Lattice and spin excitations in multiferroic h-YMnO3
C. Toulouse, L. Chaix, J. Liu, Y. Gallais, M-A. Measson, A. Sacuto, L. Pinsard-Godart, F. Willaert, J. B. Brubach, P. Roy, S. Petit, V. Simonet, S. de Brion, M. Cazayous
We used Raman and terahertz spectroscopies to investigate lattice and magnetic excitations and their cross-coupling in the hexagonal YMnO3 multiferroic. Two phonon modes are strongly affected by the magnetic order. Magnon excitations have been identified thanks to comparison with neutron measurements and spin wave calculations but no electromagnon has been observed. In addition, we evidenced two additional Raman active peaks. We have compared this observation with the anti-crossing between magnon and acoustic phonon branches measured by neutron. These optical measurements underly the unusual strong spin-phonon coupling.
We propose that concurrently magnetic and ferroelectric, i.e. multiferroic, compounds endowed with electrically-active magnetic excitations (electromagnons) provide a key to produce large directional dichroism for long wavelengths of light. By exploiting the control of ferroelectric polarization and magnetization in a multiferroic oxide Ba$_2$CoGe$_2$O$_7$, we demonstrate the realization of such a directional light-switch function at terahertz frequecies in resonance with the electromagnon absorption. Our results imply that this hidden potential is present in a broad variety of multiferroics.
We studied magnetic excitations in a low-temperature ferroelectric phase of the multiferroic YMn2O5 using inelastic neutron scattering (INS). We identify low-energy magnon modes and establish a correspondence between the magnon peaks observed by INS and electromagnon peaks observed in optical absorption [1]. Furthermore, we explain the microscopic mechanism, which results in the lowest-energy electromagnon peak, by comparing the inelastic neutron spectral weight with the polarization in the commensurate ferroelectric phase.
Low energy excitations in the multiferroic material TbMnO3 have been investigated by Raman spectroscopy. Our observations reveal the existence of two peaks at 30 cm-1 and 60 cm-1. They are observed in the cycloidal phase below the Curie temperature but not in the sinusoidal phase, suggesting their magnetoelectric origin. While the peak energies coincide with the frequencies of electromagnons measured previously by transmission spectroscopy, they show surprisingly different selection rules, with the 30 cm-1 excitation enhanced by the electric field of light along the spontaneous polarization. T…
By using polarized inelastic neutron scattering measurements, we show that the spin-lattice quantum entanglement in mutliferroics results in hybrid elementary excitations, involving spin and lattice degrees of freedom. These excitations can be considered as multiferroic Godstone modes. We argue that the Dzyaloshinskii-Moriya interaction could be at the origin of this hybridization.
Raman scattering measurements on BiFeO3 single crystals show an important coupling between the magnetic order and lattice vibrations. The temperature evolution of phonons shows that the lowest energy E and A1 phonon modes are coupled to the spin order up to the Neel temperature. Furthermore, low temperature anomalies associated with the spin re-orientation are observed simultaneously in both the E phonon and the magnon. These results suggest that magnetostriction plays an important role in BiFeO3.
