A series of manganites Tb1-xHoxMnO3 (0<=x<=0.6) with orthorhombic structure are synthesized and detailed investigations on their multiferroicity are performed. Successive magnetic transitions upon temperature variation are evidenced for all samples, and both the Mn3+ spiral spin ordering and rare-earth spin ordering are suppressed with increasing x. Significant enhancement of both the polarization and magnetoelectric response within 0.2<x<0.4 is observed, which may be ascribed to the competition possibly existing between spiral and E-type spin orders. Theoretical calculation is given based on two eg-orbital double-exchange model, and the result supports the scenario of the multiferroic phase separation.
Terahertz spectroscopy of electromagnons in Eu_{1-x}Y_xMnO_3
钇掺杂EuMnO $_3$的介电常数谱，在0 =< x =< 0.5 have been investigated in the terahertz frequency range. Magnetoelectric contributions to the permittivity were observed in all compositions for ac electric fields parallel to the crystallographic a-axis. Well defined electromagnons exist for x >= 0.2接近ν~ 20 cm^{-1}的组成范围内，并且随着掺杂，介电强度强烈增加。除了电磁振子，广泛的贡献磁电起源观察到的所有组合物。对于Eu_{0.8} Y_{0.2} MnO_3，由于外加磁场的作用，产生了一个倾斜的反铁磁相，从而抑制了电磁振子。在不同的掺杂制度的磁电效应进行了详细讨论。
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…
A theoretical description of the sequence of magnetic phases in Co3TeO6 is presented. The strongly first-order character of the transition to the commensurate multiferroic ground state, induced by coupled order parameters corresponding to different wavevectors, is related to a large magnetoelastic effect with an exchange energy critically sensitive to the interatomic spacing. The monoclinic magnetic symmetry C2' of the multiferroic phase permits spontaneous polarization and magnetization as well as the linear magnetoelectric effect. The existence of weakly ferromagnetic domains is verified exp…
Multiferroics with the coexistence of ferroelectric and ferromagnetic orders are ideal candidates for magnetoelectric applications. Unfortunately, only very few ferroelectric-ferromagnetic multiferroics (with low magnetic critical temperature) were discovered. Here we perform first principles calculations to investigate the effects of the epitaxial strain on the properties of BiMnO3 films grown along the pseudocubic [001] direction. Unlike the ground state with the centrosymmetric C2/c space group in bulk, we reveal that the tensile epitaxial strain stabilizes the ferromagnetic and ferroelectr…
In this work, we report the growth and functional characterizations of multiferroic TbMnO3 thin films grown on Nb-doped SrTiO3 (001) substrates using pulsed laser deposition. By performing detailed magnetic and ferroelectric properties measurements, we demonstrate that the multiferroicity of spin origin known in the bulk crystals can be successfully transferred to TbMnO3 thin films. Meanwhile, anomalous magnetic transition and unusual magnetoelectric coupling related to Tb moments are observed, suggesting a modified magnetic configuration of Tb in the films as compared to the bulk counterpart.…
By means of symmetry analysis, density functional theory calculations, and Monte Carlo simulations we show that goethite, $α$-FeOOH, is a linear magnetoelectric below its Néel temperature $T_{\rm N}=400$ K. The experimentally observed magnetic field induced spin-flop phase transition results in either change of direction of electric polarization or its suppression. Calculated value of magnetoelectric coefficient is 0.24 $μ$C m$^{-2}$ T$^{-1}$. The abundance of goethite in nature makes it arguably the most widespread magnetoelectric material.
