MULTIPLE MAGNETIC PHASE-TRANSITIONS IN THE TETRAGONAL GDPT2SI2 AND GDAG2 COMPOUNDS

The magnetic properties of the tetragonal GdPt2Si2 and GdAg2 compounds have been investigated by specific heat, resistivity and magnetization measurements. Both compounds exhibit at least one change of the magnetic structures at a temperature below their Neel temperature T(N) = 9.9 and 22.7 K, respectively. These structures have been determined by neutron diffraction at a very short wavelength. They are modulated incommensurate antiferromagnetic-type, the propagation vectors being Q = (1/3, 1/3, 1/2) and (0.362, 0, 1), respectively. For both compounds, the phase transitions occurring within the ordered state correspond to a sudden change of the moment direction. This feature, quite surprising for S-state compounds, reveals the existence of anisotropy terms related to the exchange interactions.