Multiferroic Compound Research Articles

Structural phase relations in perovskite-structured BiFeO3-based multiferroic compounds

In this review, the state of the art in understanding the structural phase relations in perovskite-structured BiFeO3-based polycrystalline solid solutions is presented and discussed. Issues about the close relation between the structural phase and overall physical properties of the reviewed systems are pointed out and discussed. It is shown that, by adjusting the structural symmetric arrangement, the ferroelectric and magnetic properties of BiFeO3-based polycrystalline solid solutions can be tuned to find specific multifunctional applications. However, an intrinsic mechanism linking structural arrangement and physical properties cannot be identified, revealing that this subject still deserves further discussion and investigation.

Magnetic and electric properties of CaMn7O12 based multiferroic compounds: effect of electron doping

The mixed valent multiferroic compound CaMn7O12 is studied for its magnetic and electric properties. The compound undergoes magnetic ordering below 90 K with a helimagnetic structure followed by a low temperature magnetic anomaly observed around 43 K. This study shows that the magnetic anomaly at 43 K is associated with thermal hysteresis indicating the first order nature of the transition. The compound also shows field-cooled magnetic memory and relaxation below 43 K, although no zero-field-cooled memory is present. A clear magnetic hysteresis loop is present in the magnetization versus field measurements, signifying the presence of some ferromagnetic clusters in the system. We doped trivalent La at the site of divalent Ca expecting to enhance the fraction of Mn3+ ions. The La doped samples show reduced magnetization, although the temperatures associated with the magnetic anomalies remain almost unaltered. Interestingly, the spontaneous electrical polarization below 90 K increases drastically on La substitution. We propose that the ground states of the pure as well as the La doped compositions contain isolated superparamagnetic like clusters, which can give rise to metastability in the form of field-cooled memory and relaxation. The ground state is certainly not spin glass type, as is evident from the absence of zero-field-cooled memory and frequency shift in the ac susceptibility measurements.

Competing lattice fluctuations and magnetic excitations in CuO

Lattice vibrations as well as magnetic excitations are investigated using Raman scattering to understand the high-${T}_{c}$ multiferroic properties of CuO. We observe a flat, broad spinon continuum extending to 2800 cm${}^{\ensuremath{-}1}$, being superimposed by magnon excitations with energies below 1350 cm${}^{\ensuremath{-}1}$. This allows us to estimate an exchange coupling constant of $J=108$ meV. Three Raman-active phonon modes predicted by lattice dynamical calculations are assigned and analyzed. We find a crossover temperature ${T}^{*}=140--150$ K through which new phonon modes are activated and phonon parameters show appreciable anomalies. This feature is discussed in terms of a competition of polar and nonpolar lattice distortions. Our study suggests the importance of competing lattice fluctuations for multiferroic compounds, which possess an intermediate multiferroic phase.

Temperature and pressure effects of multiferroic Bi2NiTiO6 compound

Bi2NiTiO6 compound which shows both magnetic (TM = 58 K) and ferroelectric properties (TC = 513 K) was synthesized under high pressure of 5 GPa and temperature of 1273 K. The crystal structure, as determined by X-ray powder diffraction and neutron powder diffraction, is a distorted A(B1B2)O3 type perovskite with space group Pn21a. Structural evolution of multiferroic Bi2NiTiO6 shows that there are two isostructural phase transitions at ∼2 GPa and ∼15 GPa under high pressure and at room temperature and indicates that isostructural phase transitions occurred with temperature higher than 823 K under ambient condition. All the isostructural phase transitions come from the Bi ion discontinuous shift, which identifies the phase transition at ∼15 GPa and at temperature higher than 823 K are the same. Using a modified high-T Birch-Murnaghan equation of state and a thermal-pressure approach, we have derived the thermoelastic parameters of high pressure phase Bi2NiTiO6, including the ambient bulk modulus K0, temperature derivative of bulk modulus at constant pressure, volumetric thermal expansivity, pressure derivative of thermal expansion, and temperature derivative of bulk modulus at constant volume.

