The magnetic specific heat of GdNi_{2}Sn_{2}, GdNi_{2}Sb_{2} and GdCu_{2}Sb_{2} shows a λ-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 GdNi_{2}Sn_{2} the propagation vector Q=(0.3,1/2,1/2) was found with the magnetic moments along the [1,0,0] axis.
The specific heat of (Cd_{1-x-y}Zn_{y}Mn_{x})_{3}As_{2} with low Mn concentration (x ≤ 0.067) and for two Zn contents (y = 0.14 and y = 0.34) has been measured in the temperature range of 1.5-30 K. The magnetic contribution to the total specific heat has been analysed within our generalized pair approximation model which takes into account the complicated tetragonal crystal structure of Cd_{1-x-y}Zn_{y}Mn_{x})_{3}As_{2}. Assuming that the total Mn-Mn interaction strength is a sum of superexchange and the Bloembergen-Rowland exchange, we have obtained a very good agreement between our approach and experiment by using the analytical formulae with only two adjustable parameters, i.e. the first nearest-neighbour exchange constants for both mechanisms which appear to be strongly dependent on Zn content.
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