MAGNETIC PHASE-DIAGRAM OF THE 4-SUBLATTICE ANTIFERROMAGNET TB2O2SO4 FOR A PARTICULAR FIELD DIRECTION

Tb2O2SO4 orders antiferromagnetically at 3.9 K in a four-sublattice st ructure with two of the nearest-neighbour moments antiparallel and two almost perpendicular to that of a central ion. Specific-heat and magn etization measurements were carried out and allowed to establish the p hase diagram for the external magnetic field along one of the moments' direction. Starting at the Neel temperature, the boundary of the para magnetic phase is first shifted to lower temperatures for increasing f ield and then it stays at constant temperature for further increase of field. The shift is caused by the reduction of the staggered field th at is existing in the antiferromagnetic phase. The experimental result s are corroberated to a large extent by mean-field calculations.