High static magnetic field magnetization measurements have been performed up to 23 T on Ho(0.43)Y(2.57)Fe(5)O(12) single crystals at helium temperature (T = 4.2 K) with fields applied along the three main cubic axes: [Formula: see text], [Formula: see text] and [Formula: see text]. The change from the spontaneous ferrimagnetic structure in zero magnetic field to the fully ferromagnetic one in high field takes place through several intermediate phases separated by transitions with step-like magnetization behaviour, but without any observed hysteresis. Using the effective spin Hamiltonian approximation, we show that the general features of these transitions can be accounted for by a large magnetocristalline anisotropy of the Ho(3+) moments of the uniaxial type along the local z axis of each rare-earth site. The model is in better agreement with the experiments than its Ising limit, widely used before, but is still unsuccessful in predicting the 'umbrella' magnetic structures found by previous neutron and NMR experiments.

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Some precise magnetization measurements have been performed on Ho 0.24 Y 2.76 Fe 5 O 12 single crystals under static magnetic fields up to 16 Tesla and in the temperature range (2-30 K). As previously observed in pulsed magnetic fields, the change from the spontaneous ferrimagnetic structure in zero magnetic field to the fully ferromagnetic one in high field, takes place through several intermediate phases separated by step-like magnetization jumps. Whole H-T phase diagrams have been determined for the three main cubic axes and also when the sample can rotate freely. We show that the Ising model, commonly used to account for the large magnetocristalline local anisotropy of the Ho 3+ moments, is in worst agreement with our static measurement than with the previous pulsed fields experiments and then a more realistic model is needed and will be discussed.

H–T magnetic phase diagrams of the Ho0.43Y2.57Fe5O12 garnet, due to spin-reorientation transitions, have been determined in the low temperature range (2–30 K) by magnetization measurement under high static magnetic fields (23 T) on [111] and [110] oriented single crystals. It is shown that a very good agreement between computed and observed phase diagrams can be achieved when the free energy is calculated by direct diagonalization of a Hamiltonian including the crystal field (CF) and the exchange interactions considered in the mean-field formalism.