Magnetic properties of YbNi5 from 170Yb Mössbauer and magnetisation measurements

From Yb170 Mossbauer measurements on YbNi5, we establish: the Yb3+ ground state Kramers doublet is well isolated and has uniaxial anisotropy, the compound magnetically orders at 0.55 K, the Yb3+ moments are aligned along the hexagonal axis, they have a saturated value of 3.9μB and a thermal variation close to a mean field S = 1/2 law. We obtain the strength of the Yb3+–Yb3+ coupling which we find is considerably smaller than that between the rare earths (R3+) in the other RNi5. From a study of the spin dynamics, we find that dynamic short range order is present just above the ordering temperature and that above 2 K, the relaxation rate of the paramagnetic Yb3+ spins follows a T-linear Korringa law with a relatively pronounced slope which we link to a high density of states at the Fermi level. Magnetic susceptibility and magnetisation measurements establish the ferromagnetic nature of the Yb3+–Yb3+ coupling. We comment on the Yb3+ crystal field properties.