Spin phonon coupling in hexagonal multiferroic YMnO3

Inelastic neutron scattering measurements have been performed on YMnO3, aiming to study the interplay between spin and lattice degrees of freedom in this hexagonal multiferroic material. Our study provides evidence for a strong coupling between magnons and phonons, evidenced by the opening of a gap below T(N) in the dispersion of the transverse acoustic phonon mode polarized along the ferroelectric axis. The resulting upper phonon branch is found to lock on one of the spin-wave modes. These findings are discussed in terms of a possible hybridization between the two types of elementary excitations.     

Finite-temperature properties of multiferroic BiFeO3

An effective Hamiltonian scheme is developed to study finite-temperature properties of multiferroic BiFeO3. This approach reproduces very well (i) the symmetry of the ground state, (ii) the Néel and Curie temperatures, and (iii) the intrinsic magnetoelectric coefficients (that are very weak). This scheme also predicts (a) an overlooked phase above Tc approximately 1100 K that is associated with antiferrodistortive motions, as consistent with our additional x-ray diffractions, (b) improperlike dielectric features above Tc, and (c) that the ferroelectric transition is of first order with no group-subgroup relation between the paraelectric and polar phases.     

Structure and multiferroic properties of Eu-substituted BiFeO3 ceramics

Polycrystalline Bi_1?x Eu _x FeO₃ (x=0.00–0.25) ceramics were synthesized by the solid state reaction method with the rapid liquid phase sintering process. The effects of Eu substitution on the structure, and ferroelectric and magnetic properties of BiFeO₃ ceramics were investigated. X-ray diffraction measurements reveal that the structure of BiFeO₃ was changed from rhombohedral to orthorhombic and the impurity phases were decreased both due to Eu substitution. Raman spectra results also confirm that a structure transition occurs in the Eu concentration range of 0.15–0.20. The SEM investigation has suggested that the Eu substitution hinders the grain growth. Vibrating sample magnetometer measurements indicate ferromagnetism in Eu-substituted BiFeO₃ ceramics. It is found that the room temperature magnetic moment increases with increasing Eu concentration due to the suppressed or broken cycloid spin structure. Ferroelectric measurements show that Eu substitution enhances the polarization due to the significant decrease of the electric leakage of the samples. Therefore, the Eu-substituted BiFeO₃, or more complicated substituted BiFeO₃ based on Eu substitution, will have great potential for many practical applications.     

Electric field control of magnetism in multiferroic heterostructures

We review the recent developments in the electric field control of magnetism in multiferroic heterostructures, which consist of heterogeneous materials systems where a magnetoelectric coupling is engineered between magnetic and ferroelectric components. The magnetoelectric coupling in these composite systems is interfacial in origin, and can arise from elastic strain, charge, and exchange bias interactions, with different characteristic responses and functionalities. Moreover, charge transport phenomena in multiferroic heterostructures, where both magnetic and ferroelectric order parameters are used to control charge transport, suggest new possibilities to control the conduction paths of the electron spin, with potential for device applications.     

Structural,optical, and multiferroic properties of single phased BiFeO3

A soft chemical coprecipitation method has been proposed for synthesis of nano-sized multiferroic BiFeO₃ (BFO) powders. The X-ray diffraction pattern confirms the perovskite structure of BFO and Rietveld refinement reveals the existence of rhombohedral R3c symmetry. Crystallite size and strain value are studied from Williamson–Hall (W–H) analysis. The transmission electron microscope (TEM) image shows that the particle size of BFO powders lies between 50–100 nm. 4A₁ and 7E Raman modes have been observed in the range 100–650 cm^?1 and a prominent band centered around 1150–1450 cm^?1 have also been observed corresponding to the two-phonon scattering. Differential Thermal Analysis (DTA) shows the existence of two prominent peaks at 330 ^°C and 837 ^°C corresponding to the magnetic and ferroelectric ordering, respectively. From the temperature dependent dielectric studies, an anomaly in the dielectric constant is observed at the vicinity of Neel temperature (T _N ) indicating a magnetic ordering. Also, BFO shows antiferromagnetic behavior measured from the magnetic studies.     

