The results of muon-spin relaxation and heat capacity measurements on two pyroxene compounds LiFeSi2O6 and NaFeSi2O6 demonstrate that despite their underlying structural similarity the magnetic ordering is considerably different. In LiFeSi2O6 a single muon precession frequency is observed below TN, consistent with a single peak at TN in the heat capacity and a commensurate magnetic structure. In applied magnetic fields the heat capacity peak splits in two. In contrast, for natural NaFeSi2O6, where multiferroicity has been observed in zero-magnetic-field, a rapid Gaussian depolarization is observed showing that the magnetic structure is more complex. Synthetic NaFeSi2O6 shows a single muon precession frequency but with a far larger damping rate than in the lithium compound. Heat capacity measurements reproduce the phase diagrams previously derived from other techniques and demonstrate that the magnetic entropy is mostly associated with the build up of correlations in the quasi-one-dimensional Fe3+ chains.
通式为$AM$ Si $_2$ O $_6$（$A$ =一价或二价金属，$M$ =二价或三价金属）的辉石被证明是一类新的多铁性材料。特别地，我们已经发现到目前为止，NaFeSi $_2$ O $_6$在低于$\simeq 6$ K的磁有序状态下变成铁电体。类似地，在Li同系物LiFeSi $_2$ O $_6$（$T_C \simeq 18$ K）和LiCrSi $_2$ O $_6$（$T_C \simeq 11$ K）中也检测到磁驱动铁电性。在所有这些单斜晶系中，电极化的强度都能被磁场强烈地改变。测量的磁化率，热释电流和介电常数（和它们对磁场的依赖性）使用霓石（NaFeSi $_2$ O $_6$）的天然晶体和合成晶体的LiFeSi $_2$ O $_6$和LiCrSi $_2$ O $_6$从熔体溶液中生长。对于NaFeSi $_2$ O $_6$，提出了NaFeSi $_2$ O $_6$的温度-磁场相图.交换常数的计算基于{ \it ab initio}能带结构计算。螺旋磁结构所造成的挫折起源的多铁性行为的可能性进行了讨论。我们建议，其他辉石也可能是多铁性的，这个家庭的多功能性提供了一个特殊的机会，研究一般条件和磁驱动铁电性的机制。
Magnetic order and the underlying magnetic model of the multiferroic pyroxene NaFeGe2O6 are systematically investigated by neutron powder diffraction, thermodynamic measurements, density-functional bandstructure calculations, and Monte-Carlo simulations. Upon cooling, NaFeGe2O6 first reveals one-dimensional spin-spin correlations in the paramagnetic state below about 50 K, revealed by magnetic diffuse scattering. The sinusoidal spin-density wave with spins along the a-direction sets in at 13 K, followed by the cycloidal configuration with spins lying in the (ac) plane below 11.6 K. Microscopic…
The key physical property of multiferroic materials is the existence of a coupling between magnetism and polarization, i.e. magnetoelectricity. The origin and manifestations of magnetoelectricity can be very different in the available plethora of multiferroic systems, with multiple possible mechanisms hidden behind the phenomena. In this Review, we describe the fundamental physics that causes magnetoelectricity from a theoretical viewpoint. The present review will focus on the main stream physical mechanisms in both single phase multiferroics and magnetoelectric heterostructures. The most rece…
Exploring new magnetic materials is essential for finding advantageous functional properties such as magnetoresistance, magnetocaloric effect, spintronic functionality, and multiferroicity. Versatile classes of double perovskite compounds have been recently investigated because of intriguing physical properties arising from the proper combination of several magnetic ions. In this study, it is observed that the dominant ferrimagnetic phase is coexisted with a minor multiferroic phase in single-crystalline double-perovskite Er2CoMnO6. The majority portion of the ferrimagnetic order is activated …
Multiferroic rust: magnetoelectric effect in göthite, $α$-FeOOH
通过对称性分析、密度泛函理论计算和蒙特卡罗模拟我们证明了针铁矿$α$ -FeOOH在低于Néel温度$T_{\rm N}=400$ K时是线性磁电体.实验观察到的磁场诱导自旋翻转相变的结果是电极化的强度的方向改变或抑制。磁电系数计算值为0.24 $μ$ C m $^{-2}$ T $^{-1}$。针铁矿在自然界中的丰度使其成为最广泛的磁电材料。
Phase transitions in multiferroic BiFeO3 crystals, thin-layers, and ceramics: Enduring potential for a single phase, room-temperature magnetoelectric 'holy grail'
The magnetic and dielectric properties of the multiferroic triangular lattice magnet compound alpha-NaFeO2 were studied by magnetization, specific heat, dielectric permittivity, and pyroelectric current measurements and by neutron diffraction experiments using single crystals grown by a hydrothermal synthesis method. This work produced magnetic field (in the monoclinic ab-plane, B_ab, and along the c*-axis, B_c) versus temperature magnetic phase diagrams, including five and six magnetically ordered phases in B_ab and along B_c, respectively. Comparing the polarization direction to the magnetic…
Identifying the Magnetoelectric Modes of Multiferroic BiFeO$_3$
我们已经确定了四个磁电模式的多铁性BiFeO $_3$使用太赫兹光谱测量。通过包括沿电极化的强度方向的易轴各向异性的影响，获得了与观察到的峰的极好的协议。通过扭曲摆线自旋状态，各向异性将$Ψ_{\pm 1}$模式分裂成20和21.5 cm $^{-1}$处的峰，并激活27 cm $^{-1}$处的较低$Φ_{\pm 2}$模式（T=200 K）。在21.5 cm $^{-1}$处，用上$Ψ_{\pm 1}$模式识别电磁振子。我们的结果也解释了最近的拉曼和非弹性中子散射测量。