Phase transitions in multiferroic BiFeO3 crystals, thin-layers, and ceramics: Enduring potential for a single phase, room-temperature magnetoelectric 'holy grail'
The quantitative understanding of converse magnetoelectric effects, i.e., the variation of the magnetization as a function of an applied electric field, in extrinsic multiferroic hybrids is a key prerequisite for the development of future spintronic devices. We present a detailed study of the strain-mediated converse magnetoelectric effect in ferrimagnetic Fe3O4 thin films on ferroelectric BaTiO3 substrates at room temperature. The experimental results are in excellent agreement with numerical simulation based on a two-region model. This demonstrates that the electric field induced changes of …
We have identified three of the four magnetoelectric modes of multiferroic BiFeO$_3$ measured using THz spectroscopy. Excellent agreement with the observed peaks is obtained by including the effects of easy-axis anisotropy along the direction of the electric polarization. By distorting the cycloidal spin state, anisotropy splits the $Ψ_{\pm 1}$ mode into peaks at 20 and 21.5 cm$^{-1}$ and activates the lower $Φ_{\pm 2}$ mode at 27 cm$^{-1}$ (T=200 K). An electromagnon is identified with the upper $Ψ_{\pm 1}$ mode at 21.5 cm$^{-1}$. Our results also explain recent Raman and inelastic neutron-sc…
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）
We present a phenomenological scheme to study the size-dependent electric voltage-controlled magnetic anisotropy in ferromagnetic (FM)/ferroelectric (FE) heterostructures. The FM layers are either metallic Fe(001), Ni(001), Co(0001), or half-metallic (La, Sr)MnO3 films. Two magnetoelectric mechanisms, i.e., interface-charge and strain-mediated couplings, are considered. We show that the interface-charge mediated coupling is the main mechanism for the magnetoelectic coupling when the FM film thickness is below a certain transition thickness dtr while the strain-mediated coupling dominates above…
Magnetic skyrmions, vortex-like swirling spin textures characterized by a quantized topological invariant, realized in chiral-lattice magnets are currently attracting intense research interest. In particular, their dynamics under external fields is an issue of vital importance both for fundamental science and for technical application. Whereas observations of magnetic skyrmions had been limited to metallic magnets so far, their realization was discovered also in a chiral-lattice insulating magnet Cu2OSeO3 in 2012. Skyrmions in the insulator turned out to exhibit multiferroic nature with spin-i…
Multiferroics are those materials with more than one ferroic order, and magnetoelectricity refers to the mutual coupling between magnetism and electricity. The discipline of multiferroicity has never been so highly active as that in the first decade of the twenty-first century, and it has become one of the hottest disciplines of condensed matter physics and materials science. A series of milestones and steady progress in the past decade have enabled our understanding of multiferroic physics substantially comprehensive and profound, which is further pushing forward the research frontier of this…
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 …
Pb(Fe$_{0.5}$Nb$_{0.5}$)O$_3$ (PFN), one of the few relaxor multiferroic systems, has a $G$-type antiferromagnetic transition at $T_N$ = 143 K and a ferroelectric transition at $T_C$ = 385 K. By using high-resolution neutron-diffraction experiments and a total scattering technique, we paint a comprehensive picture of the long- and short-range structures of PFN: (i) a clear sign of short-range structural correlation above $T_C$, (ii) no sign of the negative thermal expansion behavior reported in a previous study, and (iii) clearest evidence thus far of magnetoelectric coupling below $T_N$. We c…
Multiferroics with coexistent ferroelectric and magnetic orders can provide an interesting laboratory to test unprecedented magnetoelectric responses and their possible applications. One such example is the dynamical and/or resonant coupling between magnetic and electric dipoles in a solid. As the examples of such dynamical magnetoelectric effects, (1) the multiferroic domain wall dynamics and (2) the electric-dipole active magnetic responses are discussed with the overview of recent experimental observations.