Spin dynamics in the unconventional multiferroic AgCrS2

The magnetic excitation spectrum of AgCrS${}_{2}$, a layered multiferroic compound with a collinear four-sublattice antiferromagnetic order, has been investigated by means of time-of-flight inelastic neutron scattering between 5 and 300 K. Coupling mean-field and spin-wave calculations, the experimental spectrum is well reproduced introducing, in the model Hamiltonian, a ferromagnetic exchange interaction between nearest neighbors and a non-negligible antiferromagnetic in-plane next-nearest-neighbor interaction. This study shows that the sign of the magnetic interaction between chromium ions depends on the Cr-Cr distance with a threshold value above which it becomes ferromagnetic and emphasizes the role of the magnetoelastic coupling in the understanding of the physical properties of AgCrS${}_{2}$.

Magnetic compensation effect and phase reversal of exchange bias field across compensation temperature in multiferroic Co(Cr0.95Fe0.05)2O4

The multiferroic compound, Co(Cr0.95Fe0.05)2O4, displays reversal in the orientation of magnetic moments along with negative magnetization due to an underlying magnetic compensation phenomenon, which was earlier not noticed in the parent compound of CoCr2O4. This is one of the compounds with spinel structure where the exchange bias field changes sign across the compensation temperature and therefore has the potential attribute to get tuned in a preselected manner. In this letter, we shall elucidate the sign reversal of the exchange bias on the premise of the sign reversal of the magnetic moments, which is confirmed by the specific heat measurements.

Skyrmion dynamics in multiferroic insulators

Recent discovery of Skyrmion crystal phase in insulating multiferroic compound Cu$_2$OSeO$_3$ calls for new ways and ideas to manipulate the Skyrmions in the absence of spin transfer torque from the conduction electrons. It is shown here that the position-dependent electric field, pointed along the direction of the average induced dipole moment of the Skyrmion, can induce the Hall motion of Skyrmion with its velocity orthogonal to the field gradient. Finite Gilbert damping produces longitudinal motion. We find a rich variety of resonance modes excited by a.c. electric field.

Dynamics of photo-excited electrons in magnetically ordered TbMnO3

Time resolved optical spectroscopy is used to elucidate the dynamics of photodoped spin-aligned carriers in the presence of an underlying magnetic lattice in the multiferroic compound TbMnO3. The transient responses while probing d–d intersite transitions show marked differences along different crystallographic directions, which are discussed in terms of the interplay between the processes of hopping of the photo-injected electrons and the magnetic order in the material.

Bulk interface engineering for enhanced magnetization in multiferroic BiFeO3 compounds

We investigated composites of two antiferromagnetic perovskite oxides BiFeO3 and LaMnO3 to study change in bulk magnetic behavior. Composites with nominal compositions (1−x)BiFeO3-xLaMnO3 (x = 0, 5, 10, 20 wt. %) were synthesized by solid state reaction route. The structural analysis performed using X-ray diffraction and Raman spectroscopy indicated presence of phase separated compounds. Significant enhancement in magnetic moment is observed in composite samples, which was attributed to the uncompensated spins at the interface of two distinct antiferromagnetic phases.

Overlap of the intrinsic and extrinsic magnetoelectric effects in BiFeO3-PbTiO3 compounds: Potentialities for magnetic-sensing applications

The potentialities of BiFeO3-PbTiO3 multiferroic compounds for magnetic-field sensing applications is investigated by characterizing the dynamical magnetoelectric response of La doped (0.6)BiFeO3-(0.4)PbTiO3 piezoceramics. The results are compared with those obtained in Nb doped (1 wt. %) PbZr0.53Ti0.47O3 (PZT-5A) piezoceramics and the observed non-linearity of the oscillating voltage response of (0.6)BiFeO3-(0.4)PbTiO3 piezoceramics is attributed to the overlapping of the intrinsic and extrinsic magnetoelectric effects that were successfully decoupled by analyzing the in-phase and out-of-phase voltage components relative to applied oscillating magnetic field. For an alternating cosine-like magnetic field of 0.32 Oe, at the piezoelectric resonance frequency (184.2 kHz), the alternating voltage response decreases from 7.77 mV, at −10 kOe, to 2.71 mV, at 1.8 kOe, to subsequently increases until 5.97 mV at 10 kOe. This non-linear voltage response, which can be easily converted into a linear signal by using the phase difference, can be used for static magnetic field sensing in a wide range of magnetic fields.

