Anisotropy and asymmetry of the hyperfine spectrum of holmium in magnetically ordered RAl₂alloys

Abstract

The hyperfine interaction of holmium in ferromagnetically ordered RAl2 compounds has been studied by pulsed NMR at liquid helium temperatures. The seven-line NMR spectrum of 156Ho has been observed in alloys for which the spontaneous magnetisation lies in different crystallographic directions. The measured hyperfine splittings reflect the marked dependence of the ground state of the Ho3+ ion on the direction of magnetisation. Results are interpreted in terms of a single-ion effective-field model with no free parameters. Satisfactory agreement between theory and experiment is obtained only when terms of second order in the free-ion hyperfine interaction are included in the calculations. Among the second-order terms is a pseudo-octupole interaction which imparts a measurable asymmetry to the NMR spectra.