A. K. Zvezdin

Destruction of spin cycloid in (111)c-oriented BiFeO3 thin films by epitiaxial constraint: Enhanced polarization and release of latent magnetization

In BiFeO3 films, it has been found that epitaxial constraint results in the destruction of a space modulated spin structure. For (111)c films, relative to corresponding bulk crystals, it is shown (i) that the induced magnetization is enhanced at low applied fields; (ii) that the polarization is dramatically enhanced; whereas, (iii) the lattice structure for (111)c films and crystals is nearly identical. Our results evidence that eptiaxial constraint induces a transition between cycloidal and homogeneous antiferromagnetic spin states, releasing a latent antiferromagnetic component locked within the cycloid.

57Fe NMR study of a spatially modulated magnetic structure in BiFeO3

The 57 Fe spin echo spectra were studied in local magnetic fields of a BiFeO3 ferroelectric antiferromagnet over the temperature range 77–304 K. The line shape analysis confirmed the presence of a spatially modulated, incommensurate cycloidal spin structure in BiFeO3 and allowed the actual spin distribution to be reproduced throughout the cycloid length. The distribution was found to be essentially anharmonic. The modulated structure is stable over the whole temperature range studied. The cycloid wave becomes more harmonic with temperature elevation.

Magnetoelectric interactions and phase transitions in a new class of multiferroics with improper electric polarization

Symmetry analysis of magnetoelectric interactions in rare-earth orthoferrites and orthochromites has been performed. It has been shown that symmetry allows the appearance of spontaneous electric polarization or the magnetic-field-induced polarization in the region of antiferromagnetic (centro-asymmetric) ordering of the rare-earth ions. The analysis reveals a number of pronounced anomalies in the behavior of the electric polarization at the metamagnetic and spin-reorientation transitions in DyCrO3 and TbFeO3. This behavior points to a strong sensitivity of the magnetoelectric properties of such magnets to the antiferromagnetic state of the rareearth subsystem and the spin orientation of the d ions.

On the effect of inhomogeneous magnetoelectric (flexomagnetoelectric) interaction on the spectrum and properties of magnons in multiferroics

The features of the magnon spectrum in easy-plane multiferroics (such as BiFeO3), which allow inhomogeneous magnetoelectric (flexomagnetoelectric) interaction P[(L∇)L − LdivL], where L and P are the antiferromagnetic moment and electric polarization, respectively, have been theoretically analyzed. It has been shown that, in contrast to the magnon spectrum of a usual easy-plane antiferromagnet, a multiferroic with this magnetic structure is characterized by, first, the interaction between magnons of both branches propagating along the weak ferromagnetic moment and the appearance of a minimum (or zero) of the frequency of one of the branches, which reflects the instability of the system with respect to the transition to an inhomogeneous state with increasing flexomagnetoelectric interaction and, second, the nonequivalence (nonreciprocity) of the propagation of spin waves along and against the antiferromagnetism vector, which coincides with the toroidal moment in this system.

Faraday effect in Tb3Ga5O12 in a rapidly increasing ultrastrong magnetic field

The Faraday effect is measured in paramagnetic terbium gallate garnet Tb3Ga5O12 at a wavelength λ=0.63 µm at 6 K in pulsed magnetic fields up to 75 T increasing at a rate of 107 T/s for field orientation along the crystallographic direction 〈110〉. The experimental data are compared with the results of theoretical calculations taking into account the crystal fields acting on the Tb3+ ion and various contributions to the Faraday rotation. Since the measurements in pulsed fields are carried out in the adiabatic regime, the dependence of the sample temperature on the magnetic field acting during a current pulse is obtained from the comparison of the experimental dependence of Faraday rotation with the theoretically calculated dependences of the Faraday effect under isothermal conditions at various temperatures.

