Effect of Oxygen Nonstoichiometry on the Magnetic Phase Transitions in Frustrated YBaCo4O7 + x (x = 0, 0.1, 0.2) Cobaltites

Abstract

The structural and elastic characteristics of YBaCo₄O_{7 + x} (x = 0, 0.1, 0.2) cobaltites synthesized by various technologies and having various excess oxygen contents x are experimentally studied. The distortion of the crystal structure in stoichiometric samples is found to remove frustrations and to bring about a longrange magnetic order in the cobalt subsystem, which is accompanied by well-pronounced anomalies in the elastic properties in the temperature range of a magnetic phase transition T_N. At a weak deviation from oxygen stoichiometry, the structure distortion disappears, frustrations are retained, and the further development of a long-range magnetic order is hindered. As a result of an absent long-range magnetic order, the anomalies of the elastic characteristics at T_N smooth rapidly and disappear. This finding points to the suppression of structural and magnetic transitions in nonstoichiometric samples and to the conservation of only short-range correlations of order parameter. It is found that nonstoichiometric samples can be separated into two phases depending on the ceramic synthesis conditions.