Multiferroic Compound Research Articles

Structural and magnetic characterization of Bi_{0.9}Ca_{0.1}Fe_{0.98}X_{0.02}O_3 (X = Mg, Al, Ti, V) polycrystalline multiferroic compounds

The multiferroic compounds Bi_{0.9}Ca_{0.1}Fe_{0.98}X_{0.02}O_3, X = Mg, Al, Ti, V, were prepared by the solid state reaction method and were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, vibrating sample magnetometry, and differential scanning calorimetry. The compounds were found to have the rhombohedral perovskite-like structure, accompanied by the existence of small amounts of the usual impurity phases Bi_2Fe_4O_{9} and Bi_{25}FeO_{40}. The appearance of weak ferromagnetism is due to partial destruction of the spiral spin structure due to lattice distortion and suppression of oxygen vacancies. The magnetic transition temperatures of the compounds are 342, 346, 349, and 345 °C, respectively.

The Structural and magnetic properties of Bi1-xAxFe1-yByO3multiferroic system

We have prepared Bi1-xAxFe1-yByO3, where A=Ca or Gd and B=Ni or Zn. The solid state reaction technique was used to prepare the samples. The effects of the mentioned elemental substitution on the structural and magnetic properties of the multiferroic compound prepared are reported.

Facile Mechanochemical Synthesis and Magnetic Properties of pervoskite YCrxFe1-xO3, (0≤x≤1).

ABSTRACTSingle-phase samples of YCrxFe1−xO3were synthesized by a mechanochemical method. X-ray diffraction data show linear reduction in the lattice parameters of YCrxFe1−xO3perovskites with the Cr content, indicating that Cr ions substitute for Fe ions to form a solid solution. Magnetic measurements show hysteresis loops at 5K. The substitution of Cr for Fe enhances the magnetization for up to x=0.33 Cr doping level. For higher doping levels, 0.33<x<1, the system behaves as a frustrated system. At x=1, YCrO3behaves as a week ferromagnet with TN~140 K. The chloride salt based machenochemical method offers simple synthesis route for the synthesis of pure multiferroic compounds.

Evolution of spin-modulated magnetic structure in multiferroic compound Bi(1−x)SrxFeO3

BiFeO3 is a perovskite-like compound with magnetic and electrical long-range order with antiferromagnetic transition temperature about 670 K. It has space modulated magnetic structure of cycloidal type with the period λ=620±20Å incommensurate with the lattice parameter leading to specific NMR line shape. Substitution of strontium for bismuth atoms destroys the space modulated structure. Several samples with substitution of strontium for bismuth Bi1−xSrxFeO3 were studied by means of zero field NMR and Mössbauer spectroscopy. A significant broadening of 57Fe NMR spectrum was observed at x=0.10 while Mössbauer spectroscopy demonstrates this effect at x=0.07. This phenomena evidence for the destruction of the spatial modulated magnetic structure at x∼0.07–0.10 by the heterovalent substitution of strontium for bismuth without the destruction of crystal rhombohedral structure.

Zero- and finite-temperature mean field study of magnetic field induced electric polarization in Ba2CoGe2O7: Effect of the antiferroelectric coupling

We investigate the spin induced polarization in the multiferroic compound Ba${}_{2}$CoGe${}_{2}$O${}_{7}$ using variational and finite-temperature mean field approaches, with the aim to reproduce the peculiar behavior of the induced polarization in a magnetic field observed experimentally in Murakawa et al. [Phys. Rev. Lett. 105, 137202 (2010)]. The compound is usually described by a spin-3/2 Heisenberg model extended with easy-plane anisotropy and Dzyaloshinskii-Moriya (DM) interaction. By applying a magnetic field parallel to the $[110]$ axis, three phases can be distinguished in this model: (i) At high magnetic field, we find a partially magnetized phase with spins parallel to the fields and uniform polarization. (ii) Below a critical field, the ground state is a twofold-degenerate canted antiferromagnet, where the degeneracy can be lifted by a finite DM interaction. (iii) At zero field, a U$(1)$ symmetry-breaking phase takes place, exhibiting a Goldstone mode. We find that extending the Hamiltonian with an antiferroelectric term results in the appearance of a canted ferrimagnetic phase for $h\ensuremath{\lesssim}1$ T. This phase is characterized by a finite staggered polarization, as well as by a magnetization closing a finite angle with the applied field leading to torque anomalies.

