Hyperfine field anisotropy of Co in C15 RCo2 compounds (R = rare earth)

Abstract

Results of 59Co NMR investigation on the C15 type RCo2 compounds (R = rare earth) in the ferro-, ferri-, and paramagnetic states are reviewed. An enormous anisotropy in the 59Co hyperfine fields, which results in antiparallel hyperfine fields at two magnetically inequivalent Co sites in RCo2 compounds with the [111] or [011] easy axis, is attributed to unquenched orbital moment of Co atoms. Analysis of temperature dependence of 59Co Knight shifts reveals the important role of the spin-orbit coupling. Observation of well-resolved satellite lines in Tb1−xYxCo2 yields an estimate of transferred hyperfine fields due to the rare earth spins as +42 kOe in TbCo2 at 4.2 K, where the positive direction is chosen to be the direction of the Co moment. From consideration of the sign and the magnitude of lattice dipole fields, Lorentz fields, and the transferred fields, the fraction of the orbital moment on Co in the RCo2 series is found to vary from 3% in GdCo2 to 26% in PrCo2.