Magnetic properties of YbNi<Subscript>5</Subscript> from <Superscript>170</Superscript>Yb Mössbauer and magnetisation measurements

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