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

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