V. A. Sanina

Thermal vibrations and the structure of quasi-two-dimensional R2CuO4 crystals (R=La, Pr, Nd, Sm, Eu, and Gd)

Thermal vibrations of ions in R2CuO4 crystals (R=La, Pr, Nd, Sm, Eu, Gd) were studied by x-ray diffractometry. A comparative analysis of thermal displacements of the copper and rare-earth ions permitted a conclusion as to the main interactions responsible for the structural state of the CuO2 sheets and of a crystal as a whole. The structural properties were found to correlate with the magnitude of the ionic radius and with the ground state of the rare-earth ions.

Phase separation with charge self-organization in the Tb0.95Bi0.05MnO3, Gd0.75Ce0.25Mn2O5, and Eu0.8Ce0.2Mn2O5 manganite multiferroics

New doped manganite multiferroics Tb0.95Bi0.05MnO3, Gd0.75Ce0.25Mn2O5, and Eu0.8Ce0.2Mn2O5, which are semiconductors, have been grown and studied. The starting dielectric multiferroics TbMnO3 and RMn2O5 (R = Gd and Eu) have close magnetic and ferroelectric ordering temperatures of 30–40 K. The crystals studied are multiferroics in which states with giant permittivity and ferromagnetism coexist at room temperature. An analysis of the dielectric properties suggests that, at temperatures T ≥ 180 K, these crystals undergo a phase separation involving dynamic periodic alternation of quasi-2D layers of mixed-valence manganese ions, a process accounting for the onset of charge-induced ferroelectricity. At low temperatures (T < 100 K), a small phase volume in the crystals is occupied by as-grown quasi-2D layers containing dopants and carriers. Most of the crystal volume is occupied by the carrier-free dielectric phase. Thermally activated hopping conduction involving carrier self-organization in the crystal matrix with ferroelectric frustrations drives a phase transition to the state of charge-induced ferroelectricity at T ∼ 180 K.

Magnetic Properties, Magnetoresistance, and Magnetic-Field-Induced Phase Transitions in Tb 0.95 Bi 0.05 MnO 3 and Eu 0.8 Ce 0.2 Mn 2 O 5 Multiferroics

This paper reports on a study of the magnetic properties, magnetoresistance, and phase transitions in the semiconducting manganite multiferroics Tb0.95Bi0.05MnO3 and Eu0.8Ce0.2Mn2O5 whose dielectric properties have been a subject of an earlier study. An analysis of these properties has led us to the conclusion that the above crystals at temperatures T ≥ 180 K undergo phase separation with the formation of a dynamic periodic alternation of quasi-2D layers of manganese ions in different valence states, i.e., charge-induced ferroelectricity. This state exhibits a giant permittivity and ferromagnetism in the layers containing Mn3+ and Mn4+ ions. At low temperatures (T < 100 K), the phase volume is occupied primarily by the dielectric phase. Studies of the magnetic properties and the effect of the magnetic field on the dielectric properties of crystals substantiate the scenario of the formation of a state with giant permittivity put forward by us. At low temperatures, Tb0.95Bi0.05MnO3 exhibits features at the points of phase transitions in pure TbMnO3. A ferromagnetic moment is observed to exist at all the temperatures covered. At the boundaries of the quasi-2D layers, magnetic-field-induced jumps of the electrical resistivity caused by metamagnetic transitions in the layers with Mn3+ and Mn4+ ions are observed. At temperatures T ≥ 180 K, the electrical resistivity undergoes a periodic variation in a magnetic field which is a manifestation of carrier self-organization. A high magnetic field is capable of shifting the phase transition from 180 K to higher temperatures and inducing additional phase transitions.

X-ray diffraction study of ion thermal vibrations in R2CuO4 (R=Pr and Gd) crystals

An X-ray diffraction study is reported of the symmetry in the spatial distribution of thermal vibrations of Cu2+, Gd3+, and Pr3+ ions in Gd2CuO4 crystals. An analysis of the pattern of the angular thermal-vibration amplitude distribution obtained experimentally at different temperatures allows a conclusion about the character of the local Jahn-Teller effect for the Cu2+ ions, structural phase transitions, and the orbital ground state of the Cu2+ ions.

