YBaCo4O7 + x (x = 0, 0.1) System: From Antiferromagnetism to Ferromagnetism

The modification of magnetic and elastic properties of YBaCo₄O_{7 + x} (x = 0, 0.1) cobaltites at a slight controlled deviation from stoichiometry (x) has been investigated. The magnetic properties of stoichiometric YBaCo₄O₇ demonstrate nontrivial behavior, which is inconsistent with the generally accepted notion of phase transitions with long-range magnetic order. Only magnetic moment ΔM = M_FC – M_ZFC induced by an external magnetic field (an analog of thermoremanent magnetization) exhibits anomalies at magnetic phase transition temperatures T_N1 and T_N2 that coincide with those of Young’s modulus anomalies, whereas in the magnetic susceptibility curves taken in the FC and ZFC modes, phase transitions are not discerned. At a small off-stoichiometry (x = 0.1), induced moment ΔM rises by an order of magnitude and a residual ferromagnetic moment of about 10^–3 μ_B arises in the magnetization curves. Two scenarios of the cobalt subsystem magnetic behavior with increasing part of cobalt ions Co³⁺ have been discussed. It has been found that when YBaCo₄O_{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.