PHASE-DIAGRAM AND MAGNETIC-STRUCTURES OF THE 2-LEVEL SINGLET ANTIFERROMAGNET HO2O2SO4

The orthorhombic holmium oxisulphate orders as a two-sublattice antife rromagnet at T = 3.5 K. In external fields along the crystallographic a- and c-directions with large and medium-sized magnetic moment, respe ctively, a ferrimagnetic phase with 1/3 of the saturation magnetizatio n is passed before the paramagnetic phase is reached. Calculations in mean-field theory reveal that for the c-direction the ferrimagnetic ph ase is not stable at T = 0, it only exists for finite temperatures. Ma gnetization and susceptibility contain large contributions of van Vlec k paramagnetism which at any rate have to be taken into consideration. The phase diagram for the two field directions and the magnetic struc tures of the different phases are established.