The hyperfine interaction of holmium in ferromagnetic rare earth alloys

Spin echo measurements have been made, at liquid helium temperatures, on 165 Ho in ferromagnetic alloys of holmium with gadolinium, terbium and dysprosium. Intra and extraionic contributions to the hyperfine interaction are discussed, with particular emphasis on conduction electron effects. The hyperfine field B p due to conduction electrons polarized by the parent holmium ion is found to be +12 T, and the field B n due to conduction electrons polarized by neighbouring ions is -8.3 S T, where S is the mean ionic spin of the alloy. The contributions from individual neighbour ions to B n are found to be additive and of moderately long range, and to depend only on the spin of the polarizing ion. Electric quadrupole splittings in the Ho-Tb and Ho-Dy alloys indicate that the electric field gradient due to conduction electrons exceeds that due to the ionic lattice by a factor of about seven.