhombic symmetry. At room temperature (RT) and under a driving electric field of 230 kV/cm, the half substitution of Fe by Co ions is found to result in the increase in the remanent polarization $\left(2P_r\right)$ by about 35% and simultaneously the decrease in $2E_c$ by about 41%, respectively. The magnetic moment at least tripled its value by substituting half Fe ions by Co ions. The $2M_r$ of BFCT is about three thousand times the value of BFTO.     

Manipulating the magnetic structure with electric fields in multiferroic ErMn2O5

Based on measurements of soft x-ray magnetic diffraction under in situ applied electric field, we report on significant manipulation and exciting of commensurate magnetic order in multiferroic ErMn2O5. The induced magnetic scattering intensity arises at the commensurate magnetic Bragg position whereas the initial magnetic signal almost persists. We demonstrate the possibility to imprint a magnetic response function in ErMn2O5 by applying an electric field.     

Magnetoelectric coupling in RMn2O5 multiferroic: a Monte Carlo simulation

Magnetoelectric coupling in RMn₂O₅ (with R?=?non magnetic) multiferroics have been studied using the Monte Carlo simulation. The variation of magnetization and the polarization of RMn₂O₅ multiferroic have been determined. The system undergoes a magnetic transition at T_N and a further reduction of the temperature leads to a ferroelectric transition at T_C?<_? T_N depending on the coupling strength. Magnetic and ferroelectric hysteresis loops are obtained for several temperatures values. Variation of polarization with the external magnetic field of RMn₂O₅ has been given. Variation of polarization and magnetization with the electric field of RMn₂O₅ has been obtained.     

准2-2型磁电多层复合材料的磁电性能

用带底座的Pb（Zr,Ti）O₃薄片阵列作为压电相,Terfenol-D粉末与树脂的混合物作为磁致伸缩相,灌注复合得到准2-2型磁电多层三相复合材料.研究了这种准2-2型多层结构在不同偏置磁场和不同角度下的磁电效应.其面内相互垂直的两个方向磁电电压系数分别在0.15和0.225 T磁场下达到峰值,低频下分别为1.62×10⁵ V·m^-1T^-1和1.75×10⁵ V·m^-1T^-1;而在垂直于面内方向,低频下磁电系数仅有1.3×10⁴ V·m^-1T^-1,体现出显著的磁电各向异性.这种准2-2型磁电多层复合材料具有较好的交流磁场灵敏度,在谐振频率下可以探测到10^-9 T的交流磁场的变化.进行合适的设计和排列,可以探测空间磁场的大小和方向,有望应用于磁场传感器等领域. 关键词： 磁电效应 多铁性 多层复合材料