Magnetization studies have been carried out on a single crystal of TbBaCo 2 O 5.5 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 (T C = 277 K, T N1 = 210 K) and Tb (T N2 = 3.44 K) subsystems. It is shown that the spontaneous first-order phase transition at T N1 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 < T N2 for the Tb subsystem. The analysis of the magnetic properties of TbBaCo 2 O 5.5 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.

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Magnetic properties of detwinned Tb0.9Dy0.1BaCo2O5.5 single crystals have been studied. The admixture of Dy, introduced only for technological reasons, has no essential influence on the behaviour of the rare earth subsystem of this compound, which is determined by the electronic structure of the Tb3+ ions. It is shown that the Co3+ ions are Ising ions, with the easy axis along the a axis, in the temperature range 2–276 K (TC). The Tb3+ ions in the AFM phase (T