We show that strong electromagnon peaks can be found in absorption spectra of non-collinear magnets exhibiting a linear magnetoelectric effect. The frequencies of these peaks coincide with the frequencies of antiferromagnetic resonances and the ratio of the spectral weights of the electromagnon and antiferromagnetic resonance is related to the ratio of the static magnetoelectric constant and magnetic susceptibility. Using a Kagome lattice antiferromagnet as an example, we show that frustration of spin ordering gives rise to magnetoelastic instabilities at strong spin-lattice coupling, which transform a non-collinear magnetoelectric spin state into a collinear multiferroic state with a spontaneous electric polarization and magnetization. The Kagome lattice antiferromagnet also shows a ferroelectric incommensurate-spiral phase, where polarization is induced by the exchange striction mechanism.
The optical spectra in the family of multiferroic manganites RMnO3 is a great puzzle. Current models can not explain the fact that two strong electromagnons are present in the non-collinear spin cycloidal phase, with only one electromagnon surviving the transition into the collinear spin sinusoidal phase. We show that this is a signature of the presence of anomalous magnetoelectric coupling that breaks rotational invariance in spin space and generates oscillatory polarization in the ground state.
Terahertz magnetic resonance driven by the light electric field, now referred to as electromagnon, has stimulated interest due to its strong candidate for future spin-electronics. One unique characteristic of electromagnons is the terahertz magnetochromism, a color change by external magnetic field, as recently demonstrated in the hexaferrite. By taking perovskite manganites and hexaferrite as model cases, the current understating of the electromagnon activity is discussed in terms of the symmetric exchange mechanism.
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。
In the first part of this work we calculate the high frequency magnetoelectric susceptibility of a simultaneously ferroelectric and canted antiferromagnetic (also know as weak ferromagnetic) thin film with magnetostrictive magnetoelectric coupling. We show that a dynamic coupling exists between the ferroelectric and optic antiferromagnetic excitations. In the second part of the paper, we calculate using an effective medium method the susceptibility of a heterostructure comprising alternating thin films of such a material together with a ferromagnet. Dipolar magnetic fields serve to couple the …
The magnetoelectric coupling, i.e., cross-correlation between electric and magnetic orders, is a very desirable property to combine functionalities of materials for next-generation switchable devices. Multiferroics with spin-driven ferroelectricity presents such a mutual interaction concomitant with magneto- and electro-active excitations called electromagnons. TbMnO3 is a paradigmatic material in which two electromagnons have been observed in the cycloidal magnetic phase. However, their observation in TbMnO3 is restricted to the cycloidal spin phase and magnetic ground states that can support…
Magnetoelectric coupling in multiferroic materials opens new routes to control the propagation of light. The new effects arise due to dynamic magnetoelectric susceptibility that cross-couples the electric and magnetic fields of light and modifies the solutions of Maxwell equations in media. In this paper two major effects will be considered in detail: optical activity and asymmetric propagation. In case of optical activity the polarization plane of the input radiation rotates by an angle proportional to the magnetoelectric susceptibility. The asymmetric propagation is a counter-intuitive pheno…
Electromagnon excitations in modulated multiferroics
螺旋磁体铁电性的唯象理论[M. Mostovich，Phys. Rev. Lett. 96，067601（2006）]被概括为描述具有均匀铁电极化和空间调制铁电极化的一致状态。本说明书中的关键点是磁电耦合的对称部分，因为尽管与均匀分量无关，但它对极化的非均匀部分起着重要作用。我们在调制磁系统的一般例子中说明了这一重要性：纵向和横向自旋密度波状态和平面摆线相位。我们发现，即使在没有均匀铁电性的情况下，极化关联函数遵循系统的软磁行为，由于磁电效应。我们的结果可以很容易地推广到更复杂类型的磁有序，并且应用可能涉及凝聚态物理中的各种自然和人工系统（例如，可以从动态介电响应测量中提取磁振子特性）。
High resolution inelastic neutron scattering reveals that the elementary magnetic excitations in multiferroic MnWO4 consist of low energy dispersive electromagnons in addition to the well-known spin-wave excitations. The latter can well be modeled by a Heisenberg Hamiltonian with magnetic exchange coupling extending to the 12th nearest neighbor. They exhibit a spin-wave gap of 0.61(1) meV. Two electromagnon branches appear at lower energies of 0.07(1) meV and 0.45(1) meV at the zone center. They reflect the dynamic magnetoelectric coupling and persist in both, the collinear magnetic and parael…
