A. N. Sadkov

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.

Features of sound propagation in the vicinity of “symmetry-dictated” isostructural phase transitions in ferroelastics

Using a proper ferroelastic phase transition of the tension-compression type as an example, it is shown that, if the order parameter characterizing a structural phase transition allows the existence of a third-power invariant in the Landau potential, then there must be “symmetry-dictated” isostructural phase transition lines in the vicinity of the line of that structural phase transition. These isostructural transitions may manifest themselves both directly and as supercritical anomalies in the behavior of elastic moduli and lattice parameters. These effects are discovered and investigated without invoking the perturbation theory in terms of which the second-order phase transitions are commonly described. A hypothesis is made on the basis of the results obtained that the sound velocity anomalies observed in orthoclase and sanidine crystals are due to the super-critical behavior of the lattice parameters in the vicinity of a symmetry-dictated isostructural phase transition in the prototype phase of these crystals.

Theory of the isostructural extension-contraction phase transition in proper ferroelastics

Using the phenomenological Landau theory of phase transitions, a diagram of states and anomalies of physical characteristics are investigated in the area of conditions corresponding to isostructural transitions in a high-symmetric phase. A cubic phase with a two-component order parameter describing the ferroelastic extension-compression transition is considered as an example.

Phenomenological theory of phase transitions in solutions of nanotubes in liquid crystal

A phenomenological model of thermodynamic potential is constructed that describes phase transitions in homogeneous monodisperse solutions of achiral carbon tubes in liquid crystal. Possible phase diagrams of such a systems are constructed and investigated.

Symmetry induced ferroelastic isostructural phase transition and sound propagation anomaly in potassium rich feldspars

The proper ferroelastic phase transitions extension — compression type in cubic crystals are considered. The Landau potential being a polynomial function of the order parameter components includes the term of third power. This symmetry induced statement results in the line of isostructural phase transitions (so-called symmetry induced IPT) situated on the phase diagram near the lines of symmetry changing phase transition. The physical meaning of phenomenological parameters of the Landau theory and a description of the physical quantities anomalies along definite thermodynamic paths, crossing these lines are presented in this paper. One of the consequences of the developed theory is the conclusion that the rest of elastic constants anomaly, accompaning IPT in the parent phase of feldspars, must display itself in nonmonotonic dependence of the longitudinal sound velocity on external conditions. Our experimental data on potassium rich feldspars confirm this consequence of our calculations.

Metamagnetic soft mode in double sublattice antiferromagnets. II. Spin dynamics, susceptibility, thermodynamics

For pt.I see ibid., vol.19, p.4339 (1986). The theory of double sublattice antiferromagnets based on the equations dynamics, given in the first part of this paper, is developed. Normal coordinates of natural vibrations of the subsystem are plotted, when the sublattice spin value is not the integral of motion. It is shown that the almost flat motion of spins corresponds to the third (the lowest) frequency. The susceptibility to a high-frequency field chi ik( omega , T, H) is calculated and it is shown that the third branch of the spectrum should be indicated in the components of the gyration tensor, as well as in diagonal components of chi ik. The theory predicts the most quantitatively strong effects in the diagonal component chi ii perpendicular to the antiferromagnetism vector and to the external field. It is proved that damping conformed with the observed width of antiferromagnetic and paramagnetic resonance lines cannot overlap the effects of the third branch of the spectrum predicted by the theory. In conclusion the authors briefly discuss the possible indication of the third branch of the spectrum in low-temperature thermodynamics of the antiferromagnet.

Metamagnetic soft mode in double sublattice antiferromagnets. I. Phase diagram and spectrum

The linear dynamics of double-sublattice antiferromagnets is considered near the temperature of the phase transition where the value of sublattice spin is not constant. The Landau-Lifshitz equations are substituted by non-dissipative Onsager-type equations which take into account the dispersion of the kinetic coefficients and also S12 not=const. The structural stability of the equations chosen is provided. The natural frequencies of the magnetic subsystem are calculated on this basis, the thermodynamic powers and the values of the sublattice magnetisation being calculated on the basis of the Landau potential of the fourth order. The phase diagram of such a magnetic material, containing a tricritical point in the (H,T) plane, is described in detail. The magnetic subsystem has three natural frequencies. It is proved that in strong fields the frequency of the vibrations is soft which is connected with non-constant spin value. In weak fields the frequency corresponds to the anti-ferromagnetic resonance.