Origin Of Multiferroicity Research Articles

Origin of Multiferroism in VOX2 (X = Cl, Br, I) Monolayers.

Based on the proposed microscopic model, we investigate the multiferroic characteristics of VOX2 (X = Cl, Br, I) monolayers using a Green's function method. The dependence of the microscopic parameters of the ferroelectric system (pseudo-spin arrangement and flipping rate) on the magnitude and sign of the exchange magnetic interaction along the b-axis and the value of the Dzyaloshinskii-Moria vector have been investigated and qualitatively explained. The possibility of observing a spin-reorientation transition with a change in the character of spin ordering from antiferromagnetic to ferromagnetic is investigated. It is found that the antisymmetric magnetoelectric interaction may be responsible for the spin-reorientation transition without a change in the ordering of magnetic moments. Changing the sign of the exchange magnetic interaction along the b-axis leads to ferromagnetic ordering without observing a spin-reorientation transition. The dependence of isotropic and antisymmetric magnetic interactions on the microscopic parameters of the ferroelectric system is qualitatively explained. A mechanism for the occurrence of the spin-reorientation transition is presented based on the proposed microscopic model. The obtained results qualitatively coincide with Density Functional Theory calculations.

Symmetry breaking and spin lattice coupling in NdCrTiO5

The origin of multiferroicity in ${\mathrm{NdCrTiO}}_{5}$ has been examined. A first-order phase transition, due to breaking of the inversion symmetry, near the low-temperature antiferromagnetic ordering (${T}_{N}\ensuremath{\sim}21$ K) where spontaneous electric polarization also appears, is confirmed. From synchrotron x-ray powder diffraction measurements, the structure at low temperatures is found to be noncentrosymmetric (space group $Pba2$), justifying the polar behavior of this compound below ${T}_{N}$. Temperature-dependent Raman measurements reveal that the lattice is strongly coupled with the spins, pointing to the spin lattice correlations in a noncentrosymmetric space group being responsible for invoking the ferroelectricity below 21 K. Through time-of-flight neutron powder diffraction experiments, it is confirmed that the magnetic sublattices, both Nd and Cr, simultaneously order at ${T}_{N}$, and at temperatures below 15 K the Cr moments are found to be saturated while the Nd moments continue to grow until 6 K.

Effect of annealing process on dielectric and multiferroic properties of Sol-Gel derived bismuth chromate thin films

ABSTRACTMultiferroic materials such as BiCrO3, uniquely exhibit ferroelectricity and ferromagnetism simultaneously have been extensively studied for spintronics device applications. Conversely, the origin of multiferroicity remains poorly understood. In this paper, the structural phases of BiCrO3 over Pt (111)/Ti/SiO2/Si substrate and their lattice distortions after careful annealing were carefully investigated to explain their multiferroicity. Structural transition and different phases was studied from X-ray diffraction at different annealing temperature from 400°C to 700°C and the size of the grains were calculated from Scherrer's equation. SEM and AFM reveal nano-rod formation and the size and density of nano-rods tends to increase with increase of temperature. The step height variation and surface outgrowth of the films at different annealing temperature were further studied by measuring root mean square (RMS) and roughness (Rq) values. The frequency dependent dielectric behavior with terrific dielectric constant and dielectric loss were observed using LCRQ meter. Structural transformation also results an improved (well saturated) ferromagnetism at 700°C from weak ferromagnetism at 400°C due to addition of extra phases. This kind of approach could be used to probe the origin of multiferroic properties in rare-earth materials for device applications.

Hyperfine and crystal field interactions in multiferroic HoCrO3

We report a comprehensive specific heat and inelastic neutron scattering study to explore the possible origin of multiferroicity in HoCrO3. We have performed specific heat measurements in the temperature range 100 mK–290 K and inelastic neutron scattering measurements were performed in the temperature range 1.5–200 K. From the specific heat data we determined hyperfine splitting at 22.5(2) μeV and crystal field transitions at 1.379(5) meV, 10.37(4) meV, 15.49(9) meV and 23.44(9) meV, indicating the existence of strong hyperfine and crystal field interactions in HoCrO3. Further, an effective hyperfine field is determined to be 600(3) T. The quasielastic scattering observed in the inelastic scattering data and a large linear term mJ mol−1 K−2 in the specific heat is attributed to the presence of short range exchange interactions, which is understood to be contributing to the observed ferroelectricity. Further the nuclear and magnetic entropies were computed to be, ∼17.2 Jmol−1 K−1 and ∼34 Jmol−1 K−1, respectively. The entropy values are in excellent agreement with the limiting theoretical values. An anomaly is observed in the peak position of the temperature dependent crystal field spectra around 60 K, at the same temperature an anomaly in the pyroelectric current is reported. From this we could elucidate a direct correlation between the crystal electric field excitations of Ho3+ and ferroelectricity in HoCrO3. Our present study, along with recent reports, confirm that HoCrO3, and RCrO3 (R = rare earth) in general, possess more than one driving force for the ferroelectricity and multiferroicity.

Mechanisms and origin of multiferroicity

Motivated by the potential applications of their intrinsic cross-coupling properties, the interest in multiferroic materials has constantly increased recently, leading to significant experimental and theoretical advancements. From the theoretical point of view, recent progresses have allowed to identify different mechanisms responsible for the appearence of ferroelectric polarization coexisting with -- and coupled to -- magnetic properties. This chapter aims at reviewing the fundamental mechanisms devised so far, mainly in transition-metal oxides, which lie at the origin of multiferroicity.

Origin of multiferroism in the charge frustrated LuFe2O4 compound

Abstract We propose a microscopic model in order to study the multiferroic properties of LuFe2O4. It is shown that the real part of the dielectric constant ϵ has a plateau near the magnetic phase transition T N = 240 K . At room temperature ϵ decreases strongly by applying an external magnetic field H. This behavior is an evidence for a strong coupling of spins and electric dipoles in agreement with the experimental data. The observed kinks in the temperature dependence of the phonon energy and damping for the E g 3 mode around the ferroelectric and ferrimagnetic transition temperatures show that this phonon mode is closely related to the structural, magnetic and ferroelectric properties of LuFe2O4.

Theoretical study of the multiferroic properties in M-doped (M=Co, Cr, Mg) ZnO thin films

Abstract The origin of multiferroism is still an open problem in ZnO. We propose a microscopic model to clarify the occurrence of multiferroism in this material. Using Green׳s function technique we study the influence of ion doping and size effects on the magnetization and polarization of ZnO thin films. The calculations for magnetic Co- and Cr-ions are based on the s–d model, the transverse Ising model in terms of pseudo-spins and a biquadratic magnetoelectric coupling, whereas in case of nonmagnetic Mg-ions the model takes into account the Coulomb interaction and an indirect coupling between the pseudo-spins via the conduction electrons. We show that the magnetization M exhibits a maximum for a fixed concentration of the doping ions. Furthermore M increases with decreasing film thickness N . The polarization increases with increasing concentration of the dopant and decreasing N . The results are in good agreement with the experimental data.