Electromagnons in multiferroic RMn2O5 compounds and their microscopic origin
本文总结了多铁性RMn_2 O_5化合物中电磁振子的现有实验数据，其中R表示稀土离子Y或Bi，并讨论了一个现实的微观模型，这些材料的假设基础上，磁-感生铁电性和电磁振子吸收完全依赖于各向同性海森堡交换和自旋与极性晶格模式的磁致伸缩耦合，相对论效应该模型解释了许多磁性和光学性质的RMn 2 O 5锰氧化物，如自旋重取向过渡，磁致极化，外观的电磁振子峰在非共线自旋状态和光的偏振，该峰被观察到。我们比较了RMn 2 O 5和RMnO 3化合物的电磁振子的实验和理论结果。
Here we present a detailed study of the THz and FIR response of the mixed perovskite manganite system Eu$_{1-x}$Ho$_x$MnO$_3$ for holmium concentrations x = 0.1 and 0.3. We compare the magnetic excitations of the four different magnetically ordered phases (A-type antiferromagnetic, sinusoidally modulated collinear, helical phases with spin planes perpendicular to the crystallographic a and c axes). The transition between the two latter phases goes hand in hand with a switching of the ferroelectric polarization from P||a to P||c. Special emphasis is paid to the temperature dependence of the exc…
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…
Recent spectroscopic studies at terahertz frequencies for a variety of multiferroics endowed with both ferroelectric and magnetic orders have revealed the possible emergence of a new collective excitation, frequently referred to as electromagnon. It is magnetic origin, but becomes active in response to the electric field component of light. Here we give an overview on our recent advance in the terahertz time-domain spectroscopy of electromagnons or electric-dipole active magnetic resonances, focused on perovskite manganites--$R$MnO$_3$ ($R$ denotes rare-earth ions). The respective electric and…
Inelastic x-ray scattering with meV energy resolution (IXS) is an ideal tool to measure collective excitations in solids and liquids. In non-resonant scattering condition, the cross section is strongly dominated by lattice vibrations (phonons). However, it is possible to probe additional degrees of freedom such as magnetic fluctuations that are strongly coupled to the phonons. The IXS spectrum of the coupled system will contain not only the phonon dispersion (majority component) but also the so far undetected magnetic correlation function (minority component). Here we report the discovery of s…
Spin excitations of magnetoelectric LiNiPO$_4$ are studied by infrared absorption spectroscopy in the THz spectral range as a function of magnetic field through various commensurate and incommensurate magnetically ordered phases up to 33\,T. Six spin resonances and a strong two-magnon continuum are observed in zero magnetic field. Our systematic polarization study reveals that some of the excitations are usual magnetic-dipole active magnon modes, while others are either electromagnons, electric-dipole active, or magnetoelectric, both electric- and magnetic-dipole active spin excitations. Field…
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 investigated the spin excitations of magnetoelectric $\text{LiFePO}_4$ by THz absorption spectroscopy in magnetic fields up to 33$\,$T. By studying their selection rules, we found not only magnetic-dipole, but also electric-dipole active (electromagnons) and magnetoelectric resonances. The magnetic field dependence of four strong low-energy modes is reproduced well by our four-sublattice spin model for fields applied along the three orthorhombic axes. From the fit, we refined the exchange couplings, single-ion anisotropies, and the Dzyaloshinskii-Moriya interaction parameters. Additional sp…
Electric susceptibility of antiferromagnetic multiferroics with cycloidal spin order at magnetoelectric effect associated with collinear component of spins
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…
Violation of time reversal and spatial inversion symmetries has profound consequences for elementary particles and cosmology. Spontaneous breaking of these symmetries at phase transitions gives rise to unconventional physical phenomena in condensed matter systems, such as ferroelectricity induced by magnetic spirals, electromagnons, non-reciprocal propagation of light and spin waves, and the linear magnetoelectric (ME) effect - the electric polarization proportional to the applied magnetic field and the magnetization induced by the electric field. Here, we report the experimental study of the …
Regimes of optical transparency and instabilities of collinear dielectric ferromagnetic materials in the presence of the dynamic magnetoelectric effect
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…
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…
