G. G. Levchenko

Imperfection of the nanostructure, phase transitions, 55Mn NMR, and magnetoresistive properties of La0.73+Ca0.3 − x2+Srx2+MnO3 ± δ ceramics

Magnetoresistive ceramic samples La0.7Ca0.3 − xSrxMnO3 ± δ sintered at temperatures of 1150 and 1350°C are investigated using X-ray diffraction, microscopic, resistance, and magnetic (χ, 55Mn NMR) measurements. The specific features of the influence of the composition on the type and parameters of the perovskite structure, its imperfection, the porous crystallite structure, the metal-semiconductor and ferromagnetic-paramagnetic phase transitions, the 55Mn NMR spectra, and the magnetoresistance effect are established. The magnetic phase diagram is constructed. The conclusions are drawn regarding the nonuniformity of the distribution of ions and vacancies around manganese involved in the high-frequency electron-hole exchange (Mn3+ ai Mn4+) and the nanostructured separation of the perovskite structure containing anion and cation vacancies, with the concentrations and magnetoresistance effect decreasing and the lattice parameters and phase transition temperatures increasing as calcium is replaced by strontium.

Spin-wave electrodynamics of the interface between a magnetoelectric multiferroic and a nonmagnetic insulator

The features of refraction (including the Goos-Hánchen effect) of a TM or TE bulk electromagnetic wave incident from outside on the surface of a chiral magnetoelectric with the homogeneous antisymmetric magnetoelectric interaction has been analytically studied on an example of the two-sublattice model of a multiferroic. In particular, it has been shown that the requirement that the diagonal components of the permeability and permittivity tensors are simultaneously negative is not necessary for the implementation of the properties of a left-handed medium. The results have been generalized to the case of an easy-axis antiferromagnet with an antisymmetry center with the 4z±2x+I− structure in the static external magnetic field perpendicular to the easy axis of magnetization. Some results of this work were preliminarily reported in D.V. Kulagin, G.G. Levchenko, A.S. Savchenko, A.S. Tarasenko, and S.V. Tarasenko, JETP Lett. 92, 511 (2010).

Controllable dimensional crossover of magnetic behavior in single- crystal GdBaCuO with variable oxygen content

Abstract Superconducting properties of GdBa 2 Cu 3 O 6+ δ single crystals of tetragonal symmetry are studied at various oxygen contents ( δ =0.2; 0.4; 0.56; 0.75). It is shown that transition from three-dimensional behavior to quasi-2D and further to strong two- dimensional behavior occurs as oxygen content is reduced. For the first time the single crystal of oxygen-deficient 1-2-3 is found to demonstrate the strong 2D-behavior expected of a stack of ultrathin films with no coupling between them. The superconducting transition in the samples with quasi-two-dimensional behavior is found to have features of a Berezinskii- Kosterlitz-Thouless transition. Fluctuations of the order parameter in the vicinity of the transition temperature change their dimensionality from 3D to 2D while oxygen content decreases.

Structural and magnetic inhomogeneities, phase transitions, 55Mn nuclear magnetic resonance, and magnetoresistive properties of La0.6 − xNdxSr0.3Mn1.1O3-δ ceramics

The structure, lattice imperfection, and properties of ceramic samples La0.6 − xNdxSr0.3Mn1.1O3-δ (x = 0–0.4) have been investigated using the X-ray diffraction, resistive, magnetic (χac, 55Mn NMR), magnetoresistive and microscopic methods. It has been shown that there is a satisfactory agreement between the concentration decrease in the lattice parameters a of the rhombohedral (x = 0, 0.1, 0.2) and cubic (x = 0.3, 0.4) perovskite structures and the average ionic radii

