Exchange bias effect in multiferroic Eu0.75Y0.25MnO3

The exchange bias phenomenon has been investigated in multiferroic Eu_0.75Y_0.25MnO₃. The material shows a weak ferromagnetism with cone spin configuration induced by external magnetic field below 30 K. Consequently, the electric polarization coming from the cycloid spin order below 30 K can be suppressed by external magnetic fields. The magnetic hysteresis loops after cooling in a magnetic field exhibit characteristics of exchange bias below the spin glassy freezing temperature (T_g)∼16 K. The exchange bias field, coercivity field, and remanent magnetization increase with increasing cooling magnetic field. The exchange bias effect is ascribed to the frozen uncompensated spins at the antiferromagnetism/weak ferromagnetism interfaces in the spin-glass like phase.     

First-order phase transition from an antiferromagnetic ferroelectric to a cycloidal multiferroic with weak ferromagnetism during the joint action of applied magnetic and electric fields

The thermodynamics of the phase transition in a perovskite-like multiferroic, in which an antiferromagnetic ferroelectric transforms into a new magnetic state where a spiral spin structure and weak ferromagnetism can coexist in applied magnetic field H, is described. This state forms as a result of a first-order phase transition at a certain temperature (below Néel temperature T _N ), where a helicoidal magnetic structure appears due to the Dzyaloshinskii-Moriya effect. In this case, the axes of electric polarization and the helicoid of magnetic moments are mutually perpendicular and lie in the ab plane, which is normal to principal axis c. Additional electric polarization p, which decreases the total polarization of the ferroelectric P, appears in the ab plane. The effect of applied magnetic and electric fields on the properties of a multiferroic with a helicoidal magnetic structure is described. An alternating electric field is shown to cause a field-linear change in magnetic moment m, whose sign is opposite to the sign of the change of electric field E. The detected hysteretic phenomena that determine the temperature ranges of overheating and supercooling of each phase are explained. A comparison with the experimental data is performed.     

Influence of cycling electric polarization on multiferroic behaviors in heterostructural films composed of ferroelectric and ferromagnetic oxides

The CoFe₂O₄/Pb(Zr_0.52Ti_0.48)O₃ bilayer films were prepared by a sol–gel process, and the influence of cycling electric polarization on the multiferroic behaviors of the bilayer films was studied. The ferroelectric polarization hysteresis loops under various choices of magnetic bias were measured by an integrating current method. The results showed that after undergoing cycling electric polarization the ferroelectric polarization of the bilayer films enhanced and the suppression of ferroelectric polarization by external magnetic bias remarkably weakened. Based on the measurements of activation energy and leakage current, we confirmed that the oxygen vacancy migration in the bilayer films occurred during cycling electric polarization. Furthermore, we analyzed the mechanism of the influence of cycling electric polarization on the multiferroic behaviors of the bilayer films and attributed it to the oxygen vacancy migration, which could cause a part of ferroelectric domains to be unpinned from the oxygen vacancies and become more active under electric field and magnetic bias.     

单相ABO3型多铁材料的磁电耦合及磁电性质研究

实验发现多铁性钙钛矿物质YMnO₃和BiMnO₃在接近磁有序相变温度时,其介电常数和正切损失会出现异常,这些现象说明在物质的磁性和介电性质之间存在耦合.通过对系统磁性和铁电性之间可能磁电耦合方式的分析,考虑在系统哈密顿量中加入与自旋关联和极化相关的耦合项,对铁电子系统应用软模理论,对磁性运用基于海森伯模型的量子平均场近似,研究了外磁场诱导的极化、介电的变化和外电场诱导的磁化的变化等,并将以上结果与实验进行了比较和分析,较为合理地解释了一些多铁钙钛矿物质中的磁电现 关键词： 多铁 磁电耦合 铁电 铁磁     