Magnetic anisotropy and magnetoelectric properties of Tb1 − xErxFe3(BO3)4 ferroborates

Magnetic and magnetoelectric properties of ferroborate single crystals with complex composition (Tb1 − xErxFe3(BO3)4, x = 0, 0.75) and with competing exchange Tb-Fe and Er-Fe interactions are investigated. Jumps in electric polarization, magnetostriction, and magnetization are observed as a result of spin-flop transitions, as well as a considerable decrease in the critical field upon an increase in the Er concentration, in a field Hc parallel to the c axis. The observed behavior of phase-transition fields is analyzed and explained using a simple model taking into account anisotropy in g factors and exchange splitting of funda-mental doublets of the easy-axis Tb3+ ion and easy-plane Er3+ ion. It is established that magnetoelectric and magnetostriction anomalies under spin-flop transitions are mainly controlled by the Tb subsystem. The Tb subsystem makes a nonmonotonic contribution ΔPa(Ha, T) to polarization along the a axis: the value of ΔPa reverses its sign and increases with temperature due to the contribution from the excited states of the Tb3+ ion.

Field-induced phase transitions and phase diagrams in BiFeO3-like multiferroics

The incommensurate magnetic structures and phase diagrams of multiferroics has been explored on the basis of accurate micromagnetic analysis taking into account the spin flexoelecric interaction (Lifshitz invariant). The objects of the study are BiFeO_3-like single crystals and epitaxial films grown on the substrates. The main control parameters are the magnetic field, the magnetic anisotropy, and the epitaxial strain in the case of films. We predict novel quasi-cycloidal structures induced by external magnetic field or by epitaxial strain in the BiFeO_3-films. Phase diagrams representing the regions of homogeneous magnetic states and incommensurate structures stability are constructed for the two essential geometries of magnetic field (magnetic field oriented parallel to the principal crystal axis C_3 and perpendicular to this direction C_3). It is shown that the direction of applied magnetic field substantially affects a set of magnetic phases, properties of incommensurate structures, character of phase transitions. Novel conical type of cycloidal ordering is revealed during the transition from incommensurate cycloidal structure into homogeneous magnetic state. Elaborated phase diagrams allow estimate appropriate combination of control parameters (magnetic field, magnetic anisotropy, exchange stiffness) required to the destruction of cycloidal ordering corresponding to the transition into homogeneous structure. The results show that the magnitude of critical magnetic field suppressing cycloid is lowered in multiferroics films comparing to single crystals, it can be also lowered by the selection of orientation of magnetic field. Our results can be useful for strain engineering of new multiferroic functional materials on demand.

Quantum magnetoelectric effect in the molecular crystal Dy3

Magnetoelectric properties of a molecular crystal formed by dysprosium triangular clusters are investigated. The effective spin-electric Hamiltonian is derived on the base of developed quantum mechanical model of the cluster spin structure. The magnetoelectric contribution to the free energy of the crystal is calculated. The analysis reveals several distinctive features of the magnetoelectric effect, which are not typical for conventional paramagnetic systems at low temperatures. The peculiarities are explained by the chirality of the dysprosium core of the molecules.

Effect of oblique light incidence on magnetooptical properties of one-dimensional photonic crystals

We have investigated the magnetooptical properties of one-dimensional magnetic photonic crystals for the case of oblique light incidence. We developed a theoretical model based on the transfer matrix approach. We found several new effects such as transmittance resonance peak shift versus external magnetic field and the Faraday effect dependence on the incidence angle. We discuss several possible one-dimensional magnetic photonic crystals applications for the optical devices.

Magnetic phase diagrams and magnetization curves of ferrimagnets with one unstable magnetic subsystem

Abstract Magnetic phase diagrams and schematic magnetization curves are given for a ferrimagnet, one magnetic subsystem of which undergoes a metamagnetic transition from a weekly to a strongly magnetic state. In the simple model of the molecular field theory it is shown that in such systems magnetic phase transitions of different nature are possible, and the phase diagrams are of various topology depending on the relation of the system parameters. The magnetization curves for the substituted intermetallic compounds Y 1 − t R t (Co 1 − x Al x ) 2 (R is a heavy rare earth element) are calculated for the actual parameters of the system.