Pressure-induced phase transitions of multiferroic BiFeO3

Pressure-induced phase transitions of multiferroic BiFeO3 have been investigated using synchrotron radiation X-ray diffraction with diamond anvil cell technique at room temperature. Present experimental data clearly show that rhombohedral （R3c） phase of BiFeO3 first transforms to monoclinic （C2/m） phase at 7 GPa, then to orthorhombic （Pnma） phase at 11 GPa, which is consistent with recent theoretical ab initio calculation. However, we observe another peak at 2θ=7° in the pressure range of 5-7 GPa that has not been reported previously. Further analysis reveals that this reflection peak is attributed to the orthorhombic （Pbam） phase, indicating the coexistance of monoclinic phase with orthorhombic phase in low pressure range.     

Coupling of ferroelectric and antiferromagnetic order parameters in hexagonal RMnO3

Second-harmonic generation (SHG) is used as a probe for the coexisting ferroelectric and antiferromagnetic orders in hexagonal RMnO₃. SH contributions coupling to the electric and/or magnetic order parameters are identified on the basis of their spectral dependence and the symmetries of the corresponding order parameters. The SH signals from the ferroelectric and antiferromagnetic orders were employed to image the electric and magnetic domain structures separately. The transformation properties of electric and magnetic domains are discussed with respect to the transformation properties of the corresponding order parameters. An investigation of the mutual coupling between the coexisting electric and magnetic orders reveals apparently independent domain structures, which contradicts the symmetry and transformation properties of electric-dipole-induced SHG in this ferroelectromagnetic group of compounds. Apart from higher-order multipole contributions to SHG, interface contributions from clamped electric and magnetic domains can solve the contradiction. Received: 16 October 2001 / Published online: 24 April 2002     

Strain effects on multiferroic BiFeO3 films

The field of multiferroics has experienced a rapid progress resulting in the discovery of many new physical phenomena. BiFeO₃ (BFO) compound, which is one of the few room-temperature single-phase multiferroics, has contributed subsequently to this progress. As a result, significant review articles have been devoted specifically to this famous system. This chapter is dedicated to the strain effects on the structure stability and property changes of BFO thin films. It is a short and non-exhaustive topical overview that may be seen as an invitation for interested readers to go beyond. There is a very active and prolific research in this field and we apologize to the authors whose relevant work is not cited here. After a short introduction, we will thus review the effect of strain on BFO films by describing the consequences on the structure and the phase transitions as well as on polar, magnetic and magnetoelectric properties.     

Atomistic simulation of dynamical and defect properties of multiferroic hexagonal YMnO_3

Atomistic simulation has been performed to investigate the dynamical and defect properties of multiferroic hexagonal YMnO3 with newly developed interaction potentials. Dynamical calculation reveals that phonon vibrations of hexagonal YMnO3 are quite different from those of orthorhombic YMnO3. Defect calculation finds that O Frenkel is the most probable intrinsic disorder, and Mn antisite defect is favorable to exist, especially for Mn ions entering the Y2 sites. It is also found that holes prefer to localiz...     

High-temperature heat capacity of the BiFeO3 multiferroic

Data on the heat capacity of the BiFeO₃ multiferroic have been obtained within a broad temperature interval. Correlation has been established between the composition of the Bi₂O₃-Fe₂O₃ pseudobinary system and the specific heat capacity of the oxide compounds.     

Aftereffects of an electric shock in BiFeO3 multiferroic

Delayed (up to several months) transformations in the atomic structure induced by the external action (electric shock) are observed for the first time in a class of nonmetals in BiFeO₃ multiferroic.     

Magnetic properties of thick multiferroic hexagonal HoMnO3 films

Multiferroic hexagonal HoMnO₃/YSZ(1 1 1) films with a thickness of ?1 μm have been grown by pulsed laser deposition. Conventional X-ray diffraction and pole figure measurement indicate that the film is epitaxially grown without secondary phase or misorientation. Magnetization measurements reveal some differences in comparison with a single crystal. These differences are possibly related to the larger a and c lattice parameters of the film which are likely to alter the bond lengths and, thus, the magnetic superexchange interactions. A substantial change of the complex Ho³⁺ magnetic ordering below 6 K is observed.     