Local multiferroic ordering at room temperature in collinear magnetoelectric antiferromagnets induced by flexo-Zeeman coupling
Helmholtz-Zentrum Dresden-Rossendorf e.V.（德累斯顿-罗森多夫亥姆霍兹中心）；Faculty of Physics of Dresden University of Technology（德累斯顿工业大学物理学院）；Helmholtz-Zentrum Berlin für Materialien und Energie（柏林亥姆霍兹材料与能源中心）；Department of Applied Physics, KTH Royal Institute of Technology（瑞典皇家理工学院应用物理系）
Size dependent electric voltage-controlled magnetic anisotropy in multiferroic heterostructures: Interface-charge and strain co-mediated magnetoelectric coupling
We report the magnetic field dependence of the THz absorption and non-reciprocal directional dichroism spectra of BiFeO$_3$ measured on the three principal crystal cuts for fields applied along the three principal directions of each cut. From the systematic study of the light polarization dependence we deduced the optical selection rules of the spin-wave excitations. Our THz data, combined with small-angle neutron scattering results showed that i) an in-plane magnetic field rotates the $\mathbf{q}$ vectors of the cycloids perpendicular to the magnetic field, and ii) the selection rules are mos…
The Haldane spin-chain compound, Tb2BaNiO5, with two antiferromagnetic transitions, one at T1=63K, and the other at T2=25K, has been recently shown by us to be an exotic multiferroic below T2. Here, we report the results of our investigations of Sr doping at the Ba site by magnetization, heat-capacity, magnetoelectric (MDE), and pyrocurrent measurements. An intriguing finding, which we stress, is that the ferroelectricity is lost even for a doping level of 10 atomic percent, though magnetic ordering prevails. The doped specimens however retain significant magnetodielectric behaviour, but with …
We use a group theoretical approach to enumerate the possible couplings between magnetism and ferroelectric polarisation in the parent $Pm\bar{3}m$ perovskite structure. We show that third order magnetoelectric coupling terms must always involve magnetic ordering at the A and B-site which either transforms both as R-point or both as X-point time odd irreducible representations (irreps). For fourth order couplings we demonstrate that this criterion may be relaxed allowing couplings involving irreps at X, M and R-points which collectively conserve crystal momentum, producing a magnetoelectric ef…
Two dimensional multiferroics inherit prominent physical properties from both low dimensional materials and magnetoelectric materials, and can go beyond their three dimensional counterparts for their unique structures. Here, based on density functional theory calculations, a MXene derivative, i.e., i-MXene (Ta$_{2/3}$Fe$_{1/3}$)$_2$CO$_2$, is predicted to be a type-I multiferroic material. Originated from the reliable $5d^0$ rule, its ferroelectricity is robust, with a moderate polarization up to $\sim12.33$ $μ$C/cm$^2$ along the a-axis, which can be easily switched and may persist above room …
Multiferroic materials have garnered wide interest for their exceptional static and dynamical magnetoelectric properties. In particular, type-II multiferroics exhibit an inversion-symmetry-breaking magnetic order which directly induces a ferroelectric polarization through various mechanisms, such as the spin-current or the inverse Dzyaloshinskii-Moriya effect. This intrinsic coupling between the magnetic and dipolar order parameters results in record-strength magnetoelectric effects. Two dimensional materials possessing such intrinsic multiferroic properties have been long sought for harnessin…
Electronic structure and magnetic properties of pyroxenes (Li,Na)TM(Si,Ge)2O6: novel low-dimensional magnets with 90 bonds
本文给出了具有不同磁性的辉石（Li，Na）TM（Si，Ge）$_2$ O $_6$的LSDA+U计算结果，其中TM为过渡金属离子（Ti，V，Cr，Mn，Fe）。研究表明，各向异性轨道有序导致了NaTiSi $_2$ O $_6$中自旋能隙的形成.详细分析了辉石链内交换相互作用的不同贡献的解析使用微扰理论和能带结构计算的结果的基础上进行。反铁磁$t_{2g}-t_{2g}$交换被发现逐渐减少，从Ti到Fe。结果表明，在Cr基辉石中，半满$t_{2g}$和空$e_g$轨道之间的铁磁相互作用几乎可以补偿这种铁磁相互作用。NaCrGe $_2$ O $_6$的铁磁性是由于晶体结构对相互作用参数的微调所致。进一步增加$e_g$亚壳层的电子总数和占据使$t_{2g}-e_g$贡献和总交换相互作用对Mn和Fe基辉石的反铁磁作用。铁基辉石具有较强的氧极化。结果表明，这种效应导致了相当大的减少反铁磁链内交换。所得结果可作为分析辉石的各种磁性的基础，包括那些最近发现的多铁性行为。
Magnetoelectric (ME) properties under rotating magnetic field H are comparatively investigated in two representative hexaferrites Y-type Ba0.5Sr1.5Zn2(Fe0.92Al0.08)12O22 and Z-type Ba0.52Sr2.48Co2Fe24O41, both of which have exhibited a similar transverse conical spin structure and giant ME coupling near room temperature. When the external H is rotated clockwise by 2pi, in-plane P vector is rotated clockwise by 2pi in the Y-type hexaferrite and counterclockwise by 4pi in the Z-type hexaferrite. A symmetry-based analysis reveals that the faster and opposite rotation of P vector in the Z-type hex…