Anisotropic multiferroic properties of SrMnGe2O6 pyroxene single crystals were systematically investigated by means of magnetization, heat capacity, pyroelectric current measurement and elastic and inelastic neutron scattering experiments. Single crystal neutron diffraction allows us to unambiguously reveal the presence of two incommensurate magnetic orderings: a non-polar amplitude-modulated collinear sinusoidal magnetic structure emerges at TN1=4.36(2)K followed by a polar elliptical cycloidal spin structure below TN2=4.05(2)K. Pyroelectric current measurements on single crystal confirm the …
Chirality, i.e., the right- and left-handedness of structure, is one of the key concepts in many fields of science including biology, chemistry and physics, and its manipulation is an issue of vital importance. The electron spins in solids can form chiral configurations. In perovskite manganites RMnO3 (R=Tb, Dy,...etc), the Mn-spins form a cycloidal structure, which induces ferroelectric polarization (P) through the relativistic spin-orbit interaction. This magnetism-induced ferroelectricity (multiferroics) and associated infrared-active spin waves (electromagnons) open a promising route to co…
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…
Evidence for a strong spin-lattice coupling in multiferroic HoMnO_3 is derived from thermal expansion measurements along a- and c-axis. The magnetoelastic effect results in sizable anomalies of the thermal expansivities at the antiferromagnetic (T_N) and the spin rotation (T_{SR}) transition temperatures as well as in a negative c-axis expansivity below room temperature. The coupling between magnetic orders and dielectric properties below T_N is explained by the lattice strain induced by the magnetoelastic effect. At T_{SR} various physical quantities show discontinuities that are thermodynami…
The idea of embedding and transmitting information within the fluctuations of the magnetic moments of spins (spin waves) has been recently proposed and experimentally tested. The coherence of spin waves, which describes how well defined these excitations are, is of course vital to this process, and themost significant factor that affects the spin-wave coherence is temperature. Here we present neutron inelastic scattering measurements of the full threedimensional spin-wave dispersion in BiFeO3, which is one of the most promising functional multiferroic material, for temperatures from 5K to 700K…
The magnetic excitations in multiferroic TbMnO3 have been studied by inelastic neutron scattering in the spiral and sinusoidally ordered phases. At the incommensurate magnetic zone center of the spiral phase, we find three low-lying magnons whose character has been fully determined using neutron-polarization analysis. The excitation at the lowest energy is the sliding mode of the spiral, and two modes at 1.1 and 2.5meV correspond to rotations of the spiral rotation plane. These latter modes are expected to couple to the electric polarization. The 2.5meV-mode is in perfect agreement with recent…
Magnetic excitations in cycloidal magnet LiCu$_2$O$_2$ are explored using THz absorption spectroscopy in magnetic fields up to 12 T. Below the spin ordering temperature we observe eight optically active transitions in the spin system of LiCu$_2$O$_2$ in the energy range from 4 to 30 cm$^{-1}$. In magnetic field the number of modes increases and the electric polarization flop is seen as a change in magnetic field dependence of mode energies. The polarization dependence of two of the modes in zero magnetic field fits the selection rules for the cycloid tilted by $θ=41\pm1^{\circ}$ from the bc pl…
Multiferroic rare earth manganites attracted recent attention because of the coexistence of different types of magnetic and ferroelectric orders resulting in complex phase diagrams and a wealth of physical phenomena. The coupling and mutual interference of the different orders and the large magnetoelectric effect observed in several compounds are of fundamental interest and bear the potential for future applications in which the dielectric (magnetic) properties can be modified by the onset of a magnetic (dielectric) transition or the application of a magnetic (electric) field. The physical mec…
Magnetodielectric materials are characterized by a strong coupling of magnetic and dielectric properties and in rare cases simultaneously exhibit both, magnetic and polar order. Among other multiferroics, TbMnO3 and GdMnO3 reveal a strong magneto-dielectric (ME) coupling and as a consequence fundamentally new spin excitations exist: Electro-active magnons, or electromagnons, i. e. spin waves which can be excited by ac electric fields. Here we show that these excitations appear in the phase with an incommensurate (IC) magnetic structure of the manganese spins. In external magnetic fields this I…
The phenomenological theory of ferroelectricity in spiral magnets presented in [M. Mostovoy, Phys. Rev. Lett. 96, 067601 (2006)] is generalized to describe consistently states with both uniform and modulated-in-space ferroelectric polarizations. A key point in this description is the symmetric part of the magnetoelectric coupling since, although being irrelevant for the uniform component, it plays an essential role for the non-uniform part of the polarization. We illustrate this importance in generic examples of modulated magnetic systems: longitudinal and transverse spin-density wave states a…
Electromagnon is the spin wave in multiferroic materials and is known to accompany electric polarization due to the cross correlation between the charge and spin. Here, we theoretically show that the electromagnons also induce dc current upon their photoexcitations. The proposed dc current response originates from the shift current mechanism which is characterized by the so called shift vector, a geometric quantity of the Bloch wavefunctions.
