Dynamics of domain walls under the action of pulse and gradient magnetic fields in rare-earth orthoferrites

Experimental and theoretical investigations of solitary domain wall dynamics in an yttrium orthoferrite plate under the action of a pulse magnetic field were carried out. The investigations are performed under conditions in which the change in the gradient magnetic field is comparable to the magnitude of the pulse magnetic field shifting the domain walls when the latter are displaced from their equilibrium position.     

Observation of Bloch lines in yttrium orthoferrite

Utilizing the dark field method, parts of different brightness within domain walls of yttrium orthoferrite have been observed. Dark segments between these parts and the dependence of their positions on an applied magnetic field indicate the presence of Bloch lines.     

Ferroelectricity Driven by Dzyaloshinskii-Moriya Interaction in an Anisotropic Heisenberg Antiferromagnetic Chain

We have made a theoretical study on the ferroelectricity and ferromagnetism in an antiferromagnetic Heisenberg chain with a Dzyaloshinskii-Moriya interaction which may induce ferroelectricity in some low-dimensional magnetic materials. Based on the transfer-matrix method, we obtain the analytical results of the average magnetization, polarization and magnetic susceptibility. And these physical quantities as functions of temperature and applied field are discussed respectively under various conditions. We find that the temperature dependence of the magnetic susceptibility exhibits different responses for the external field applied along the chain and perpendicular to the chain, demonstrating the important role of anisotropy.     

Dynamics of a Néel domain wall with a fine structure in rare-earth orthoferrites

The dynamics of an isolated domain wall (DW) with a fine structure moving at a supersonic velocity in a rare-earth orthoferrite is studied. A set of nonlinear equations of motion of the center of a DW structure line is derived. A steady-state solution to these equations adequately describes the experimental data for yttrium orthoferrite. The effect of an external magnetic field on the steady-state velocity of a DW with structural lines is investigated.     

Generation of pairs of antiferromagnetic vortices and their dynamics at a domain wall in yttrium orthoferrite

The stable generation of pairs of antiferromagnetic vortices at a domain wall moving at a velocity of 12 km/s is investigated at the instant it passes through a defect in a thin plate of yttrium orthoferrite. The velocities of a vortex and an antivortex moving in opposite directions along the domain wall and being accompanied by solitary flexural waves are ±16 km/s. The total velocity of antiferromagnetic vortices is close to the maximum velocity of the domain wall, 20 km/s. Such a high velocity can only be due to the action of a quite large gyroscopic force. An external dc magnetic field (±400 Oe) applied along the b axis of the orthoferrite affects this velocity insignificantly. The effective magnetic field that violates the Lorentz invariance of the dynamics considerably exceeds this value.     

The observation of the contribution of longitudinal susceptibility to the frequency of the soft magnetoresonance mode in SmFeO<Subscript>3</Subscript>

The behavior of the spectrum of the soft magnetoresonance mode in samarium orthoferrite was experimentally and theoretically studied in the region of the Γ₄?Γ₂₄ orientation phase transition induced by an external magnetic field. The special features of this behavior can be explained by the relative contributions of interaction between ferrite subsystems and longitudinal susceptibility. It is also shown that the contribution of longitudinal susceptibility to the gap of the soft magnetoresonance mode in samarium orthoferrite can also be substantial in low fields; that is, in the vicinity of spontaneous orientation phase transitions. This is explained by the occurrence of spontaneous orientation phase transitions in samarium orthoferrite at high temperatures, at which longitudinal susceptibility is comparable in magnitude with transverse susceptibility.     

An investigation on magnetism, spin–phonon coupling, and?ferroelectricity in multiferroic GdMn2O5

The magnetization, Raman spectroscopy, and ferroelectricity of multiferroic GdMn₂O₅ as a function of temperature and magnetic field are investigated. The complicated magnetic transitions at low temperatures are featured with anomalous Raman mode shifts, dielectric response, and ferroelectricity generation, indicating the significant spin–phonon coupling. It is argued that this coupling is possibly responsible for the electrical polarization generation associated with the incommensurate–commensurate transition.      

