Dilution effects in spin 7/2 systems. The case of the antiferromagnet GdRhIn5

Abstract We report the structural and magnetic characterization of La-substituted Gd 1 − x La x RhIn 5 ( x ≤ 0.50 ) antiferromagnetic (AFM) compounds. The magnetic responses of pure GdRhIn 5 are well described by a S =7/2 Heisenberg model. When Gd 3+ ions are substituted by La 3+ , the maximum of the susceptibility and the inflection point of the magnetic specific heat are systematically shifted to lower temperatures accompanied by a broadening of the transition. The data is qualitatively explained by a phenomenological model which incorporates a distribution of magnetic regions with different transition temperatures ( T N ). The universal behaviour of the low temperature specific heat is found for La (vacancies) concentrations below x =0.40 which is consistent with spin wave excitations. For x =0.5 this universal behaviour is lost. The sharp second order transition of GdRhIn 5 is destroyed, as seen in the specific heat data, contrary to what is expected for a Heisenberg model. The results are discussed in the context of the magnetic behaviour observed for the La-substituted (Ce,Tb,Nd)RhIn 5 compounds.