Experimental observations of ferroelectricity in double pyrochlore Dy2Ru2O7

Double pyrochlore Dy2Ru207 is synthesized and its magnetism and ferroelectricity below the Ru4＋ spin ordering temperature （NI00 K） are investigated. The ferroelectric transition appears at -18 K, much higher than the Dy3＋ spin ordering point at -1.8 K and lower than the Ru4＋ spin ordering point at -100 K. The measured electric polarization at ,-2 K is as big as 145℃/m2 in the polycrystalline samples. It is argued that the ferroelectricity is possibly ascribed to the electric dipole ordering arising from the collective monopole excitations in the Dy3＋ tetrahedrons in prior to the Dy3＋ spin ordering into spin-ice like state below -1.8 K.     

Magnetoelectric memory effect in the Y-type hexaferrite BaSrZnMgFe_(12)O_(22)

We report on the magnetic and magnetoelectric properties of the Y-type hexaferrite BaSrZnMgFe_(12)O_(22),which undergoes transitions from a collinear ferrimagnetic phase to a proper screw phase at 310 K and to a longitudinal conical phase at 45 K.Magnetic and electric measurements revealed that the magnetic structure with spiral spin order can be modified by applying a magnetic field,resulting in magnetically controllable electric polarization.It was observed that BaSrZnMgFe_(12)O_(22)exhibits an anomalous magnetoelectric memory effect:the ferroelectric state can be partially recovered from the paraelectric phase with collinear spin structure by reducing magnetic field at 20 K.We ascribe this memory effect to the pinning of multiferroic domain walls,where spin chirality and structure are preserved even in the nonpolar collinear spin state.     

Spontaneous ferroelectricity in strained low-temperature monoclinic Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>: A first-principles study

As a single-phase multiferroic material, Fe₃O₄ exhibits spontaneous ferroelectric polarization below 38 K. However, the nature of the ferroelectricity in Fe₃O₄ and effect of external disturbances such as strain on it remains ambiguous. Here, the spontaneous ferroelectric polarization of low-temperature monoclinic Fe₃O₄ was investigated by first-principles calculations. The pseudo-centrosymmetric FeB42–FeB43 pair has a different valence state. The noncentrosymmetric charge distribution results in ferroelectric polarization. The initial ferroelectric polarization direction is in the -x and -z directions. The ferroelectricity along the y axis is limited owing to the symmetry of the Cc space group. Both the ionic displacement and charge separation at the FeB42–FeB43 pair are affected by strain, which further influences the spontaneous ferroelectric polarization of monoclinic Fe₃O₄. The ferroelectric polarization along the z axis exhibits an increase of 45.3% as the strain changes from 6% to -6%.     

Multiglass order and magnetoelectricity in Mn<Superscript>2+</Superscript> doped incipient ferroelectrics

“Multiglass” materials with simultaneous occurrence of two different glassy states extend the frame of conventional multiferroicity, which is devoted to crystalline materials with coexisting uniform long-range electric and magnetic ordering. The concept applies to Sr_0.98Mn_0.02TiO₃ ceramics, where A-site substituted Mn²⁺ ions are at the origin of both a polar and a spin cluster glass. Spin freezing is initiated below the dipolar glass temperature, T_g ^e ≈ 38 K, which is seemingly indicated by a divergence of the nonlinear susceptibility, χ₃. Below T_g ^m ≈ 34 K both glass phases are independently verified by memory and rejuvenation effects. Biquadratic interaction of the Mn²⁺ spins with ferroelectric correlations of their off-center pseudospins in the incipient ferroelectric host crystal SrTiO₃ explains the high spin glass temperature and comparably strong third-order magnetoelectric coupling between the polar and the magnetic degrees of freedom. Preliminary results on the related compound K_0.97Mn_0.03TaO₃ favorably comply with the magnetoelectric multiglass concept.     

