Specific heat and magnetic structure of GdT2X2 compounds (T = Cu, Ni, X = Sn, Sb)

The magnetic specific heat of GdNi2Sn2, GdNi2Sb2 and GdCu2Sb2 shows a lambda-type anomaly at T-N with a noticeable reduction of the discontinuity compared to that expected for a simple antiferromagnet. It can be associated with the existence of amplitude modulated magnetic structure just below T-N. Above T-N a weak magnetic contribution is observed, evidencing the short-range ordering. From neutron diffraction studies it can be also concluded that investigated compounds have not simple antiferromagnetic structures. For GdNi2Sn2 the propagation vector Q = (0.3, 1/2, 1/2) was found with the magnetic moments along the [1,0,0] axis.