Spin-dependent electric dipole operators are investigated group-theoretically for the emergence of an electric dipole induced by a single spin or by two spins, where the spin dependences are completely classified up to the quadratic order. For a single spin, a product of spin operators behaves as an even-parity electric quadrupole operator, which differs from an odd-parity electric dipole. The lack of the inversion symmetry allows the even- and odd-parity mixing, which leads to the electric dipole described by the electric quadruple operators. Point-group tables are given for classification of…
We present a formalism to calculate the macroscopic magnetoelectric monopolization from first principles within the density functional theory framework. An expression for the monopolization in the case of insulating collinear magnetism is derived first in terms of spin-polarized Wannier functions then recast as a Berry phase. We propose an extension to the general, non-collinear case which we implement computationally in the Wannier function form and use to calculate the magnetoelectric monopolizations of LiMnPO$_4$ and Cr$_2$O$_3$. We find that, while the former is well approximated by a summ…
Collinear-to-Spiral Spin Transformation without Changing Modulation Wavelength upon Ferroelectric Transition in Tb1-xDyxMnO3
用X射线衍射和中子衍射研究了铁电居里温度T_C附近的磁电化合物Tb_（1-x）Dy_xMnO_3的晶格调制和磁结构。对于0.50< x < 0.68的化合物，通过T_C观察到温度无关的调制矢量。这表明，铁电性与极化（P）沿c轴的RMnO 3系列不能归因于这样的ininadenate相称的过渡的反铁磁秩序，如先前预期的。X=0.59的单晶的中子衍射研究表明，FE转变伴随着Mn自旋取向从正弦（共线）反铁磁性到横向螺旋结构的转变。在T_C以下观察到的螺旋结构预期沿c轴产生P，并具有“逆”Dzialoshinski-Moriya相互作用，这与观察一致。
We have investigated magnetoelectric coupling in the single-molecule magnet $\mathrm{Mn}_{4}\mathrm{Te}_{4}(\mathrm{P}\mathrm{Et}_{3})_{4}$ with tetrahedral spin frustration. Our density functional studies found that an electric dipole moment can emerge with various non-collinear spin orderings. The forms of spin-dependent dipole are determined and consistent with that in non-centrosymmetric magnets driven by the Dzyaloshinskii-Moriya interaction. Writing a parameterized spin Hamiltonian, after solving for eigenvalues and eigenstates we quantified the magnetoelectric coupling by calculating th…
We present results from ab initio calculations based on density functional theory for bismuth-based double perovskite Bi2MnTiO6. Using total energy calculation with stress and force minimization we have predicted the equilibrium crystal structure for Bi2MnTiO6 considering potential structures into the calculation. We have predicted that the ground state of Bi2MnTiO6 will be a noncentrosymmetric rhombohedral structure with space group R3c. Our spin polarized calculation for different possible collinear magnetic configurations we found that Bi2MnTiO6 will be an insulator with G-type antiferromag…
Magnetoelectric materials are attractive for several applications, including actuators, switches, and magnetic field sensors. Typical mechanisms for achieving a strong magnetoelectric coupling are rooted in transition metal magnetism. In sharp contrast, here we identify CsEr(MoO4)2 as a magnetoelectric material without magnetic transition metal ions, thus ensuring that the Er ions play a key role in achieving this interesting property. Our detailed study includes measurements of the structural, magnetic, and electric properties of this material. Bulk characterization and neutron powder diffrac…
Magnetoelectric (ME) materials are of fundamental interest and have been investigated for their broad potential for technological applications. The search for, and eventually the theoretical design of, materials with large ME couplings present challenging issues. First-principles methods have only recently been developed to calculate the full ME response tensor $α$ including both electronic and ionic (i.e., lattice-mediated) contributions. The latter is proportional to both the Born dynamical electric charge $Z^{\rm e}$ and its analogue, the dynamical magnetic charge $Z^{\rm m}$. Here we prese…
We show that using epitaxial strain and chemical pressure in orthorhombic YMnO3 and Co-substituted (YMn0.95Co0.05O3) thin films, a ferromagnetic response can be gradually introduced and tuned. These results, together with the measured anisotropy of the magnetic response, indicate that the unexpected observation of ferromagnetism in orthorhombic o-RMnO3 (R= Y, Ho, Tb, etc) films originates from strain-driven breaking of the fully compensated magnetic ordering by pushing magnetic moments away from the antiferromagnetic [010] axis. We show that the resulting canting angle and the subsequent ferro…