Ferroelectricity of polycrystalline GdMnO3 and multifold magnetoelectric responses

The multiferroic behaviors of polycrystalline GdMnO₃ are investigated by focusing on the ferroelectric response to the spin ordering sequence and external magnetic field. The polarization current shows sensitive response to both the Mn cycloidal spin order and Gd antiferromagnetic (AFM) order. The complicated magnetoelectric behaviors suggest that the Mn cycloidal spin order can be modulated by the Gd AFM order at low temperature via the Gd–Mn spin interaction. Due to the possible disorder and defects in polycrystalline nature, polycrystalline GdMnO₃ may accommodate the cycloidal spin order in addition to the A-type AFM order at Mn sites, as illustrated by simulation based on the two-orbit double exchange model and measured hysteresis loops of polarization against magnetic field, indicating the switching of the ferroelectric domains coupled with the magnetic domains in response to magnetic field.     

BiFeO3中各离子在铁电相变中作用本质的第一性原理研究

采用基于密度泛函理论的广义梯度近似方法和赝势平面波法,对多铁材料BiFeO₃的铁电反铁磁相和可能的高温顺电相的电子结构进行了第一性原理研究.计算验证了BiFeO₃基态为G型反铁磁有序,Fe离子的理论磁矩与实验值相符.铁电相变发生后,Bi-6s态和6p态发生了电荷转移,Bi-6s电子的作用更加明显.Born有效电荷的研究表明铁电畸变主要表现为Bi原子的位移,并且电极化强度计算值很好地符合薄膜实验结果.部分态密度的计算表明Bi-6p态的成键轨道与反键轨道间的能量劈裂 关键词： 第一性原理 铁电性 铁电畸变 反铁磁性     

Structural and magnetic properties of Mn3−xCdxTeO6 (x=0, 1, 1.5 and 2)

Mn₃TeO₆ exhibits a corundum-related A₃TeO₆ structure and a complex magnetic structure involving two magnetic orbits for the Mn atoms [Ivanov et al., 2011 [3]]. Mn_3−xCd_xTeO₆ (x=0, 1, 1.5, and 2) ceramics were synthesized by solid state reaction and investigated using X-ray powder diffraction, electron microscopy, and calorimetric and magnetic measurements. Cd²⁺ replaces Mn²⁺ cations without greatly affecting the structure of the compound. The Mn and Cd cations were found to be randomly distributed over the A-site. Magnetization measurements indicated that the samples order antiferromagnetically at low temperature with a transition temperature that decreases with increasing Cd doping. The nuclear and magnetic structure of one specially prepared ¹¹⁴Cd containing sample: Mn_1.5¹¹⁴Cd_1.5TeO₆, was studied using neutron powder diffraction over the temperature range 2-295 K. Mn_1.5¹¹⁴Cd_1.5TeO₆ was found to order in an incommensurate helical magnetic structure, very similar to that of Mn₃TeO₆ [Ivanov et al., 2011 [3]]. However, with a lower transition temperature and the extension of the ordered structure confined to order 240(10) Å.     

Evolution of the antiferromagnetism vector of a multiferroic BiFeO<Subscript>3</Subscript> during switching its ferroelectric polarization

The evolution of the antiferromagnetism vector of multiferroic BiFeO₃ during switching of its ferroelectric polarization by an electric field has been studied by numerical simulation in the framework of the phenomenological model for the magnetic anisotropy energy. Optimal variants have been found for the cut of electrosensitive BiFeO₃ layer, the deformation induced by a substrate, and the direction of applying electric field for the development of prototypes of new-generation marnetoresistive memory.     

