Structural and magnetic transformations in the system YBaCo4O7+X (x = 0, 0.1, 0.2)

The modification of magnetic and elastic properties of YBaCo ₄O _{7 + x} ( x = 0, 0.1) cobaltite series at a small controlled deviation x = 0, 0.1, 0.2 from stoichiometry has been investigated. The non-stoichiometric cobaltites with x > 0.1 are found to be separated into two structural phases with different values of x under certain synthesis and heat-treatment conditions of the YBaCo ₄O _7+x system ( x = 0, 0.1, 0.2). A non-trivial magnetic behavior is observed in stoichiometric YBaCo ₄O ₇. Magnetic moment Δ M = M_FC– M_ZFC induced by an external magnetic field (an analog of thermo-residual magnetization) exhibits anomalies at magnetic phase transition temperatures T_{N 1} and T_{N 2} that coincide with Young’s modulus anomalies. Conversely, in the magnetic susceptibility curves taken in the FC and ZFC modes, phase transitions are not discerned. When x deviates from stoichiometry, the ratio between two magnetic ions Co ³⁺/Co ²⁺ changes, the induced moment Δ M increases, and a residual ferromagnetic moment of ∼ 10 ^-2μ _B appears in the magnetization curves at low temperatures. At a slight deviation from the oxygen stoichiometry in the samples, the structural distortion disappears, and the anomalies in the elastic characteristics near the T_N region rapidly diffuse and vanish. It has been found that when YBaCo ₄O _7+x cobaltites deviate from stoichiometry, the evolution of their magnetic properties is similar to that observed at the transition from Y-based to Ca-based cobaltite. Two scenarios of the cobalt subsystem magnetic behavior with an increasing fraction of cobalt Co ³⁺ ions have been discussed.