MAGNETIC-ORDERING AND HYBRIDIZATION IN YBAUCU4

We report on a Yb-170 Mossabauer spectroscopy investigation of the low temperature magnetic properties of the cubic heavy electron material YbAuCu4. In the antiferromagnetically ordered phase (T(N) = 1 K), we measure a 20 % reduction of the Yb3+ saturated spontaneous moment with respect to that of the crystal field GAMMA(7) ground state and we evidence a sizeable anisotropy of the interionic magnetic interaction. We interpret these features in terms of 4f electron - conduction electron hybridisation, which accounts for the heavy electron properties. The energy scale k(B)T0 of hybridisation is estimated to be 0.3 K. The Mossbauer spectra in the ordered and paramagnetic phases show inhomogeneous line broadenings which are interpreted as arising from a distribution of local strains along the <111> crystal axes, of mean magneto-elastic energy 0.2-0.4 K.