Solid State Phenomena Vol. 215

Abstract: A model for the description of two-subsystem Heisenberg ferrimagnet with frustrated intersubsystem exchange and competition between exchange interactions in a subsystem is proposed. The conditions of the existence of noncollinear Yafet-Kittel state and partially ordered magnetic structure are investigated. The phase diagram of competition parameter vs temperature is obtained in the mean field approximation. The peculiarities of the succesive magnetic phase transitions are considered.

Abstract: The ground state of a rhombohedrally distorted magnetic cubic lattice with frustrated exchange in the thriangular base plane of spins and competition of exchanges between the nearest and next nearest planes was investigated. The dependence of helix vector components on the exchange parameters was calculated. The phase diagram of ground state was obtained.

Abstract: It is shown that the magnetocaloric effects are absent in the spin-flop phase of an isotropic antiferromagnet at the T<T_N (T_N the Neel temperature) and appear only when an applied magnetic field exceeds the critical field of the spin-flip transition. It is displayed as well that the direct magnetocaloric effects in an antiferromagnet above T_N are much less that the analogous effects in a ferromagnet above the Curie point T_C.

Abstract: Phase transitions between incommensurate spin states in ferroelectromagnet TbMnO₃ in magnetic field along the modulation vector are phenomenologically analyzed. It is shown that the competition between different terms of the fourth order in magnetic anisotropy energy is a reason of the reorientation of spin-cycloid plane and electric polarization by 90⁰ (polarization flop). Calculated phase diagram is in a good agreement with experimental data.

Abstract: Using the resistivity method it was found that temperature dependence of the upper critical field for underdoped Nd_1.86Ce_0.14CuO_4+δ have an anomalous upward curvature of H_c2(T) dependence and can be consistently explained by the two-band/two-gap model of a dirty superconductor. Near antiferromagnetic-superconducting phase boundary the critical temperature remains constant with the change of the disorder parameter and the slope of B_c2 increases with increasing of the disorder parameter. This behavior is completely different from dependencies for pure superconducting phase at optimal doping region. This difference may indicate the change of the type of the paring: from the predominance of the anisotropic s-wave component (may be due to unstable competition between antiferromagnetic (AF) and superconducting (SC) regions) in underdoped (x=0.14) region to the prevalence of d-wave part in optimal doped regions (x=0.15) because of residual spin fluctuations.

Abstract: The effect of cobalt deficiency in a single crystal of double layered cobaltite GdBaCo_1.86O_5.32 on the magnetic properties was investigated. We have found that in the region 210 - 400 K the sample has the ferrimagnetic state with the Neel temperature (T_N ~ 400 K) which is above the Curie temperature (T_C ~ 262 K). In the paramagnetic region T = 410 - 650 K the effective magnetic moment is μ_eff = 8.78 μ_B, that points out the intermediate spin state (IS) of Co³⁺ ions.

Abstract: Within the framework of the two-phase grain model (by the example of γ-Fe203 cobalt coated particles) a theoretical analysis of the effect of uniaxial mechanical stresses on the magnetization of the system of hetero-phase interacting nanoparticle was held. It is shown that stretching leads to reduction when compression increases the coercive force Hc and doesnt change the residual saturation magnetization Irs of the system of interacting particles. "Smoothing" the hysteresis loop and "lowering" the magnetization curve, the magnetostatic interaction between particles decreases both Hc, and Irs, giving the fact that hysteresis characteristics of an ensemble of interacting nanoparticles change much more in stretching than in compression.

Abstract: The magnetization processes of HoAl₃(BO₃)₄ rare-earth aluminum borates have been studied theoretically. Magnetic properties of the crystals were examined. The dependencies of the magnetic susceptibility on the magnitude and direction of magnetic field were calculated. Study of a magnetoelectric effect was performed and the dependencies of the polarization on the strength and orientation of a magnetic field and temperature were obtained. A comparison of the theoretical and experimental data was performed, their consistency has been ascertained.

Abstract: We investigated transport characteristics of composite that consist of the superconductor MgB₂ and ferromagnetic half-metallic nanoparticles La_0.7Ca_0.3MnO₃ (LCMO). The experiments were carried out in a wide temperature range with different volume content of the magnetic impurity. It was found that over 25% of the impurity manganite breaches percolation paths of the diboride magnesium. This leads to a considerable broadening of the MgB₂ superconducting transition and the formation of the network of contacts through the ferromagnetic inclusions. This result shows the possibility of the spin - activated surface realization on the superconductor - manganite boundary. It's allowing the free transition of Cooper pairs with s wave symmetry of the order parameter in a ferromagnet with 100% spin-polarized carriers.

Abstract: The matrix Greens function of coupled spin and elastic waves for media with the 3D inhomogeneities of the coupling parameter is studied. It has been shown that the difference in shapes of the Greens functions for the cases of 1D and 3D inhomogeneities appears only for the sufficiently large correlation wave number of the inhomogeneities.