Magnetic phase transitions in TbBaCo₂O_5.5single crystals

Abstract

Magnetization studies have been carried out on a single crystal of TbBaCo2O5.5 in the temperature range 2–350 K and magnetic fields up to 50 kOe. Several spontaneous magnetic phase transitions have been observed and analysed in both Co (TC = 277 K, TN1 = 210 K) and Tb (TN2 = 3.44 K) subsystems. It is shown that the spontaneous first-order phase transition at TN1 from an antiferromagnetic to a weak ferromagnetic state is accompanied by an intermediate state. The properties of this state are strongly determined by the thermo-magnetic history of the sample. Metamagnetic type transitions have been observed at 140 K< T<190 K for the Co subsystem and at T<TN2 for the Tb subsystem. The analysis of the magnetic properties of TbBaCo2O5.5 points to the strongly anisotropic character of the Tb and Co subsystems. The Tb subsystem was shown to be of the Ising type. For this subsystem, the exchange interactions between nearest neighbours and next-nearest neighbours were determined. The observed properties of the Co subsystem also point to the Ising character of the Co ions. The effect of the twinning structure on magnetization processes of the weak ferromagnetic phase, especially hysteresis curves, is discussed.