Raman scattering study of delafossite magnetoelectric multiferroic compounds: CuFeO2 and CuCrO2

Ultrasonic velocity measurements on the magnetoelectric multiferroic compound CuFeO2 reveal that the antiferromagnetic transition observed at TN1 = 14 K might be induced by an pseudoproper ferroelastic transition []. In that case, the group theory states that the order parameter associated with the structural transition must belong to a two-dimensional irreducible representation Eg (x2 − y2, xy). Since this type of transition can be driven by a Raman Eg mode, we performed Raman scattering measurements on CuFeO2 between 5 and 290 K. Considering that the isostructural multiferroic compound CuCrO2 might show similar structural deformations at the antiferromagnetic transition TN1 = 24.3 K, Raman measurements have also been performed for comparison. At ambient temperature, the Raman modes in CuFeO2 are observed at ωEg = 352 cm−1 and ωA1g = 692 cm−1, while these modes are detected at ωEg = 457 cm−1 and ωA1g = 709 cm−1 in CuCrO2. The analysis of the temperature dependence of the modes in both compounds shows that the frequencies of all modes increase with decreasing temperature. This typical behavior is attributed to anharmonic phonon–phonon interactions. These results clearly indicate that none of the Raman active modes observed in CuFeO2 and CuCrO2 drive the pseudoproper ferroelastic transitions observed at the Néel temperature TN1. Finally, a broad band at about 550 cm−1 observed in the magnetoelectric phase of CuCrO2 below TN2 could be associated with magnons.

Symmetry and magnetic field driven transitions in the 2D triangular lattice compound RbFe(MoO4)2

The temperature versus magnetic field phase diagram of the 2D triangular lattice and multiferroic compound RbFe(MoO4)2 is analysed from the point of view of symmetry. The paramagnetic space group and its irreducible representations are used in order to obtain the magnetic symmetry of the possible modulated phases and characterize the restrictions imposed by this symmetry on the corresponding magnetic structures. Superspace symmetry is considered in the case of incommensurate phases. It is shown that the experimentally observed phases correspond to different isotropy subgroups originating in the same irreducible representation of the paramagnetic symmetry group. The relevant couplings between the primary transverse spin modulation and the electric polarization, the in-plane magnetization and the secondary longitudinal magnetic modulation are discussed. The mechanisms for the destabilization of the improper ferroelectric chiral phase and the origin of the different orientation of the spins with respect to the external field in the two collinear phases are analysed from a symmetry based perspective.

Temperature-dependent multi-k magnetic structure in multiferroic Co 3TeO 6

A complex magnetic order of the multiferroic compound Co 3TeO 6 has been revealed by neutron powder diffraction studies on ceramics and crushed single crystals. The compound adopts a monoclinic structure (s.g. C2/ c) in the studied temperature range 2–300 K but exhibits successive antiferromagnetic transitions at low temperature. Incommensurate antiferromagnetic order with the propagation vector k 1 = (0, 0.485, 0.055) sets in at 26 K. A transition to a second antiferromagnetic structure with k 2 = (0, 0, 0) takes place at 21.1 K. Moreover, a transition to a commensurate antiferromagnetic structure with k 3 = (0, 0.5, 0.25) occurs at 17.4 K. The magnetic structures have been determined by neutron powder diffraction using group theory analysis as a preliminary tool. Different coordinations of the Co 2+ ions involved in the low-symmetry C2/ c structure of Co 3TeO 6 render the exchange-interaction network very complex by itself. The observed magnetic phase transformations are interpreted as an evidence of competing magnetic interactions. The temperature dependent changes in the magnetic structure, derived from refinements of high-resolution neutron data, are discussed and possible mechanisms connected with the spin reorientations are described.

Femtoscale Magnetically Induced Lattice Distortions in Multiferroic TbMnO 3

Magneto-electric multiferroics exemplified by TbMnO(3) 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 used an electric field to produce a single magnetic domain state, and a magnetic field to induce ionic displacements. Under these conditions, interference between charge and magnetic x-ray scattering arose, encoding the amplitude and phase of the displacements. When combined with a theoretical analysis, our data allow us to resolve the ionic displacements at the femtoscale, and show that such displacements make a substantial contribution to the zero-field ferroelectric moment.

Concurrent transition of ferroelectric and magnetic ordering near room temperature

Strong spin-lattice coupling in condensed matter gives rise to intriguing physical phenomena such as colossal magnetoresistance and giant magnetoelectric effects. The phenomenological hallmark of such a strong spin-lattice coupling is the manifestation of a large anomaly in the crystal structure at the magnetic transition temperature. Here we report that the magnetic Néel temperature of the multiferroic compound BiFeO(3) is suppressed to around room temperature by heteroepitaxial misfit strain. Remarkably, the ferroelectric state undergoes a first-order transition to another ferroelectric state simultaneously with the magnetic transition temperature. Our findings provide a unique example of a concurrent magnetic and ferroelectric transition at the same temperature among proper ferroelectrics, taking a step toward room temperature magnetoelectric applications.

