Solid State Phenomena Vols. 168-169

Abstract: The problem of frustrations and phase transition appearance or suppression is studied on the base of exact analytical solutions for maximum eigenvalue of Kramers-Wannier transfer matrix in the Ising model on a square lattice [1], a triangular and honeycomb lattices [2], and kagome lattice [3] with allowance for the interactions between nearest J is studied. We also studied these phenomena by the numerical “replica Monte Carlo method”, taking also into account the interactions between next-nearest neighbors J'.

Abstract: The crystal structure of manganite under pressure is investigated. The influence of external hydrostatical pressure is studied. The theoretical study of magnetic interactions and NMR spectra under pressure is carried out. The influence of Ga-doping is discussed.

Abstract: We present a short overview of different microscopic models for nonrelativistic and relativistic magnetoelectric coupling in 3d oxides including so-called "spin current scenario" and ab-initio calculations.

Abstract: There are presented core-level 3s X-ray photoelectron spectra of some 3d oxides. The possibility to extract additional information on intra- and interatomic exchange effects from 3s X-ray photoelectron spectra is demonstrated. The results pointed to the possibility of using X-ray photoelectron 3s spectra for estimation of local spin magnetic moments localized on 3d elements in manganese and iron oxides. We have found that for doped manganites, the Mn formal valency from 3+ to 3.3+ is not accompanied by any decrease in the Mn 3s splitting. This fact can be explained by the holes in the O 2p states (d4L ground-state configuration). For Cu, Ni, and Co oxides, the charge-transfer effect dominates the multiplet effect in the 3s spectra which allows to estimate the occupancy of 3d states.

Abstract: Results of structural neutron diffraction study and data on the transport and magnetic properties (the linear and nonlinear (second and third order) susceptibilities) are reported for the La1-xSrxCoO3 (x = 0.18 and 0.2) polycrystals. The data on the linear and third-order susceptibilities show indications of a long - range ferromagnetic ordering with the Curie temperature (TC) about 180 K. Measurements of the second harmonic of the magnetization in the parallel ac and steady (H) magnetic fields (M2(H,T)) reveal the appearance of ferromagnetic clusters in a paramagnetic matrix below some temperature (T*) above TC. The characteristics of this clustered state and the stages of its temperature evolution are reliably determined since the non-ferromagnetic matrix affects weakly this state. It is found that a spontaneous nucleation of the ferromagnetic clusters occurs below T*, and their concentration increases sharply on cooling. Upon further decreasing temperature, the isolated clusters coalesce, leading to a percolative - type origination of a ferromagnetism. The M2(H,T) response of isolated clusters and its T-variation, including the onset of the coalescence, are very close to those found in the doped manganites, which supports an assumption on a common origin of the clustered state in these compounds.

Abstract: We consider a metamagnetic phase transition of itinerant electrons in the metamagnetic- ferromagnetic metal junction. The current flow between a ferromagnetic metal and a metamagnetic metal produces the non-equilibrium spin imbalance acting as an effective magnetic field and initiating the first-order type transition from low- to high-magnetization states of the metamagnet in the vicinity of the ferromagnet. We show that the current dependence of the length of high-magnetization state region diverges at some threshold value, due to nonequilibrium shift, generated in a contact between the high and low magnetization states of the metamagnetic metal.

Abstract: The magnetic/spin-state phase diagram of the (Pr1-yEuy)0.7Ca0.3CoO3 series was obtained on the basis of measurements of the specific heat, thermal expansion, magnetization and resistivity. The phase diagram reveals three different states depending on the static distortions (Eu content), the oxygen-isotope mass, and the temperature. The samples with the lower Eu concentrations are ferromagnetically ordered up to moderate temperatures (about 50 K),, most probably, due to the low-spin Co4+ – intermediate-spin Co3+ interaction of the double-exchange type. As the Eu doping increases, the Co3+ LS (S = 0) state becomes stabilized and the magnetic ordering of the Co4+ ions is suppressed up to temperatures well below 5 K, resulting in a low-temperature anomaly in Cp. At higher temperatures, we observe a first-order spin-state transition from the LS to the IS state of Co3+, which is accompanied by a decrease in the electrical resistivity, an increase in the magnetization, and a strong lattice expansion.

Abstract: The Hubbard model with strong correlations is treated in the many-electron representation of the Hubbard's operators. The regions of stability of saturated and non-saturated ferromagnetism in the n–U plane for the square and simple cubic lattices are calculated. The role of the singularities of bare density of states for the magnetic phase diagram is discussed. A comparison with the results of previous works is performed.

Abstract: Magnetic properties of undoped samples of lanthanum manganites in the temperature range 77-650K are reported. The ferromagnetic Curie temperature increases when the unit-cell volume decreases and the number of cation defects increases. Uncorrelated polarons and noninteracting moments of Mn ions are present in the region TC < T < 440K. Above 2TC < 440 K < T, the nonstoihiometric LaMnO3 compound passes into the homogeneous paramagnetic state.

Abstract: Ferrogarnets of the system {Y3}[Fe2-xScx](Fe3)O12 were investigated by neutron diffraction and magnetic methods. Parameters of crystal structure were refined. It was found that deviations from a collinear ferrimagnetic structure emerged at x ~ 0.7. At high concentrations of Sc a growth of the high-field magnetic susceptibility and an abnormal temperature dependences of dynamic susceptibility typical of local angular spin structures were revealed. The fracture temperatures for the local angular structures were determined.