Gyroscopic dynamics of antiferromagnetic vortices in domain boundaries of yttrium orthoferrite

It is established experimentally that the magnetic field directed along the b axis has little effect on the velocities of antiferromagnetic vortices in the domain boundary (DB) of yttrium orthoferrite and fails to explain the presence of an appreciable gyroscopic force acting on these vortices. This force is induced by the dynamic canting of magnetic sublattices proportional to the DB velocity. Due to the canting, the velocities of antiferromagnetic vortices depend initially quadratically on the DB velocity, as was experimentally found in this work. The dynamics of antiferromagnetic vortices in the yttrium orthoferrite DBs is gyroscopic and quasi-relativistic, with the limiting velocity of 20 km/s equal to the velocity of spin waves at the linear portion of their dispersion curve.     

脉冲强磁场下的电极化测量系统

多铁性材料是当前物质科学研究的热点,具有重要的科学研究意义和应用前景.低温和强磁场实验环境为研究多铁性材料提供了一种有效途径.脉冲强磁场下的电极化测量系统能实现最高磁场强度60 T、最低温度0.5 K的铁电特性测量.该系统采用热释电方法,具有磁场强度高、控温范围广、转角测量等特点,可用于强磁场下的磁电特性研究.本文介绍了该系统的测量装置和实验原理,并展示了其在多铁性材料研究中的一系列应用,揭示了脉冲强磁场电极化测量系统在磁电特性探索中的重要作用.     

Dual nature of improper ferroelectricity in a magnetoelectric multiferroic

Using first-principles calculations, we study the microscopic origin of ferroelectricity (FE) induced by magnetic order in the orthorhombic HoMnO3. We obtain the largest ferroelectric polarization observed in the whole class of improper magnetic ferroelectrics to date. We find that the two proposed mechanisms for FE in multiferroics, lattice and electronic based, are simultaneously active in this compound: a large portion of the ferroelectric polarization arises due to quantum-mechanical effects of electron orbital polarization, in addition to the conventional polar atomic displacements. An interesting mechanism for switching the magnetoelectric domains by an electric field via a 180 degrees coherent rotation of Mn spins is also proposed.     

Spin-reorientation, ferroelectricity, and magnetodielectric effect in YFe(1-x)Mn(x)O3(0.1 ≤ x ≤ 0.40)

We report the observation of magnetoelectric and magnetodielectric effects at different temperatures in Mn-substituted yttrium orthoferrite, YFe(1-x)Mn(x)O(3)(0.1 ≤ x ≤ 0.40). Substitution of Mn in antiferromagnetic YFeO(3)(T(N) = 640 K) induces a first-order spin-reorientation transition at a temperature, T(SR), which increases with x whereas the Néel temperature (T(N)) decreases. While the magnetodielectric effect occurs at T(SR) and T(N), the ferroelectricity appears rather at low temperatures. The origin of magnetodielectric effect is attributed to spin-phonon coupling as evidenced from the temperature dependence of Raman phonon modes. The large magnetocapacitance (18% at 50 kOe) near T(SR) = 320 K and high ferroelectric transition temperature (～115 K) observed for x = 0.4 suggest routes to enhance magnetoelectric effect near room temperature for practical applications.     

A nonfrustrated magnetoelectric with incommensurate magnetic order in magnetic field

We discuss a model nonfrustrated magnetoelectric in which a sufficiently strong magnetoelectric coupling produces an incommensurate magnetic order leading to ferroelectricity. Properties of the magnetoelectric in the magnetic field directed perpendicular to the wave vector describing the spin helix are considered in detail. Analysis of the classical energy shows that in contrast to the naive expectation, the onset of ferroelectricity occurs at a field H _c1 that is lower than the saturation field H _c2. We have H _c1 = H _c2 at large enough magnetoelectric coupling. We show that at H = 0, ferroelectricity occurs at T = T _FE < T _N. A qualitative discussion of the phase diagram in the H-T plane is presented within the mean-field approach. The text was submitted by the author in English.     

Nonlinear magnetoacoustic interactions in weak ferromagnets

The pinning and interaction of a single domain wall with normal magnetoelastic waves excited during its motion in a single-crystal yttrium orthoferrite plate, were discovered and investigated by a method based on the magneto-optical Faraday effect. The dependences of the bending wave amplitude and the spectra of shear waves, which can be excited by a moving domain wall, were calculated. The results obtained are interpreted with allowance for the interactions of excited oscillations in both the magnetic and elastic subsystems of the orthoferrite.     

