Magnetocaloric effect and magnetic ordering in GdFe_1-TSi, T = Cr, V, Ni

The intermetallic compounds GdFe_1-CrSi, x = 0-0.8, GdFe_1-VSi, x = 0-0.4, and GdFe_1-NiSi, x = 0-0.4 with a tetragonal CeFeSi (P4/nmm) structure type have been synthesized. The Curie temperature, T_C, sharply increases from 130 K to 255 K and 250 K for the GdFe_1-CrSi and GdFe_1-VSi compounds and decreases to 104 K for GdFe_1-NiSi. Within the framework of the model of effective d-f exchange interaction in R3̄d intermetallics, these changes in T_C can be caused by the corresponding changes in the density of states. The electronic structure, magnetic moments and types of magnetic orderings of the GdFe_1-TSi, T = Cr, V, Ni intermetallic compounds were calculated using the DFT+U theoretical method. For the GdFe_1-NiSi system, the transformation of a ferromagnet with the composition x = 0 into an antiferromagnet with the composition x = 0.3 was established experimentally and using first-principles calculations. The correlation of the ferromagnetic or antiferromagnetic type of the magnetic state in the GdFe_1-NiSi compounds with the value of the lattice parameter c to greater or less than the critical value c = 6.72 Å for the GdCoSi antiferromagnet has been experimentally established. The magnetic structures of the antiferromagnets GdFe_0.7Ni_0.3Si and GdCoSi were found to be different. GdFe_0.7Ni_0.3Si is characterized by a collapse in the magnetocaloric effect via a change in the isothermal magnetic entropy ΔS_M(T_C). The compounds GdFe_0.4Cr_0.6Si with -ΔS_M(T_C) = 2.37 J kg^-1 K^-1 at T_C = 255 K and R_C = 82.07 J kg^-1 and GdFe_0.7V_0.3Si with -ΔS_M(T_C) = 2.06 J kg K^-1 at T_C = 250 K and RC = 96.02 J kg^-1 in a field changing to 17 kOe could be of practical interest due to the T_C being close to room temperature.