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…
We report the magnetic and electric properties of Ba$_3$NiNb$_2$O$_9$, which is a quasi-two-dimensional spin-1 triangular lattice antiferromagnet (TLAF) with trigonal structure. At low $T$ and with increasing magnetic field, the system evolves from a 120 degree magnetic ordering phase (A phase) to an up-up-down ($uud$) phase (B phase) with a change of slope at 1/3 of the saturation magnetization, and then to an "oblique" phase (C phase). Accordingly, the ferroelectricity switches on at each phase boundary with appearance of spontaneous polarization. Therefore, Ba$_3$NiNb$_2$O$_9$ is a unique T…
We present data on the magnetic and magneto-elastic coupling in the hexagonal multiferroic manganite LuMnO3 from inelastic neutron scattering, magnetization and thermal expansion measurements. We measured the magnon dispersion along the main symmetry directions and used this data to determine the principal exchange parameters from a spin-wave model. An analysis of the magnetic anisotropy in terms of the crystal field acting on the Mn is presented. We compare the results for LuMnO3 with data on other hexagonal RMnO3 compounds.
Based on temperature dependent far infrared transmission spectra of YMn2O5 and TbMn2O5 single crystals, we report the observation of electric dipole-active magnetic excitations, or electromagnons, in these multiferroics. Electromagnons are found to be directly responsible for the step-like anomaly of the static dielectric constant at the commensurate--incommensurate magnetic transition and are the origin of the colossal magneto-dielectric effect reported in these multiferroics.
We propose a model for the pair of electromagnon excitations observed in the class of multiferroic materials {\it R}MnO$_3$ ({\it R} is a rare-earth ion). The model is based on a harmonic cycloid ground state interacting with a zone-edge magnon and its twin excitation separated in momentum space by two times the cycloid wave vector. The pair of electromagnons is activated by cross coupling between magnetostriction and spin-orbit interactions. Remarkably, the spectral weight of the twin electromagnon is directly related to the presence of a magnetically induced oscillatory polarization in the g…
Both the Dzyaloshiskii-Moriya interaction and the exchange-striction are shown to affect dynamically the magnetoelectric excitations in the perovskite multiferroic RMnO3. The exchange-striction results in a biquadratic interaction between the spins and the transverse phonons, giving rise to quantum fluctuations of the ferroelectric polarization P. This leads to low-lying phonon modes that are perpendicular to P and to the helical spins at small wave vector but are parallel to P at a wave vector close to the magnetic modulation vector. For spin-1/2 helimagnet, the local polarization can be comp…
We studied the novel multiferroic material Sr$_2$FeSi$_2$O$_7$, and found 3 absorption modes above the magnetic ordering transition temperature using time-domain terahertz spectroscopy. These absorption modes can be explained as the optical transitions between the spin-orbit coupling and crystal field split 3d$^6$ Fe$^{2+}$ ground state term in this material. Consideration of the compressed tetrahedral environment of the Fe$^{2+}$ site is crucial to understand the excitations. We point out, however, discrepancies between the single-site atomic picture and the experimental results.