Phase separation in strained cation- and anion-deficient Nd0.52Sr0.48MnO3 films

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Structural and magnetic inhomogeneity, phase transitions, magnetoresonance and magnetoresistive properties of La0.6 − xPrxSr0.3Mn1.1O3 (x = 0–0.6)

for the lattice containing anion vacancies, cation vacancies, and nanostructured clusters with Mn2+ ions in A-positions. With an increase in the neodymium concentration x, the vacancy-type imperfection increases, the cluster-type imperfection decreases, the temperatures of metal-semiconductor phase transition Tms and ferromagnetic-paramagnetic phase transition TC decrease, and the content of the ferromagnetic phase decreases. The anomalous hysteresis is associated with the appearance of unidirectional exchange anisotropy induced in a clustered perovskite structure consisting of a ferromagnetic matrix and a planar antiferromagnetic cluster coherently coupled with it. An analysis of the asymmetrically broadened 55Mn NMR spectra has revealed a high-frequency electronic double exchange (Mn3+-O2−-Mn4+) ↔ (Mn4+-O2−-Mn3+) and an inhomogeneity of the magnetic and charge states of manganese due to the heterogeneous environment of the manganese ions by other ions and defects. The observed changes in the resonant frequency and width of the resonance curve are caused by changes in the ratio Mn3+/Mn4+ and magnetic inhomogeneity. An increase in the neodymium concentration x leads to a decrease in the ferromagnetic phase content determined from the dependences 4πNχac(T) and the 55Mn NMR curves. The phase diagram characterizes an interrelation between the composition, the imperfection of the structure, and the transport, magnetic, and magnetoresistive properties of lanthanum neodymium manganite perovskites. It has been found that there is a correlation between the imperfection, magnetic inhomogeneity, coercive force, and magnetoresistance effect exhibited by the perovskite structure.

Properties of Evanescent Waves in Polarized Media in a Constant External Electric Field: II. The Noncompensated Antiferromagnetic

Microstructure and magnetic properties of BiMnO3 and BiFe0.5Mn0.5O3 films, prepared by rf magnetron sputtering on LaAlO3 (001) single-crystalline substrate, are investigated. The selected-area electron diffraction analysis allows us to identify the crystal structure of the BiMnO3 film as orthorhombic, while the BiFe0.5Mn0.5O3 film has a hexagonal lattice symmetry. High-resolution electron microscopy study reveals the presence of strip-domain phase with a periodic spacing of about 3c in both films. Magnetic measurements show that in addition to the basic paramagnetic phase the films exhibit Griffiths phase behavior in a wide temperature range. We argue that the observed weak ferromagnetism is due to the strip-domain layered inclusions, rather than intrinsic physical origin of the films.

Structural and magnetic heterogeneities, phase transitions, and magnetoresistance and magnetoresonance properties of the composition ceramic La0.7Pb0.3 − xSnxMnO3

The magnetic and transport properties of anion- and cation-deficient Nd0.52Sr0.48MnO3 films with different thicknesses, as well as of two films from this system grown on different SrTiO3 and LaAlO3 substrates, are studied. Below Curie temperature TC, the films with different thicknesses exhibit phase separation: they represent magnetic clusters (drops) embedded in a nonconducting paramagnetic (at T > TN, where TN is the Néel temperature) or antiferromagnetic (T < TN) matrix. The temperature dependences of the resistivity of the films are well described in terms of the polaron mechanism of conduction. In external magnetic field H = 0.01 T, the drops may reach 15 nm in size. They consist of magnetic polarons with a small radius (1–2 nm). The drops are shown to interact with each other in the films. Because of competition between drop-drop dipole interaction and the magnetic energy, the drops disintegrate into droplets with a size comparable to that of a magnetic polaron in a field of 1 T. An explanation is given for the discrepancy between our results and the frequently observed growth of the drops with a rise in the external magnetic field. As the film gets thicker, the fraction of the ferromagnetic phase grows with thickness nonlinearly. In the film grown on SrTiO3 (compressed by 0.9%), the characteristic Néel and Curie temperatures are lower than in the film grown on LaAlO3. The diameters of ferromagnetic drops (both maximal at H = 0.01 T and minimal at H = 1 T) turn out to be roughly the same as in the films with different thicknesses.