Emergent phenomena in perovskite-type manganites

Perovskite-type manganites exhibit various interesting phenomena arising from complex interplay among spin, charge, orbital, and lattice degrees of freedom. One such example is the keen competition between phases with different spin/charge/orbital orders. Keen competition between antiferromagnetic metal and orbital-ordered insulator is found in the slightly electron-doped regime near Mn⁴⁺ state which is stabilized by the high oxygen-pressure condition. Another one is the emergence of ferroelectricity either induced by the magnetic ordering or independently of the magnetic ordering. As the respective examples, perovskite-type YMnO₃ and Sr_1−xBa_xMnO₃ are discussed. In the YMnO₃, the ferroelectric lattice distortion associated with the E-type spin order is observed for the first time. Displacement-type ferroelectricity with off-center magnetic ions is discovered for Sr_0.5Ba_0.5MnO₃, which shows both large polarization value and strong coupling between ferroelectricity and magnetism.     

Numerical simulation of magnetic interactions in polycrystalline YFeO3

The magnetic behavior of polycrystalline yttrium orthoferrite was studied from the experimental and theoretical points of view. Magnetization measurements up to 170 kOe were carried out on a single-phase YFeO₃ sample synthesized from heterobimetallic alkoxides. The complex interplay between weak-ferromagnetic and antiferromagnetic interactions, observed in the experimental M(H) curves, was successfully simulated by locally minimizing the magnetic energy of two interacting Fe sublattices. The resulting values of exchange field (H_E=5590 kOe), anisotropy field (H_A=0.5 kOe) and Dzyaloshinsky–Moriya antisymmetric field (H_D=149 kOe) are in good agreement with previous reports on this system.     

Magnetic field influence on ferroelectric phase transitions in BaTiO3-type systems

The effect of an external magnetic field on ferroelectric characteristics of BaTiO₃-type crystals is discussed and estimated in the framework of the vibronic theory of ferroelectricity.     

On the magneto-optics of orthoferrites with spin reorientation

The results of magneto-optical investigations into the Γ₂–Γ₄ spin reorientation in an Sm_0.6Tb_0.2Tm_0.2FeO₃ orthoferrite single crystal are reported. The magneto-optical parameters are determined taking into account the dichroism. The orientation of the antiferromagnetic vector in the canted phase is investigated as a function of the magnetic field and temperature. The magnetic anisotropy field strengths are determined by the processing of the results. The orientation contribution to the birefringence is estimated.     

Solid solutions of bismuth-based Aurivillius oxides: structural and dielectric characterization

Bismuth layered perovskite structures show interesting physical properties varying as a function of external parameters (temperature, frequency, electric and magnetic fields). When a magnetic ion is incorporated in some of these materials, some of the structures show simultaneous ferroelectricity and ferro/antiferromagnetism. Thus, they exhibit magnetoelectric properties under the influence of an external magnetic field. This paper compares the structural (XRD and SEM) and electrical properties of two eight-layered Aurivillius oxides.     

Magnetic field induced dehybridization of the electromagnons in multiferroic TbMnO₃

We have studied the impact of the magnetic field on the electromagnon excitations in TbMnO? crystal. Applying a magnetic field along the c axis, we show that the electromagnons transform into pure antiferromagnetic modes, losing their polar character. Entering in the paraelectric phase, we are able to track the spectral weight transfer from the electromagnons to the magnon excitations and we discuss the magnetic excitations underlying the electromagnons. We also point out the phonons involved in the phase transition process. This reveals that the Mn-O distance plays a key role in understanding the ferroelectricity and the polar character of the electromagnons. Magnetic field measurements along the b axis allow us to detect a new electromagnon resonance in agreement with a Heisenberg model.     

Hybrid improper ferroelectricity: a mechanism for controllable polarization-magnetization coupling

First-principles calculations are presented for the layered perovskite Ca3Mn2O7. The results reveal a rich set of coupled structural, magnetic, and polar domains in which oxygen octahedron rotations induce ferroelectricity, magnetoelectricity, and weak ferromagnetism. The key point is that the rotation distortion is a combination of two nonpolar modes with different symmetries. We use the term "hybrid" improper ferroelectricity to describe this phenomenon and discuss how control over magnetism is achieved through these functional antiferrodistortive octahedron rotations.