We have performed Raman measurements on TbMnO3 single crystal under magnetic field along the three crystallographic directions. The flip of the spin spiral plane creates an electromagnon excitation. In addition to the electromagnons induced by the Heisenberg coupling, we have detected the electromagnon created by the Dzyaloshinskii-Moriya interaction along the c axis. We have identified all the vibrational modes of TbMnO3. Their temperature dependences show that only one phonon observed along the polarization axis is sensitive to the ferroelectric transition. This mode is tied to the Tb3+ ion …
We report the magnetic diffraction pattern and spin wave excitations in (CD$_3$)$_2$ND$_2$[Mn(DCO$_2$)$_3$] measured using elastic and inelastic neutron scattering. The magnetic structure is shown to be a G-type antiferromagnet with moments pointing along the $b$ axis. By comparison with simulations based on linear spin wave theory, we have developed a model for the magnetic interactions in this multiferroic metal-organic framework material. The interactions form a three-dimensional network with antiferromagnetic nearest-neighbour interactions along three directions of $J_1=-0.103(8)$~meV, $J_…
We investigate magnetoelectric coupling and low-energy magnetic excitations in multiferroic $α$-Cu$_2$V$_2$O$_7$ by detailed thermal expansion, magnetostriction, specific heat and magnetization measurements in magnetic fields up to 15~T and by high-field/high-frequency electron spin resonance studies. Our data show negative thermal expansion in the temperature range $\leq 200$~K under study. Well-developed anomalies associated with the onset of multiferroic order (canted antiferromagnetism with a significant magnetic moment and ferroelectricity) imply pronounced coupling to the structure. We d…
Competing interactions and geometric frustration provide favourable conditions for exotic states of matter. Such competition often causes multiple phase transitions as a function of temperature and can lead to magnetic structures that break inversion symmetry, thereby inducing ferroelectricity [1-4]. Although this phenomenon is understood phenomenologically [3-4], it is of great interest to have a conceptually simpler system in which ferroelectricity appears coincident with a single magnetic phase transition. Here we report the first such direct transition from a paramagnetic and paraelectric …
Magnetoelectric phase diagrams of the perovskite manganites, Eu1-xYxMnO3 and Gd1-xTbxMnO3, are theoretically studied. We first construct a microscopic model, and then analyze the model using the Monte-Carlo method. We reproduce the diagrams, which contain two different multiferroic states, i.e., the ab-plane spin cycloid with electric polarization P//a and the bc-plane spin cycloid with P//c. We reveal that their competition originates from a conflict between the single-ion anisotropy and the Dzyaloshinsky-Moriya interaction, which is controlled by the second-neighbor spin exchanges enhanced b…
We discuss a model scenario for multiferroic systems of type II (collinear spins) where the electric dipolar order competes with a frustrated magnetic order in determining the elastic distortions of the lattice ion positions. High magnetic frustration due to second neighbors exchange and small spin easy-axis anisotropy lead to the appearance of the so called quantum magnetic plateau states. Increasing the magnetic field above the plateau border produces composite excitations, where fractionalized spin tertions arise together with spontaneous dipolar flips (in the form of domain walls) and enha…
We studied spin excitations of the multiferroic Ba2CoGe2O7 in high magnetic fields up to 33 T. In the electron spin resonance and far infrared absorption spectra we found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. We show that a multi-boson spin-wave theory can capture these unconventional modes, that include spin-stretching modes associated with an oscillating magnetic dipole (or only quadrupole) moment. The lack of the inversion symmetry allows these modes to become electric dipole active. We expect that the spin-stre…
We provide a detailed study of the reflectivity of multiferroic TbMnO_3 for wave numbers from 40 cm^{-1} to 1000 cm^{-1} and temperatures 5 K < T < 300 K. Excitations are studied for polarization directions E || a, the polarization where electromagnons are observed, and for E || c, the direction of the spontaneous polarization in this material. The temperature dependencies of eigenfrequencies, damping constants and polar strengths of all modes are studied and analyzed. For E || a and below the spiral ordering temperature of about 27 K we observe a transfer of optical weight from phonon excitat…
We show that electronic correlations decimate the intrinsic ferroelectric polarization of the recently discovered class of multiferroic manganites RMn$_2$O$_5$, where R is a rare earth element. Such is manifest from {\it ab initio} bandstructure computations that account for the strong local Coulomb interactions between the manganese 3d electrons --the root of magnetism in these materials. When including these the computed electronic, magnetic and lattice structure of multiferroic HoMn$_2$O$_5$ results in an amplitude and direction of polarization that is in accordance with experiment. The mic…
