Effect of Crystal Fields on the Magnetic Properties of Samarium Intermetallic Compounds

Neither of the intermetallics SmAl₂ (a ferromagnet below TC=120 K) and SmSn₃ (an antiferromagnet below TN=11 K) show the sign reversal predicted by White and Van Vleck for the $4f$-induced Knight shift in the paramagnetic region. On the other hand, no particular anomaly has been observed in the temperature dependence of the susceptibility. An explanation for this is given in terms of mixing of excited J levels of Sm³⁺ into the J=5/2 ground multiplet by cubic crystal fields. The calculation is described of the crystal field matrix elements of Sm³⁺ between any J and J^', and expressions are derived for the Knight shift and the susceptibility in the presence of crystalline and molecular fields. It is shown that the sixth-order component of the crystal field is important, and that the anomalous behavior is enhanced by ferromagnetic exchange between the Sm ions. A study of the lattice constants and the ²⁷Al quadrupolar coupling in comparison with other RAl₂ (R=rareearth) compounds excludes the possibility of Sm in SmAl₂ not being trivalent.