Improved multiferroic properties of La-doped 0.6BiFeO3—0.4SrTiO3 solid solution ceramics

The 0.6（Bi1-xLax）FeO 3-0.4SrTiO 3（x = 0,0.1） multiferroic ceramics are prepared by a modified Pechini method to study the effect of substitution of SrTiO3 and La in BiFeO3.The X-ray diffraction patterns confirm the single phase characteristics of all the compositions each with a rhombohedral structure.The magnetic properties of the ceramics are significantly improved by a solid solution with SrTiO3 and substitution of La.The values of the dielectric constant ε r and loss tangent tan δ of all the samples decrease with increasing frequency and become constant at room temperature.The La-doped 0.6BiFeO3-0.4SrTiO3 ceramics exhibit improved dielectric and ferroelectric properties,with higher dielectric constant enhanced remnant polarization（Pr） and lower leakage current at room temperature.Compared with a anti-ferromagnetic BiFeO3 compound,the 0.6（Bi0.9La0.1）FeO3-0.4SrTiO3 sample shows the optimal ferromagnetism with remnant magnetization M r ～ 0.135 emμ/g and ferroelectricity with Pr ～ 5.94 μC/cm 2 at room temperature.     

Spin lattice coupling in multiferroic hexagonal YMnO3

Aiming to shed light on the possible existence of hybrid phonon-magnon excitations in multiferroic manganites, neutron scattering measurements have been undertaken at LLB and ILL on the particular case of hexagonal YMnO₃. Our experiments focused on a transverse acoustic phonon mode polarized along the ferroelectric axis. The neutron data show that below the magnetic transition, this particular phonon mode splits in two different branches. The upper branch is found to coincide with a spin wave mode. This manifestation of a strong spin-lattice coupling is discussed in terms of a possible hybridization between the two types of elementary excitations, the phonon and magnons.      

微纳尺度多铁异质结中电驱动磁反转

近年来,多铁异质结中电控磁性研究引起了广泛关注,已成为多铁领域的热点.现代自旋电子学器件(如磁内存)通常利用电流产生的磁场或自旋转移扭矩效应驱动磁反转来实现数据擦写,但这带来高额能耗和热量,成为亟待解决的关键难题.而利用多铁异质结实施电场驱动磁反转则有望大幅降低能耗,从而实现高速、低能耗、高稳定性新型高密度磁存储、逻辑及其他自旋电子学器件.在当前器件发展的微型化趋势下,探索可集成化的微纳尺度电场驱动磁反转方案显得越发重要.本文针对发展新型磁电器件所面临的微型化关键问题,回顾了微纳尺度电场驱动磁反转研究的新进展,主要关注小尺度多铁异质结中电控磁的新特点、新方法及相关物理机理的实验和理论成果,讨论了进入纳米尺度将面临的挑战,并对未来研究工作提出一些展望.     

Investigation of complex impedance and modulus properties of Nd doped 0.5BiFeO3-0.5PbTiO3 multiferroic Composites

0.5BiNd _x Fe_1?x O ₃ ? 0.5PbTiO₃ (BN _x F_1?x ? PT)(x = 0.05, 0.10, 0.15, 0.20) composites were successfully synthesized by a solid state reaction technique. At room temperature, X-ray diffraction shows tetragonal structure for all concentrations of Nd doped 0.5BiFeO₃ ? 0.5PbTiO₃ composites. The nature of Nyquist plot confirms the presence of bulk effects only for BN _x F_1?x ? PT (x = 0.05, 0.10, 0.15, 0.20) composites. The bulk resistance is found to decreases with the increasing temperature as well as Nd concentration and exhibits a typical negative temperature coefficient of resistance (NTCR) behavior. Both the complex impedance and modulus studies have suggested the presence of non-Debye type of relaxation in the composites. Conductivity spectra reveal the presence of hopping mechanism in the electrical transport process of the composites. The activation energy calculated from impedance plot of the composite decreases with increasing Nd _x concentration and found to be 0.89, 0.76, 0.71 and 0.70 eV for x=0.05, 0.10, 0.15 and 0.20 respectively.     

Mechanisms of exchange bias with multiferroic BiFeO3 epitaxial thin films

We have combined neutron scattering and piezoresponse force microscopy to show that the exchange field in CoFeB/BiFeO_{3} heterostructures scales with the inverse of the ferroelectric and antiferromagnetic domain size of the BiFeO3 films, as expected from Malozemoff's model of exchange bias extended to multiferroics. Accordingly, polarized neutron reflectometry reveals the presence of uncompensated spins in the BiFeO3 film at the interface with CoFeB. In view of these results, we discuss possible strategies to switch the magnetization of a ferromagnet by an electric field using BiFeO3.