We have investigated magnetoelastic effects in multiferroic YMnO$_3$ below the antiferromagnetic phase transition, $T_N \approx 70$ K, using neutron powder diffraction. The $a$ lattice parameter of the hexagonal unit cell of YMnO$_3$ decreases normally above $T_N$, but decreases anomalously below T$_N$, whereas the $c$ lattice parameter increases with decreasing temperature and then increases anomalously below T$_N$. The unit cell volume also undergoes an anomalous contraction below $T_N$. By fitting the background thermal expansion for a non-magnetic lattice with the Einstein-Grüneisen equati…
We used Raman scattering to study the lattice and magnetic excitations in the hexagonal HoMnO3 single crystals. The E2 phonon mode at 237 cm-1 is affected by the magnetic order. This mode is related to the displacement of Mn and O ions in a-b plane and modulates the Mn-O-Mn bond angles in a-b plane and the in-plane Mn-Mn superexchange interaction. The mode at 269 cm-1 associated to the displacement of the apical Ho3+ ions along the c direction presents an abrupt change of slope at TN showing that the role of the rare earth ions can not be neglected in the magnetic transition. We have identifie…
We report inelastic neutron scattering measurements that provide a distinct dynamical "fingerprint" for the multiferroic ground state of 3.5% Ga-doped CuFeO_2. The complex ground state is stabilized by the displacement of the oxygen atoms, which are also responsible for the multiferroic coupling predicted by Arima. By comparing the observed and calculated spectrum of spin excitations, we conclude that the magnetic ground state is a distorted screw-type spin configuration that requires a mechanism for magnetoelectric coupling different from the generally accepted spin-current model.
Lattice and spin excitations have been studied by Raman scattering in hexagonal YbMnO3 single crystals. The temperature dependences of the phonon modes show that the E2 mode at 256 cm-1 related to the displacement of Mn and O ions in a-b plane is coupled to the spin order. The A1 phonon mode at 678 cm-1 presents a soft mode behavior at the Neel temperature. Connected to the motion of the apical oxygen ions along the c direction, this mode controls directly the Mn-Mn interactions between adjacent Mn planes and the superexchange path. Crystal field and magnon mode excitations have been identifie…
We apply ultrafast X-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO$_3$ after above-bandgap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO$_3$: the relevant excited charge carriers must rem…
The spin-wave excitations of the multiferroic $\rm MnWO_4$ have been measured in its low-temperature collinear commensurate phase using high-resolution inelastic neutron scattering. These excitations can be well described by a Heisenberg model with competing long-range exchange interactions and a single-ion anisotropy term. The magnetic interactions are strongly frustrated within the zigzag spin chain along c axis and between chains along the a axis, while the coupling between spin along the b axis is much weaker. The balance of these interactions results in the noncollinear incommensurate spi…
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…
Magneto-electric multiferroics exemplified by TbMnO3 possess both magnetic and ferroelectric long-range order. The magnetic order is mostly understood, whereas the nature of the ferroelectricity has remained more elusive. Competing models proposed to explain the ferroelectricity are associated respectively with charge transfer and ionic displacements. Exploiting the magneto-electric coupling, we use an electric field to produce a single magnetic domain state, and a magnetic field to induce ionic displacements. Under these conditions, interference charge-magnetic X-ray scattering arises, encodi…
The magnetoelastic coupling in multiferroic $DyMn_2O_5$ is investigated by magnetostriction measurements along the three crystallographic orientations. Strong lattice anomalies as a function of the magnetic field are detected at the low temperature magnetic and ferroelectric phase transitions. The sign and magnitude of the magnetostrictive coefficient as well as the lattice anomalies at the transitions are correlated with the Dy moment order and with the sharp changes of the dielectric constant and the ferroelectric polarization. With the magnetic field applied along the c-axis a new high-fiel…
The synthesis and structural studies on the polycrystalline La1-xBixCrO3 (x =0, 0.2, 0.25, 0.3, 0.35) have been done using XRD and TEM. The results of the Reitveld refinement of XRD data shows that these compounds crystallize in an orthorhombic structure with Pnma space group. With increasing Bi content, the unit cell volume and lattice parameters undergo non monotonous changes within the tolerance factor from 0.84092 to 0.84072. This has been attributed to the anisotropy induced by lone pair of Bi3+, which may distorted the lattice.