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…
Various phenomena related to inhomogeneous magnetoelectric interaction are considered. The interrelation between spatial modulation of order parameter and electric polarization, known as flexoelectric effect in liquid crystals, in the case of magnetic media appears in a form of electric polarization induced by spin modulation and vice versa. This flexomagnetoelectric interaction is also related to the influence of ferroelectric domain structure on antiferromagnetic vector distribution, and to the magnetoelectric properties of micromagnetic structures. The influence of inhomogeneous magnetoelec…
Hexagonal RMnO3 is a multiferroic compound with a giant spin-lattice coupling at an antiferromagnetic transition temperature [1]. Despite extensive studies over the past two decades, however, the origin and underlying microscopic mechanism of the strong spin-lattice coupling remain still very much elusive. In this study, we have tried to address this problem by measuring the thermal expansion and dielectric constant of doped single crystals Y1-xLuxMnO3 with x = 0, 0.25, 0.5, 0.75, and 1.0. From these measurements, we confirm that there is a progressive change in the physical properties with do…
By an electrical-pump--optical-probe technique we show that the electric-field-induced reversal of the magnetic order parameter in multiferroic MnWO$_4$ occurs on the time scale of milliseconds and maintains a rigid coupling of the magnetization to the magnetically induced electric polarization. The temporal progression of the spatially resolved domain structure was imaged with nanosecond resolution by optical second harmonic generation and compared to the quasi-static domain reversal. A qualitative model gives an estimate of why the magnetoelectric order-parameter reversal in the magnetically…
We theoretically discover that unique eigenmodes of skyrmion crystal (SkX) are not only magnetically active to ac magnetic field (H^w) but also electrically active to ac electric field (E^w) in a multiferroic chiral-lattice magnet Cu2OSeO3, which amplifies the dynamical magnetoelectric coupling between E^w and the spin texture. The resulting intense interference between their electric and magnetic activation processes can lead to unprecedentedly large diode effect on the microwave, i.e., its absorption by SkX changes up to ~20% when the incident direction is reversed. Our results demonstrate t…
Some magnetoelectric multiferroics have a canted spin structure that can be described by a Dzyaloshinkii-Moriya coupling. We calculate properties and features expected for surface and bulk magnon polaritons in such media with a linear magnetoelectric interaction for the case of transverse electric polarisation. The dielectric polarisation and magnetisation of weak ferromagnetism are constrained to lie in the plane parallel to the surface. We examine a geometry with the polarisation oriented in the film plane and present numerical results for the transverse electric polarisation. Particular att…
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 …
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…
The question of how ferroelectric polarization is coupled to magnetism in magnetoelectric multiferroics, in which both types of order are simultaneously present, is of considerable scientific and practical interest. A recent Letter \cite{fennie} presents an analysis of the important ``ABO$_3$'' class of perovskite multiferroics. This Letter argues that antiferromagnetic multiferroics with magnetic ions on the B site, such as the well-studied room-temperature multiferroic bismuth ferrite (A=Bi, B=Fe), cannot show linear magnetoelectric coupling of the form ${\bf P} \cdot ({\bf L} \times {\bf M}…
A phenomenological explanation of the magnetoelectric behavior of Co_3TeO_6 is developed. We explain the second harmonic generation data and the magnetic field induced spontaneous polarization in the magnetically ordered phase below 20K.
For disordered Heisenberg systems with small single ion anisotropy, two spin glass transitions below the long range ordered phase transition temperature has been predicted theoretically for compositions close to the percolation threshold. Experimental verification of these predictions is still controversial for conventional spin glasses. We show that multiferroic spin glass systems can provide a unique platform for verifying these theoretical predictions via a study of change in magnetoelastic and magnetoelectric couplings, obtained from an analysis of diffraction data, at the spin glass trans…
We have discovered strong magnetoelectric (ME) effects in the single chiral-helical magnetic state of single-crystalline langasite Ba3NbFe3Si2O14 that is crystallographically chiral. The ferroelectric polarization, predominantly aligned along the a axis below the Neel temperature of ~27 K, changes in a highly non-linear fashion in the presence of in-plane magnetic fields (H) perpendicular to the a axis (H//b*). This ME effect as well as smaller ME effects in other directions exhibit no poling dependence, suggesting the presence of a self-formed single ME domain. In addition, these ME effects a…
The giant magnetoelectric coupling is a very rare phenomenon which has gained a lot of attention for the past few decades because of fundamental interest as well as practical applications. Here, we have successfully achieved the giant magnetoelectric coupling in PbTi1-xVxO3 (x= 0-1) with the help of a series of generalized-gradient-corrected (GGA), GGA including on-site coulomb repulsion (U) corrected spin polarized calculations based on accurate density functional theory. Our total energy calculations show that PbTi1-xVxO3 stabilizes in C-type antiferromagnetic ground state for x>0.123. With …
All-electrical control of a dynamic magnetoelectric effect is demonstrated in a classical multiferroic manganite DyMnO3, a material containing coupled antiferromagnetic and ferroelectric orders. Due to intrinsic magnetoelectric coupling with electromagnons a linearly polarized terahertz light rotates upon passing through the sample. The amplitude and the direction of the polarization rotation are defined by the orientation of ferroelectric domains and can be switched by static voltage. These experiments allow the terahertz polarization to be tuned using the dynamic magnetoelectric effect.
