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

• The magnetic properties of the Gd1−xLaxRhIn5(x≤0.50) AFM compounds are discussed.
• GdRhIn5 can be modelled by a J=7/2 Heisenberg model on a cubic lattice.
• For x≠0x≠0, weak disorder can be described through a distribution of transition T.
• For larger x, results are not compatible with a classical spin Heisenberg model.
• This is a simple 4f (L=0) AFM system for the study of disorder and short range order.

We report the structural and magnetic characterization of La-substituted Gd1−xLaxRhIn5(x≤0.50) antiferromagnetic (AFM) compounds. The magnetic responses of pure GdRhIn5 are well described by a S=7/2 Heisenberg model. When Gd3+ ions are substituted by La3+, 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 (TN). 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 GdRhIn5 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)RhIn5 compounds.