Electrophoretic deposition of multiferroic BiFeO3 sub-micrometric particles from stabilized suspensions

The electrophoretic deposition (EPD) was applied to BiFeO3 (BFO) powders, one of the most interesting multiferroic compounds characterized by simultaneous magnetic and ferroelectric activity, to form homogeneous films. The preparation and characterization of stable BFO colloidal suspensions in aqueous, organic and mixed solvents were investigated by zeta potential measurements at room temperature in the presence of surfactants. BFO thin films were then deposited on steel substrates from stabilized BFO suspensions, by adjusting the preparative parameters to optimize the film quality. The compositional, morphological and electrical characteristics of the obtained BFO films, together with thickness measurements, were studied using SEM, XRD, AFM, EIS and optical surface profilometer. EPD method applied to BFO stable suspensions produced homogeneous thickness BFO films, free from pinholes and cracks, that were successively sintered and characterized also in terms of photocatalytic response.

Magnetoelectricity in the system RAl3 (BO3)4 (R = Tb, Ho, Er, Tm)

The magnetoelectric effect in the system RAl3(BO3)4 (R = Tb, Ho, Er, Tm) is investigated between 3 K and room temperature and at magnetic fields up to 70 kOe. We show a systematic increase of the magnetoelectric effect with decreasing magnetic anisotropy of the rare earth moment. A giant magnetoelectric polarization is found in the magnetically (nearly) isotropic HoAl3(BO3)4. The polarization value in transverse field geometry at 70 kOe reaches 3600 μC/m2 which is significantly higher than reported values for the field-induced polarization of linear magnetoelectric or even multiferroic compounds. The results indicate a very strong coupling of the f-moments to the lattice. They further indicate the importance of the field-induced ionic displacements in the unit cell resulting in a polar distortion and a change in symmetry on a microscopic scale. The system RAl3(BO3)4 could be interesting for the technological utilization of the high-field magnetoelectric effect.

Structural, Optical and Ferroelectric Properties of BiCoO<sub>3</sub>:BiFeO<sub>3</sub> Composite Films

Multiferroic materials, which simultaneously exhibit ferroelectricity and ferromagnetism, have recently stimulated a sharply increasing number of research activities for their scientific interest and significant technological promise in the novel multifunctional devices. Natural multiferroic single phase compounds are rare, and their magnetoelectric response are relatively weak at room temperature. In contrast, multiferroic composites improve the magnetoelectric coupling at room temperature which can have potential applications in data storage, sensors, spintronics and filters. In view of this, Multiferroic BiFeO3 –BiCoO3 (BF-BC) composite thin films have been prepared by the spray pyrolysis method, where (110) - oriented texture was obtained. X-ray diffraction analyses confirmed that BF-BC composite films were highly (110) textured. The AFM images show that the films were uniform, dance and of nearly spherical shape nanoparticle with size of 18 nm. The (110) - texture BF-BC composite films exhibits improvement in remanent polarization and coercive field with very low leakage current. The optical properties of the composite films have been studied and correlated with their structural parameters.

Observation of different spin behavior with temperature variation and Cr substitution in a multiferroic compound YMn2O5

In this article, the collective response of the spins is explored through low field bulk magnetic measurement for the series YMn2−xCrxO5 (x=0.0, 0.05). Low field ac susceptibility and dc magnetization of YMn2O5 shows multiple transitions in analogy to those observed in electrical measurement of the compound. Perturbing the parent compound with a small doping (2.5% Cr) causes a drastic change in the long-range magnetic behavior. It is observed that, YMn1.95Cr0.05O5 undergoes a ferrimagnetic ordering with an enhanced magnetic ordering temperature as compared to the parent, which undergoes an antiferromagnetic ordering. Appearance of spontaneous magnetization without any major change in the atomic structure is rather significant since the parent compound is an important multiferroic material. In addition, magnetic memory effect is observed in the Cr substituted compound whereas it is absent in the parent compound.

