Anisotropy of the magnetic susceptibility of CeCu6-xAux near the quantum phase transition

We report on the low-temperature dc magnetic susceptibility of CeCu6-xAux (x=0.1) in very small fields B=100 μT and 1 mT. For x=0.1, i.e. for the quantum critical concentration, we confirm the anomalous exponent α of the susceptibility along the easy c-direction, χc-1∼θ+Tα with α=0.8 observed previously in moderate fields B=100 mT (see, A. Schroeder, et al., Nature 407 (2000) 351). However, below 240 mK we see a clear additional contribution to χc. At the lowest measuring temperature T=40 mK, the ratio χc:χa:χb=4:1.5:1 of the susceptibility anisotropy is weaker than at higher T where the ratio is 10:2:1. This is caused by a steeper increase of χa and χb as compared to χc