We report a comprehensive inelastic neutron scattering study of the hybrid molecule-based multiferroic compound (ND4)2FeCl5D2O in the zero-field incommensurate cycloidal phase and the high-field quasi-collinear phase. The spontaneous electric polarization changes its direction concurrently with the field-induced magnetic transition, from mostly aligned with the crystallographic a-axis to the c-axis. To account for such change of polarization direction, the underlying multiferroic mechanism was proposed to switch from the spin-current model induced via the inverse Dzyalloshinskii-Moriya interac…
We report on the low-frequency optical excitations in the multiferroic ground state of polycrystalline FeCr2S4 in the frequency range 0.3-3~THz and their changes upon applying external magnetic fields up to 7~T. In the ground state below the orbital-ordering temperature T_OO=9K we observe the appearance of several new modes. By applying the external magnetic field parallel and perpendicular to the propagation direction of the THz radiation, we can identify the strongest absorptions to be of predominantly electric-dipole origin. We discuss these modes as the low-energy electronic excitations of…
The field-induced multiferroic transition in TbMnO3 has been studied by neutron scattering. Apart strong hysteresis, the magnetic transition associated with the flop of electronic polarization exhibits a memory effect: after a field sweep, TbMnO3 does not exhibit the same phase as that obtained by zero-field cooling. The strong changes in the magnetic excitations across the transition perfectly agree with a rotation of the cycloidal spiral plane indicating that the inverse Dzyaloshinski-Moriya coupling causes the giant magnetoelectric effect at the field-induced transition. The analysis of the…
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…
Broadband microwave spectroscopy has been performed on single-crystalline GaV$_4$S$_8$, which exhibits a complex magnetic phase diagram including cycloidal, Néel-type skyrmion lattice, as well as field-polarized ferromagnetic phases below 13 K. At zero and small magnetic fields two collective modes are found at 5 and 15 GHz, which are characteristic of the cycloidal state in this easy-axis magnet. In finite fields, entering the skyrmion lattice phase, the spectrum transforms into a multi-mode pattern with absorption peaks near 4, 8, and 15 GHz. The spin excitation spectra in GaV$_4$S$_8$ and t…
The coexisting magnetic and ferroelectric orders in multiferroic materials give rise to a handful of novel magnetoelectric phenomena, such as the absorption difference for the opposite propagation directions of light called the non-reciprocal directional dichroism (NDD). Usually these effects are restricted to low temperature, where the multiferroic phase develops. In this paper we report the observation of NDD in the paramagnetic phase of Sr2CoSi2O7 up to temperatures more than ten times higher than its Néel temperature (7 K) and in fields up to 30 T. The magnetically induced polarization and…
We have demonstrated that ferroelectric polarization in a spin-driven multiferroic CuFe1-xGaxO2 with x = 0.035 can be controlled by the application of uniaxial pressure. Our neutron diffraction and in-situ ferroelectric polarization measurements have revealed that the pressure dependence of the ferroelectric polarization is explained by repopulation of three types of magnetic domains originating from the trigonal symmetry of the crystal. We conclude that the spin-driven anisotropic lattice distortion and the fixed relationship between the directions of the magnetic modulation wave vector and t…