Multiferroic properties and enhanced magnetoelectric coupling in (1 − x)PbTiO3 − xNdFeO3

Multiferroic solid solution of (1 − x)PbTiO3 − xNdFeO3 (PT − NF) compounds are synthesized by a conventional solid state reaction. XRD investigations indicate the phase PT − NF transition from tetragonal to pseudo-cubic and then orthorhombic phase by increasing the dopant NdFeO3. The multiferroic and magnetoelectric (ME) behaviors in (1 − x)PbTiO3 − xNdFeO3 solid solutions are observed in the tetragonal phase (0 ≦ x ≦ 0.3). Further addition of the dopant NdFeO3 results into severe leakage currents and phase transitions. The solid solution compounds achieve strong ferromagnetism with high remnant magnetization (Mr = 0.786 emu/g) at x = 0.8. This work demonstrates a way to tailor the ferroelectric, magnetic properties and ME coupling in multiferroic PT-based compounds.

Observation of Field-induced Anomaly in High-field Magnetization on a Complex Spin-Driven Multiferroic Compound, LiCu2−z Zn z O2

We performed high-field magnetization measurements in the pulse magnetic fields up to 55 T at low temperatures on a doped LiCu2−z Zn z O2 (z=0.07). When H // c-axis, one anomaly was clearly observed in \(\frac{dm}{dH}\) curves at ∼9 T. The anomaly broadens as temperature increases and disappears at ∼15 K, which is close to the critical temperature of spiral spin phase and the formation of isolated spin-dimer. Interestingly, an additional peak shows up at 4.2 K, indicating a possible boundary at ∼5 K. On the other hand, when H // a-axis, one broad anomaly can be distinguish which is centered around 11.8 T. However, this anomaly only appears at T≤5.5 K, which is much lower than 15 K, indicating the origin of this anomaly could be different with those in H // c-axis. It is probably comes from the broken one-dimensional chain and the complex inter-chain interactions.

Apoplexie adénomateuse : mode de guérison spontanée d’une maladie de Cushing (à propos d’un cas)

A complex magnetic order of the multiferroic compound Co3TeO6 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 k1 = (0, 0.485, 0.055) sets in at 26 K. A transition to a second antiferromagnetic structure with k2 = (0, 0, 0) takes place at 21.1 K. Moreover, a transition to a commensurate antiferromagnetic structure with k3 = (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 Co2+ ions involved in the low-symmetry C2/c structure of Co3TeO6 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.

Formation of Magnetic Microphases inCa3Co2O6

We study a frustrated quantum Ising model relevant for Ca$_3$Co$_2$O$_6$ that consists of a triangular lattice of weakly-coupled ferromagnetic (FM) chains. According to our quantum Monte Carlo simulations, the chains become FM and form a three-sublattice "up-up-down" structure for $T \leq T_\text{CI}$. In contrast, long-period spin-density-wave (SDW) {\it microphases} are stabilized along the chains for $T_\text{CI} < T < T_c$. Our mean field solutions reveal a quasi-continuous temperature dependence of the SDW wavelength, implying the existence of metastable states that explain the very slow dynamics observed in Ca$_3$Co$_2$O$_6$. We also discuss implications of microphases for the related multiferroic compounds Ca$_3$CoMnO$_6$ and Lu$_2$MnCoO$_6$.

Thermal properties of a novel compound PbVO3

Effect of pressure on thermal properties of a multiferroic (MF) novel compound PbVO3 is studied using an atomistic approach. The temperature dependence of the specific heat (CP) (20K ≤ T ≤ 300 K) of PbVO3 has been studied and compared with the earlier experimental work. Also, this work reports, probably for the first time the relation of pressure to the Debye temperature (θD) along with other properties like bulk modulus (B), molecular force constant (f), Reststrahlen frequency (ν0), thermal expansion (α) and Grüneisen parameter of PbVO3 with the Modified Rigid Ion Model (MRIM).

Photoexcitation of gigahertz longitudinal and shear acoustic waves in BiFeO3 multiferroic single crystal

Using femtosecond laser pulses, coherent GHz acoustic phonons are efficiently photogenerated and photodetected in BiFeO3 (BFO) multiferroic single crystal. Due to the crystal lattice symmetry, longitudinal as well as two transverse acoustic modes are generated and detected, and the corresponding sound velocities are determined. This provides the opportunity to experimentally evaluate the elastic coefficients of the multiferroic compound BiFeO3 that have been estimated so far only through ab initio calculations. The knowledge of the elastic properties of BFO is highly desired for BFO integration in nanoelectronic devices. Moreover, our findings highlight also that BFO may be a good candidate for light-controlled coherent acoustic phonons sources.