Sign reversal of the 4f contribution to the hyperfine field at Sm in paramagnetic samarium compounds

In paramagnetic rare-earth compounds, in the presence of an applied magnetic field, the $4f$ electrons produce a field at the rare-earth nuclear site, which, for the free Sm³⁺ ion, is positive at all temperatures. It is shown that because of crystal-field effects, the $4f$ contribution to the hyperfine field at Sm reverses its sign at one or two temperatures, unlike in the free ion. Exchange interaction between Sm³⁺ ions, in the presence of crystal fields, may induce or suppress the sign reversals in the hyperfine field.