Yu. D. Zavorotnev

Interrelation of structural and magnetic order parameters in crystals

The paper presents a theoretical definition of conditions for which the change of the type of magnetic phase transition (PT) from a disordered state to an ordered magnetic state becomes possible, when the temperatures of the structural and magnetic phase transitions are close. We have found the dependence of the magnitude of the magnetization jump and the transition temperature on the parameters of phenomenological potential at superposition of the structural and magnetic PT. Forced reconstruction of the structural order parameter (OP) under periodic changes of the magnetic OP modulus in phase with a spiral structure has been studied.

Dielectric, magnetoelectric, structure, and dissipative properties and the Mössbauer effect in PbFe1/2Nb1/2O3 ceramics in wide frequency and temperature ranges

High-quality fine-grained ceramic samples of classical multiferroics PbFe1/2Nb1/2O3 (PFN) were synthesized. Their dielectric, magnetoelectric, and magnetic characteristics, including the Mössbauer effect, were measured over wide ranges of temperatures (10–1000 K) and field frequencies (from 25 Hz to 1 MHz). The temperature dependence of the dielectric loss exhibits a maximum between 150 and 170 K, likely due to magnetic ordering. The dependence of ɛ on the magnetic field displays an anomalous increase near the Curie temperature (370 K) that rises to 40% upon heating.

“Resonance” phenomena in a magnetic system under simultaneous torsion and pressure

The types of distributions of the structural and magnetic order parameters under torsion and pressure applied perpendicular to the “easy axis” of a ferromagnet are studied. It is found that these interactions cause two-sided single-phase and antiphase spatial amplitude modulations of the ordering parameters. Increased pressure leads to an increase in the amplitude of the modulations of the order parameter. Possible cases of resonance phenomena in the structural and magnetic subsystems are pointed out.

Exercising control by changing the modulus of the ferromagnetic order parameter of twisting deformation

The distribution of the ferromagnetic order parameter (OP) is studied for an easy-axis type of ferromagnet under the effect of intense flow twisting applied along and perpendicular to the easy axis of a sample. The periodic changes in the OP modulus and the emergence of a perpendicular component of the magnetic OP are described. It is established that the law of the distribution of the magnetic moment depends on the temperature.

Interaction of the structural and magnetic subsystems in crystals upon severe plastic deformation by torsion

Interaction between the structural and magnetic subsystems of a crystal has been studied near phase transition lines. The consideration has been performed in terms of the Landau phenomenological theory. It has been shown that severe plastic deformation by torsion leads to a forced formation of a heterogeneous distribution in the magnetic subsystem. A nonsine space modulation of the magnitudes of the magnetic and structural parameters appears near the lines of the magnetic and structural phase transitions. An analysis of the obtained dependences has demonstrated a possibility of designing various distributions of the ferromagnetic vector in crystals under action of severe plastic deformation by torsion.

Effect of dislocations on the structural order parameter in a crystal upon torsional strain

The interaction of the structural subsystem with dislocations upon applying an elastic torsional strain has been studied. The consideration is performed in the terms of the Landau phenomenological theory with the refusal from the approximation that the irreducible vector magnitudes are constant. It is shown that additional spatial amplitude and frequency oscillations related to the change in the magnitudes of the structural order parameter and the dislocation density appear.

Structure, dielectric, magnetoelectric, and dissipative properties of AFe2/3W1/3O3 (A = Ba, Sr, Pb) ceramics in wide frequency and temperature ranges

High-density impurity-free AFe2/3W1/3O3 (A = Ba(BWF), Sr(SWF), Pb(PWF)) ceramics with well-developed microstructures were synthesized. Their structures and dielectric properties were investigated over wide temperature (10–973 K) and frequency (25 Hz-1 MHz) ranges. These materials are found to be almost transparent in the microwave range. They were characterized by weak magnetoelectric interactions; ɛ(BWT) ≈ 50 up to 400 K and ɛ(BWT) and ɛ(PWT) have second maxima between 400 and 600 K. All properties are easily reproduced, regardless of slight variations in synthesis conditions.

Resonances of the Spatial Distribution of Dislocations upon Interaction with the Structural Order Parameter

The interaction of a structural subsystem with dislocations under the superposition of an elastic torsion strain and an additional uniaxial spatially nonuniform time-constant pressure is studied. The analysis is carried out within of Landau’s phenomenological theory with refusal of the approximation of constant magnitudes of irreducible vectors. The rise of spatial resonances of the dislocation density in these conditions is shown.

Peculiarities of the magnetic structure of MnCoSi

Possible mechanisms of the formation of a long-period structure in the MnCoSi compound are considered. It is established that the invariants containing the first spatial derivative of the irreducible magnetic vectors in different powers play a key role in the theoretical description. It is shown that when a spiral structure is formed by only antiferromagnetic vectors, the ferromagnetic vector can also be spontaneously involved. In this case, the orthogonality of rotating vectors is violated. The consideration is performed taking into account the approximated constancy of the irreducible-vector moduli and disregarding it. This approach made it possible to consider the magnetic superstructure evolution with a change in temperature.

Characteristics of irreducible vectors rotating in superstructures with two one-component order parameters

It is shown that incommensurate long-period structures formed by two synchronously rotating irreducible magnetic vectors are unstable and split into two interacting superstrucures. The behavior of such states away from and close to the phase-transition line is considered.