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

Neither of the intermetallics Sm${\mathrm{Al}}_2$ (a ferromagnet below $T_C=120\mathrm{}$ K) and Sm${\mathrm{Sn}}_3$ (an antiferromagnet below $T_N=11\mathrm{}$ 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 ${\mathrm{Sm}}^{3+}$ into the $J=\frac{5}{2}$ ground multiplet by cubic crystal fields. The calculation is described of the crystal field matrix elements of ${\mathrm{Sm}}^{3+}$ between any $J$ and $J^{\prime }$, 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 ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$ quadrupolar coupling in comparison with other $R{\mathrm{Al}}_2$ ($R=\mathrm{rare}\mathrm{earth}$) compounds excludes the possibility of Sm in Sm${\mathrm{Al}}_2$ not being trivalent.