Tunneling magnetoresistance and electroresistance properties of composite‐barrier ferroelectric tunnel junctions

The multiferroic properties of ferroelectric tunnel junctions with a composite barrier comprising a fully epitaxial La_0.7Sr_0.3MnO₃/BaTiO₃/SrTiO₃/La_0.7Sr_0.3MnO₃ heterostructure are reported in this study. The patterned junctions having extended top electrodes show tunnel magnetoresistance ratios ranging from 20% to 110% at 77 K. Furthermore, tunneling electroresistance – induced by ferroelectric polarization switching and showing two‐state memory effect in the dynamic resistance – has also been observed in these junctions. Thus, with the concurrence of tunneling electroresistance and magnetoresistance, these tunnel junctions serve as memory devices with four resistance states. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhanced multiferroic characteristics in Fe-doped Bi4Ti3O12 ceramics

Modification of Bi₄Ti₃O₁₂ multiferroic ceramics prepared by a conventional solid state reaction method were investigated by substituting Ti partly with Fe. The introduction of Fe does not change the layered perovskite structure of Bi₄Ti₃O₁₂. Upon increasing Fe content, the remnant polarization of the samples is enhanced. The magnetism of the ceramics at room temperature develops from diamagnetism to weak ferromagnetism with increasing Fe doping. The largest variations of 15% and 6% in remnant polarization and magnetization, achieved in a Bi₄Ti₁Fe₂O_12?δ sample after poling it in a magnetic field at 1 T and a DC electric field at 30 kV/cm for 10 min, are evidence of magnetoelectric coupling between the electric dipoles and magnetic dipoles at room temperature. The present results suggest a new candidate for a room temperature multiferroic material with enhanced properties.     

Anomalously strong relaxation of the polarization of muons in the magnetically ordered and paramagnetic states of the TbMnO<Subscript>3</Subscript> multiferroic

An anomalously strong relaxation of the muon polarization in a magnetically ordered state in the TbMnO₃ multiferroic has been revealed by the method below the μSR Néel temperature (42 K). Such a relaxation is due to the muon channel of relaxation of the polarization and the interaction of the magnetic moment of the muon with inhomogeneities of the internal magnetic field of an ordered state in the form of a cycloid. Above the Néel temperature, beginning with temperatures depending on the applied magnetic field, a two-phase state has been revealed where one phase has an anomalously strong relaxation of the muon polarization for a paramagnetic state. These features of the paramagnetic state are due to short-range magnetic order domains that appear in strongly frustrated TbMnO₃. A true paramagnetic state has been observed only at T ≥ 150 K.     

Magnetic and ferroelectric properties of exchange-frustrated multiferroics (Ni1 ? xTx)3V2O8 (T = Co, Mn, Zn)

The effect of the substitution of Co²⁺, Mn²⁺, and Zn²⁺ ions for Ni²⁺ ions on the magnetic, dielectric, and ferroelectric properties of vanadate single crystals (Ni_{1 − x} T _x )₃V₂O₈ has been analyzed. It has been found that the low-level (x ≤ 0.1) substitution of both magnetic and nonmagnetic ions stabilizes the ferroelectric state with a cycloidal magnetic structure. The existence region of this state is expanded to low temperatures down to 3 K for Zn²⁺ and below 1.8 K for Co²⁺ and Mn²⁺ owing to the suppression of a low-temperature weak ferromagnetic phase. At the same time, the ferroelectric phase disappears completely at large concentrations of Co and Mn. The effect of magnetic fields on the magnetic and ferroelectric states has been analyzed. It has been shown that the magnetic field along the c axis suppresses the ferroelectric state, whereas the magnetization along the antiferromagnetism axis (a axis) induces the reentrant phase transition from a paraelectric weak ferromagnetic structure to a ferroelectric structure. The corresponding H-T phase diagrams have been drawn.     

Spontaneous reorientation of the electric polarization in Eu1 − x Ho x MnO3 multiferroics

The magnetic, dielectric, and ferroelectric properties of Eu_{1 ? x} Ho _x MnO₃ single crystals (0 < x ≤ 0.5), where magnetic ordering can be varied from the canted antiferromagnetic phase to modulated spin structures, have been studied. It has been found that a ferroelectric state appears at x ≥ 0.2 and low temperatures. As the temperature decreases and the holmium content increases, the electric polarization in this state is reoriented from the a axis to the c axis. It has been shown that the polarization is reoriented owing to a change in the spin rotation plane in the cycloidal phase from the ab to cb plane because of the stabilization of the latter upon an increase in the rare-earth contribution to the anisotropy energy. The T-x phase diagram of magnetic and ferroelectric states has been constructed.     