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 …
We developed numerical calculations to simulate the magnetoelectric coupling in multiferroic compounds, using the Monte Carlo technique. Two simple models were used to simulate the compounds. In the first one, the magnetic ions are represented by a spin 1/2 2D Ising lattice of ions, and the electric lattice by classical moments, coupled one to one with the magnetic moments. The coupling between both lattices allows to the leading lattice, that is, the magnetic one, to change the orientation of the electrical dipoles in one direction perpendicular to the magnetic dipoles. This direction was cho…
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 obse…
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…
We report the results of our investigations on the influence of partial substitution of Er and Gd for Dy on the magnetic and magnetoelectric properties of DyFe0.5Cr0.5O3, which is known to be a multiferroic system. Magnetic susceptibility and heat capacity data, apart from confirming the occurrence of magnetic transitions at ~ 121 and 13 K in DyFe0.5Cr0.5O3, bring out that the lower transition temperature only is suppressed by rare-earth substitution. Multiferroic behavior is found to persist in Dy0.4Ln0.6Fe0.5Cr0.5O3 (Ln= Er and Gd). There is an evidence for magnetoelectric coupling in all th…
By using broadband dielectric spectroscopy in the radiofrequency and microwave range we studied the magnetoelectric dynamics in the multiferroic chiral antiferromagnet MnWO$_4$. Above the multiferroic phase transition at $T_{N2} \approx 12.6$ K we observe a critical slowing down of the corresponding magnetoelectric fluctuations resembling the soft-mode behavior in canonical ferroelectrics. This electric field driven excitation carries much less spectral weight than ordinary phonon modes. Also the critical slowing down of this mode scales with an exponent larger than one which is expected for m…
We present spin transport studies on a low-field, room-temperature magnetoelectric multiferroic polycrystalline $\text{Sr}_{3}\text{Co}_{2}\text{Fe}_{24}\text{O}_{41}$ (SCFO)|Pt heterostructure wherein a highly tunable transverse conical magnetic phase is responsible for static and dynamic magnetoelectric coupling. We measured angular dependence of spin Hall magnetoresistance (SMR) at constant magnetic fields ($H$) in the range of 50 to 100 kOe. Application of field below the critical value (2.5 kOe), yielded negative SMR and the $H$-evolution of normalized SMR exhibited a negative gradient. F…
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…
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…
Magnetoelectric coupling in helical multiferroics allows to steer spin order with electric fields. Here we show theoretically that in a helical multiferroic chain quantum information processing as well as quantum phases are highly sensitive to electric (E) field. Applying E-field, the quantum state transfer fidelity can be increased and made directionally dependent. We also show that E field transforms the spin-density-wave/nematic or multipolar phases of frustrated ferromagnetic spin-1/2 chain in chiral phase with a strong magnetoelectric coupling. We find sharp reorganization of the entangle…
The magnetic field induced rearrangement of the cycloidal spin structure in ferroelectric mono-domain single crystals of the room-temperature multiferroic BiFeO$_3$ is studied using small-angle neutron scattering (SANS). The cycloid propagation vectors are observed to rotate when magnetic fields applied perpendicular to the rhombohedral (polar) axis exceed a pinning threshold value of $\sim$5\,T. In light of these experimental results, a phenomenological model is proposed that captures the rearrangement of the cycloidal domains, and we revisit the microscopic origin of the magnetoelectric effe…
We have established detailed magnetoelectric phase diagrams of (Eu$_{0.595}$Y$_{0.405}$)$_{1-x}$Tb$_x$MnO$_3$ ($0 \le x \le 1$) and (Eu,Y)$_{1-x}$Gd$_x$MnO$_3$ ($0 \le x \le 0.69$), whose average ionic radii of $R$-site ($R$: rare earth) cations are equal to that of Tb$^{3+}$, in order to reveal the effect of rare earth 4$f$ magnetic moments on the magnetoelectric properties. In spite of the same $R$-site ionic radii, the magnetoelectric properties of the two systems are remarkably different from each other. A small amount of Tb substitution on $R$ sites ($x \sim 0.2$) totally destroys ferroel…
