Temperature Dependence of the Sm3+ Spin 〈 S(T) 〉 in Intermetallic Compounds

The temperature dependence of the Sm3+ spin 〈 S(T) 〉 in the NaCl cubic intermetallic compounds SmP, SmAs, SmSb and SmBi has been studied by the observation of the nuclear magnetic resonance (NMR) of the nonmagnetic constituent, e.g., 31P:SmP etc. The Knight shifts in these intermetallic compounds were found to consist of two principal contributions: (1) a temperature-independent term K0 equal to the Knight shifts in the nonmagnetic isomorphs, e.g., 31P:LaP etc., and (2) a temperature-dependent term, proportional to the Sm3+ spin 〈 S(T) 〉. In all the compounds, the temperature-dependent part of the Knight shift reversed sign at T≃300°K in agreement with theory. The temperature-dependence of the Sm3+ spin 〈 S(T) 〉 was calculated for a Sm3+ ion in an octahedral cubic field and an exchange energy of the form 2βHexS. The 31P Knight shift in SmP was measured between 4.0 and 575°K with the result that good agreement between theory and experiment was obtained using: (1) a crystal-field splitting of 130°K between an upper quartet and lower doublet of the J=52 ground state and (2) an exchange field Hex=λS of strength λβ/k=1.5°K.