Spin-glass behaviour with extended short-range ferromagnetic order in U₂RhSi₃

For U₂RhSi₃, which crystallizes in a disordered derivative of the hexagonal AIB₂-type structure, we present a detailed investigation of magnetic properties by measurements of AC and DC susceptibility, magnetization, magnetic relaxation, specific heat and electrical resistivity. The results provide evidence for the formation of extended short-range ferromagnetic order at the temperature T_c ≈ 25 K, as indicated by a specific heat anomaly, and for a low-temperature spin-glass state with a freezing temperature T_f = 19.6 K. According to systematic specific heat experiments on isostructural spin-glass systems, short-range ferromagnetic correlations above the spin-freezing temperature are observed in U₂RhSi₃ (T_f ≈ 25 K, T_f = 19.6 K) and U₂CoSi₃ (T_c ≈ 10 K, T_f ≈ 8 K), but not in U₂PdSi₃ (T_f = 13.5 K) and U₂NiSi₃ (T_f ≈ 22 K). Different degrees of structural disorder of transition metal and silicon atoms, both on the same crystallographic site, seem to be the origin for the distinct different magnetic behaviour.