Theory of High-Temperature Multiferroicity in Cupric Oxide

The incommensurate-commensurate phases reported in cupric oxide below 230 K are shown theoretically to realize an inverted sequence of symmetry-breaking mechanisms with respect to the usual sequence occurring in low-temperature multiferroic compounds. The sequence inversion results from a strong triggering-coupling mechanism between two antiferromagnetic order parameters inducing a first-order transition to the multiferroic phase. Such mechanism is favored by the large antiferromagnetic superexchange interactions, responsible of the high-T(N) temperature, and implies a preeminence of these interactions on the magnetocrystalline anisotropy. The magnetic structures of the equilibrium phases and the microscopic interactions giving rise to the polarization are determined.

Giant magnetoelectric effect in HoAl3(BO3)4

A giant magnetoelectric polarization is found in HoAl$_3$(BO$_3$)$_4$. The polarization in transverse field geometry at 70 kOe reaches 3600 $\mu C/m^2$ which is significantly higher than reported values of linear magnetoelectric or even multiferroic compounds. The magnetostrictive effect is also measured and compared with the magnetoelectricity. The results show that spin-lattice coupling in HoAl$_3$(BO$_3$)$_4$ is extremely strong and that the magnetic field causes a large polar distortion of the ionic positions in the unit cell.

FERROELECTRIC SWITCHING PATH IN MONODOMAIN RHOMBOHEDRAL BiFeO3 CRYSTAL: A FIRST-PRINCIPLES STUDY

Based on density-functional calculations, we have studied possible ferroelectric switching path in monodomain single crystal of rhombohedral BiFeO3 , a prototypical multiferroic compound. By carefully studying the behaviors of FeO6 corner-sharing double-tetrahedrons, we find abrupt changes in total energy and oxygen atomic positions, and therefore polarizations, occur in the ferroelectric switching path of rhombohedral BiFeO3 . Detailed analyses suggest that such behavior might be caused by the frustrated magnetic ordering in the paraelectric phase of rhombohedral BiFeO3 , where three O atoms and the Bi atom are in the same plane perpendicular to the polarization direction. This is supported by the fact that the ferroelectric switching for paramagnetic BiFeO3 is smooth and has a much lower energy barrier than that of antiferromagnetic BiFeO3 .

Aliovalent-Ion and Magnetic Field Induced Phase Transition in Multiferroic BiFe<SUB>1−<I>x</I></SUB>Ti<SUB><I>x</I></SUB>O<SUB>3</SUB> System

Multiferroic compounds with general formula BiFe(1-x)Ti(x)O3 (x = 0.1, 0.2, 0.3 and 0.35) have been synthesized by conventional solid state reaction method. The effect of Ti substitution on ferroelectric and magnetic properties is studied. From X-ray diffraction (XRD) analysis, a rhombohedral to orthorhombic phase transition for x > 0.3 was observed. From SQUID measurements, a magnetic field induced phase transition has been observed in the BiFe(1-x)T(x)O3 system for x = 0.3. An anomaly in dielectric constant and dielectric loss in the vicinity of antiferromagnetic Néel temperature (T(N)) and a small enhancement in magnetization have been observed. Magnetization measurements above room temperature showed no systematic variation in antiferromagnetic Néel temperatures on Ti substitution. Further it is seen that this system shows the coupling between electric and magnetic dipoles exhibiting magnetoelectric (ME) effect at room temperature and possess high dielectric constant.

Relaxation dynamics of multiferroic domain walls in DyMnO3with cycloidal spin order

Dynamic responses of various types of spin-cycloidal domain walls (DWs) have been investigated for the multiferroic compound DyMnO${}_{3}$ by the dielectric measurements. By exploiting the multiferroic nature of this material, we were able to drive the spin-cycloidal DWs by an electric field and hence elucidate the dynamics in terms of the frequency-dependent dielectric susceptibility. In the 10${}^{3}$--10${}^{9}$-Hz range, all types of DWs exhibit relaxation motion. In some DWs, the relaxation motion presents critical behavior near the first-order flop transition of the ferroelectric spin-cycloidal plane; that is, the dielectric constant is enhanced, and the dynamics slow down. The theory of DW relaxation ascribes this behavior to the growing Larkin length. This observation of the Larkin length along with generic considerations of DW widths and flop transitions implies that the critical behavior of DW relaxation is universal in materials that exhibit a flop transition, for example, of spin or polarization. Multiferroic compounds can thus provide an ideal laboratory for the study of magnetic DW dynamics.