Random anisotropy in R2CuO4 (R=Nd, Pr, Sm, Gd, Eu) quasi-2D antiferromagnets

The mechanisms of random anisotropy produced by an an effective coupling between rare-earth ion moments and orbital momenta of Cu2+ ions through spin fluctuations is studied in R2CuO4 crystals. The effective random-anisotropy fields are estimated from an analysis of experimental data for R2CuO4 crystals (R=Eu, Pr, Gd).

Nonlinear magnetic susceptibility and microwave spin dynamics of R2CuO4 quasi-2D antiferromagnets (R=Eu, Pr, Gd)

An experimental study of nonlinear ac magnetic susceptibility and microwave spin dynamics of R2CuO4 quasi-2D Heisenberg antiferromagnets (R=Eu, Pr, Gd) has been carried out. The data obtained can be accounted for if one assumes the existence of a random-field (RF) state in the Eu2CuO4 and Pr2CuO4 tetragonal crystals within the 77⩽T⩽350 K range covered. If this is so, 3D antiferromagnetic order persists only within limited regions, while in CuO2 layers there are 2D Heisenberg antiferromagnetic spin fluctuations with large correlation lengths. In the Gd2CuO4 crystal there exists, besides uniform 3D antiferromagnetic long-range order with weak ferromagnetism, an admixture of an RF-type state.

Magnetic and electric properties of Eu0.62Bi0.38MnO3 and Eu0.53Bi0.32Sr0.15MnO3 crystals

Single crystals of Eu0.62Bi0.38MnO3 and Eu0.53Bi0.32Sr0.15MnO3 solid solutions crystallizing in an orthorhombically distorted perovskite structure were prepared. At temperatures above 120 K, Eu0.62Bi0.38MnO3 exhibits the properties of structural glass while remaining a dielectric at all temperatures. There is no long-range magnetic order in this compound. Eu0.53Bi0.32Sr0.15MnO3 behaves as a semiconductor above 120 K and exhibits a jump in conductivity at T = 175 K associated with a metal-insulator transition occurring within limited regions of the crystal. In these regions, there appears a ferromagnetic moment (due to double exchange mediated by charge carriers) and local electric polarization.

Spin dynamics and phase transitions in quasi-2D antiferromagnets R2CuO4 (R=Pr, Nd, Sm, Eu, Gd)

The spin and lattice dynamics of the R2CuO4 quasi-2D antiferromagnetic crystals (R=Pr, Nd, Sm, Eu, Gd) were studied in the millimeter-range electromagnetic wave band. Strong variations of the absorption coefficient were observed to occur at temperatures T≈T0. Absorption lines of electrical nature due to lattice dynamics were also revealed near the T0 temperatures. The observed anomalies are assumed to originate from phase transitions at T≈T0, which entail changes in the structural and magnetic properties.

Polarization Interaction and Phase Transitions in Crystals with Two Interacting Subsystems

The influence of the polarization interactions on the state and phase transitions in magnetic-ordered and dielectric crystals with two interacting order parameters has been investigated. Consideration is given to the case when the interaction in one of the subsystems is considerably weaker than that in the other subsystem. It is demonstrated that the polarization interactions in the weak subsystem can substantially affect the state and the character of phase transitions in the strong subsystem. These interactions can bring about the disordering (formation of the random-field state or the state of spin glass) in the critical region near the second-order phase transition in the main subsystem and also the smearing of the phase transition. At the same time, the polarization interactions can give rise to the ordered and disordered states in the weak subsystem.

Jahn-Teller effect and the orbital ground state of Cu2+ ions in Eu2CuO4 and La2CuO4 crystals

Local ion displacements and their anharmonicity in Eu2CuO4 and La2CuO4 crystals are studied by high-precision x-ray diffractometry. A symmetry analysis and the temperature behavior of the probability density function of Cu2+ ions reveal a degenerate ground orbital state in the Eu2CuO4 crystal (tetragonal doublet dxz, dyz). The pattern and anharmonicity of the local displacements are found to be determined not only by lattice vibrations but by 2D antiferromagnetic spin fluctuations in the CuO2 sheet as well. By contrast, in the La2CuO4 crystal the orbital ground state of the Cu2+ ion is a singlet (dz2). The pattern of Eu3+ local displacements and their effect on the properties of the Cu subsystem in the Eu2CuO4 crystal are also studied.