Spin chirality and electric polarization in multiferroic compounds (, Er)

Polarized neutron diffraction experiments have been performed on multiferroic materials RMn₂O₅ (R=Ho, Er) under electric fields in the ferroelectric commensurate (CM) and the low-temperature incommensurate (LT-ICM) phases, where the former has the highest electric polarization and the latter has reduced polarization. It is found that, after cooling in electric fields down to the CM phase, the magnetic chirality is proportional to the electric polarization. Also we confirmed that the magnetic chirality can be switched by the polarity of the electric polarization in both the CM and LT-ICM phases. These facts suggest an intimate coupling between the magnetic chirality and the electric polarization. However, upon the transition from the CM to LT-ICM phase, the reduction of the electric polarization is not accompanied by any reduction of the magnetic chirality, implying that the CM and LT-ICM phases contain different mechanisms of the magnetoelectric coupling.     

Block antiferromagnetism and possible ferroelectricity in KFe2Se2

Superconductors and multiferroics are two of the hottest branches in condensed matter physics. The connections between those two fields are fundamentally meaningful to unify the physical rules of correlated electrons. Recently, BaFe₂Se₃, was predicted to be multiferroic [Phys. Rev. Lett. 113 , 187204 (2014)] due to its unique one‐dimensional block‐type antiferromagnetism. Here, another iron‐selenide KFe₂Se₂, a parent state of iron‐based superconductor, is predicted to be multiferroic. Its two‐dimensional block‐type antiferromagnetism can generate a moderate electric dipole for each Fe–Se layer via the Fe–Se–Fe exchange striction. Different stacking configurations of these magnetic blocks give closely proximate energies and thus the ground state of KFe₂Se₂ may be switchable between antiferroelectric and ferroelectric phases.

Crystal structure of KFe₂Se₂. (a) Purple: K; green: Se; brown: Fe. Two Fe sheets in a minimum unit cell are indicated as A and B. (b) One Fe–Se layer with magnetism. Brown: spin up; blue: spin down. (c) A side view of Fe–Se bonds. The ionic displacements driven by exchange striction are indicated by arrows.     

Nature of unusual spontaneous and field-induced phase transitions in multiferroics RMn2O5

Complex magnetic, magnetoelectric and magnetoelastic studies of spontaneous and field-induced phase transitions in TmMn₂O₅ were carried out. In the vicinity of spontaneous phase transition temperatures (35 and 25 K) the magnetoelectric and magnetoelastic dependences demonstrated the jumps of polarization and magnetostriction induced by the field ∼150 kOe. These anomalies can be attributed to the influence of magnetic field on the conditions of incommensurate-commensurate phase transition at 35 K and the reverse one at 25 K. In b-axis dependences the magnetic field-induced spin-reorientation phase transition was also observed below 20 K. Finally the magnetoelectric anomaly associated with metamagnetic transition is observed below the temperature of rare-earth subsystem ordering at relatively small critical fields of 5 kOe. This variety of spontaneous and induced phase transitions in RMn₂O₅ stems from the interplay of three magnetic subsystems: Mn³⁺, Mn⁴⁺, R³⁺. The comparison with YMn₂O₅ highlights the role of rare earth in low-temperature region (metamagnetic and spin-reorientation phase transitions), while the phase transition at higher temperatures between incommensurate and commensurate phases should be ascribed to the different temperature dependences of Mn³⁺ and Mn⁴⁺ ions. The strong correlation of magnetoelastic and magnetoelectric properties observed in the whole class of RMn₂O₅ highlights their multiferroic nature.