Papers by Author: A.I. Kurbakov

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 presented for the La_1-xSr_xCoO₃ and La_1-xCa_xCoO₃ at x = 0.15 single crystals. It is founf that these compounds are insulators and exhibit the rhombohedral R3-c space group. According to the magnetic measurements, the development of the ferromagnetic (F) clustered state proceeds into three stages. On cooling, during the first stage, the F clusters nucleate at the preferential sites that are likely produced by variation in oxygen and doping stoichiometry. The second stage is characterized by sharp increasing the concetraion of the isolated F clusters on cooling. This is the process of the homogeneous nucleation. A coalescence of the isolated F clusters into some large-scale complexes containig some ammount of the domains is associated with the third stage. Note that it is the typical behavior observed at a first order phase transition. The data allow one to determined reliably the temperature boundaries of the stages.

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.