We report on THz time-domain spectroscopy on multiferroic GeV4S8, which undergoes orbital ordering at a Jahn-Teller transition at 30.5 K and exhibits antiferromagnetic order below 14.6 K. The THz experiments are complemented by dielectric experiments at audio and radio frequencies. We identify a low-lying excitation close to 15 cm-1, which is only weakly temperature dependent and probably corresponds to a molecular excitation within the electronic level scheme of the V4 clusters. In addition, we detect complex temperature-dependent behavior of a low-lying phononic excitation, closely linked to…
We report on the different roles of two orbital-active Fe$^{2+}$ at the A site and V$^{3+}$ at the B site in the magnetic excitations and on the anomalous spin wave broadening in FeV$_{2}$O$_{4}$. FeV$_{2}$O$_{4}$ exhibits three structural transitions and successive paramagnetic (PM)-collinear ferrimagnetic (CFI)-noncollinear ferrimagnetic (NCFI) transitions. The high-temperature tetragonal/PM -orthorhombic/CFI transition is accompanied by the appearance of an energy gap with a high magnitude in the magnetic excitations due to strong spin-orbit coupling induced anisotropy at the Fe$^{2+}$ site…
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…
An effective model is developed to explain the phase diagram and the mechanism of magnetoelectric coupling in multiferroics, $RMn_2O_5$. We show that the nature of magnetoelectric coupling in $RMn_2O_5$ is a coupling between two Ising-type orders, namely, the ferroelectric order in the b axis, and the coupled magnetic order between two frustrated antiferromagnetic chains. The frustrated magnetic structure drives the system to a commensurate-incommensurate phase transition, which can be understood as a competition between a collinear or col-plane order stemming from the `order by disorder' mech…
Interplay of Spin Waves, Crystal-Field Excitations, and Phonons in Multiferroic Ba3HoRu2O9 revealed by Inelastic Neutron Scattering, Crystal-Field Analysis, and Machine-Learned Phonon Calculations
Rajiv Gandhi Institute of Petroleum Technology（Rajiv Gandhi石油技术学院）；ISIS Neutron and Muon Source, STFC, Rutherford Appleton Laboratory（ISIS中子与缪子源，STFC，卢瑟福阿普顿实验室）；School of Physical Sciences, Indian Institute of Technology Goa（印度理工学院果阿分校物理科学学院）；UGC - DAE Consortium for Scientific Research Mumbai Centre, Bhabha Atomic Research Centre（UGC-DAE科学研究中心孟买中心，巴哈原子研究中心）；Highly Correlated Matter Research Group, Physics Department, University of Johannesburg（约翰内斯堡大学高度关联物质研究组物理系）
Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review article, we outline some of the progressive milestones in this st…
Using inelastic neutron scattering technique, we measured the spin wave dispersion over the entire Brillouin zone of room temperature multiferroic BiFeO3 single crystals with magnetic excitations extending to as high as 72.5 meV. The full spin waves can be explained by a simple Heisenberg Hamiltonian with a nearest neighbor exchange interaction (J=4.38 meV), a next nearest neighbor exchange interaction (J'=0.15 meV), and a Dzyaloshinskii-Moriya-like term (D=0.107 meV). This simple Hamiltonian determined, for the first time, for BiFeO3 provides a fundamental ingredient for understanding of the …
Single-crystal samples of LiCu2O2 with spin 1/2 Cu2+ chains of edge-sharing CuO4 square planes (ribbon chains), have been characterized by X-ray diffraction, thermogravimetric analysis, and magnetic measurements. Neither the atomic deficiency nor the mixing of Cu and Li atoms has been found, indicating that lattice defects conceived as a possible origin of the reported multiferroic behavior can be excluded. Anomalies found in the data of specific heat and neutron magnetic Bragg reflections show clear evidence that the system exhibits successive magnetic transitions at TN1=24.5 K and TN2=22.8 K…
