Calorimetric Investigation of Magnetic Transitions in GdPdAl and TbPdAl

A comparative specific heat (C\(_{P}\)) study of GdPdAl and TbPdAl compounds crystallizing in hexagonal ZrNiAl-type crystal-structure (space group P\(\bar{6}\)2 m) is presented. Consistent with earlier reports both the compounds show the signature of two magnetic transitions in magnetization data. The magnitude of the jump in C\(_{P}\) at the high-temperature transition at T\(_{N1}\) (\(\sim \)47 K ) in GdPdAl indicates ordering into an amplitude-modulated magnetic structure. The analysis of magnetic entropy change (S\(_{4f}\)) showed that about one-half of total S\(_{4f}\) occurs below low temperature transition at T\(_{N2}\). This is in contrast to that seen in TbPdAl, where only one-third of the entropy of transition occurs below T\(_{N2}\). For both the compounds S\(_{4f}\) tends to saturate to about 84% (for T > T\(_{N1}\)) of that expected for complete ordering of the rare earth moments, indicating incomplete removal of geometrical frustration.