Towards next-generation spintronics devices, such as computer memories and logic chips, it is necessary to satisfy high thermal stability, low-power consumption and high spin-polarization simultaneously. Here, from first-principles, we investigate thermal stability (both structure and magnetization) and the electric field control of magnetic anisotropy on Co2FeAl (CFA)/MgO. A phase diagram of structural thermal stability of the CFA/MgO interface is illustrated. An interfacial perpendicular-anisotropy, coming from the Fe-O orbital hybridization, provides high magnetic thermal stability and a lo…
The layered perovskite YBaCuFeO5 (YBCFO) is considered one of the best candidates to high-temperature chiral multiferroics with strong magnetoelectric coupling. In RBaCuFeO5 perovskites (R: rare-earth or Y) A-site cations are fully ordered whereas their magnetic properties strongly depend on the preparation process. They exhibit partial cationic disorder at the B-site that generates a magnetic spiral stabilized through directionally assisted long range coupling between canted locally frustrated spins. Moreover the orientation of its magnetic spiral can be critical for the magnetoelectric respo…
In this study we show from first principles calculations the possibility to induce multiferroic and magnetoelectric functional properties in the Pnma NaMnF3 fluoroperovskite by means of epitaxial strain engineering. Surprisingly, we found a very strong non-linear polarization-strain coupling that drives an atypical amplification of the ferroelectric polarization for either compression or expansion of the cell. This property is associated with a non-collinear antiferromagnetic ordering, which induces a weak ferromagnetism and thus, making the strained NaMnF3 fluoroperovskite multiferroic. We al…
While ferromagnets are at the heart of daily life applications, their large magnetization and resulting energy cost for switching bring into question their suitability for reliable low-power spintronic devices. Non-collinear antiferromagnetic systems do not suffer from this problem and often possess remarkable extra functionalities: non-collinear spin order may break space-inversion symmetry and thus allow electric-field control of magnetism, or produce emergent spin-orbit effects, which enable efficient spin-charge interconversion. To harness these unique traits for next-generation spintronic…
Colossal Type-II Multiferroic Polarization Driven by Collinear Spin Orders
Nanjing University of Science and Technology（南京理工大学）；School of Physics, Nanjing University（南京大学物理学院）；Collaborative Innovation Center of Advanced Microstructures, Nanjing University（南京大学先进微结构协同创新中心）；Hefei National Laboratory（合肥国家实验室）；Jiangsu Physical Science Research Center, Nanjing University（南京大学江苏省物理科学研究中心）
The coupling between magnetic order and ferroelectricity has been under intense investigation in a wide range of transition-metal oxides. The strongest coupling is obtained in so-called magnetically-induced multiferroics where ferroelectricity arises directly from magnetic order that breaks inversion symmetry. However, it has been difficult to find non-oxide based materials in which these effects occur. Here we present a study of copper dimethyl sulfoxide dichloride (CDC), an organo-metallic quantum magnet containing $S = 1/2$ Cu spins, in which electric polarization arises from non-collinear …
Antiferromagnetic thin films are currently generating considerable excitement for low dissipation magnonics and spintronics. However, while tuneable antiferromagnetic textures form the backbone of functional devices, they are virtually unknown at the submicron scale. Here we image a wide variety of antiferromagnetic spin textures in multiferroic BiFeO3 thin films that can be tuned by strain and manipulated by electric fields through room temperature magnetoelectric coupling. Using piezoresponse force microscopy and scanning NV magnetometry in self-organized ferroelectric patterns of BiFeO3, we…
Antisymmetric Dzyaloshinskii-Moriya (DM) interactions generating from the spin-orbit coupling induce various fascinating properties, like magnetoelectric (ME) effect, weak ferromagnetism and non-trivial topological spin textures like skyrmions, in real materials. Compared to their symmetric isotropic exchange counterpart, these interactions are generally of a weaker order of strength, creating modest twisting in the spin structure which results in weak ferromagntism or weak linear ME effect. Our proposed two-sublattice model, in contrast, predicts a hitherto unobserved, charge ordered non-coll…