Effect of La doping in the multiferroic compound BiFeO 3

We have performed magnetization and magnetocapacitance measurements on ceramic samples of the multiferroic series Bi 1− x La x FeO 3 for 0≤ x≤.0.25. We show that doping with La reduces the transition magnetic field from the spatially modulated state to a homogenous one and increases the magnetocapacitance, these effects being the strongest for x=0.15, which is the highest concentration for maintaining the non-centrosymmetric rhomboedral structure (R3c) of BiFeO 3. For highest La content ( x≥0.17), analysis of the XRD patterns shows that the lattice symmetry gradually changes to orthorhombic (C222), giving rise to an enhancement of the latent magnetization and to a drop of the magnetodielectric constant.

Robust ferroelectric state in multiferroicMn1−xZnxWO4

We report the remarkably robust ferroelectric state in the multiferroic compound Mn$_{1-x}$Zn$_x$WO$_4$. The substitution of the magnetic Mn$^{2+}$ with nonmagnetic Zn$^{2+}$ reduces the magnetic exchange and provides control of the various magnetic and multiferroic states of MnWO$_4$. Only 5 % of Zn substitution results in a complete suppression of the frustrated collinear (paraelectric) low temperature phase. The helical magnetic and ferroelectric phase develops as the ground state. The multiferroic state is stable up to a high level of substitution of more than 50 %. The magnetic, thermodynamic, and dielectric properties as well as the ferroelectric polarization of single crystals of Mn$_{1-x}$Zn$_x$WO$_4$ are studied for different substitutions up to x=0.5. The magnetic phases have been identified in single crystal neutron scattering experiments. The ferroelectric polarization scales with the neutron intensity of the incommensurate peak of the helical phase.

Multiferroic Compounds with Double-Perovskite Structures.

New multiferroic compounds with double-perovskite structures were synthesized. Bi2NiMnO6 was synthesized in bulk form by high-pressure synthesis and also in a thin-film form by epitaxial growth. The material showed both ferromagnetic and ferroelectric properties, i.e., the multiferroic property at low temperature. Bi2FeCrO6 was also fabricated in a (1 1 1) oriented BiFeO3/BiCrO3 artificial superlattice, with a 1/1 stacking period. The superlattice film showed ferromagnetic behavior and polarization switching at room temperature. In the compounds, Bi3+ ion, located at the A site in the perovskite structure, caused ferroelectric structural distortion, and the B-site ordering of the Ni2+ and Mn4+ ions (Fe3+ and Cr3+ ions) in a rock-salt configuration led to ferromagnetism according to the Kanamori-Goodenough rule.

A simple model for the magnetoelectric interaction in multiferroics

The (anti)ferromagnetic and ferroelectric transitions in some multiferroic compounds seem to be strongly correlated. Even for systems that do not show spontaneous ferroelectricity such as the LiMPO4 (M = Mn, Fe, Co, Ni) compounds, the coupling between magnetic and electric degrees of freedom is evident experimentally. Here, we present a simple numerical calculation to simulate this coupling that leads to the two transitions. We assume a magnetic sublattice consisting of classical magnetic moments coupled to a separated nonmagnetic sublattice consisting of classical electric dipoles. The coupling between them is realized through a phenomenological spin-lattice Hamiltonian, and the solution is obtained using the Monte Carlo technique. In the simplest version, the magnetic system is 2D Ising (anti)ferromagnetic lattice, with nearest neighbors interactions only, and the electric moments are permanent moments, coupled electrically. Within this approximation, the second order magnetic transition induces ferroelectricity in the electric dipoles. We show that these calculations can be extended to other magnetic systems, (x-y model and 3D Heisenberg) and to systems where the electric moments are created by strains, generated via spin-lattice coupling, so the model can be applied to model realistic systems such as the olivines mentioned above.

Magnetic and dielectric properties of multiferroic Tb0.5Eu0.5MnO3

The structural, magnetic and dielectric properties of multiferroic compound Tb0.5Eu0.5MnO3 are reported near the boundary of the multiferroic phase of the Eu-doped Tb1−xEuxMnO3 system. The Mn3+ moments order in an incommensurate antiferromagnetic manner below TN = 50 K, followed by a spin canting-like response around 25 K. Below TN, extra hysteresis loops were observed for applied fields Ba around 1-2 T, with decreasing magnetic / K moment for increasing magnetic fields. The dielectric constant ϵ' showed a peak at 28, which was attributed to the ferroelectric transition in analogy with the parent compound TbMnO3. Structural analysis indicates the Mn-O2-Mn bond angle within the ab-plane is 145.9° suggesting that there exists a critical angle in the perovskite manganate multiferroic systems.