Magnetic properties of YbNi 5 from 170 Yb Mössbauer and magnetisation measurements

From Yb 170 Mössbauer measurements on YbNi 5 , we establish: the Yb 3+ ground state Kramers doublet is well isolated and has uniaxial anisotropy, the compound magnetically orders at 0.55 K, the Yb 3+ moments are aligned along the hexagonal axis, they have a saturated value of 3.9μ B and a thermal variation close to a mean field S=1/2 law. We obtain the strength of the Yb 3+ –Yb 3+ coupling which we find is considerably smaller than that between the rare earths (R 3+ ) in the other RNi 5 . 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 Yb 3+ 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 Yb 3+ –Yb 3+ coupling. We comment on the Yb 3+ crystal field properties. Copyright EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2007