Low temperature Kondo reduction of quadrupolar and magnetic moments in theYbCu5-xAlx series

We present measurements in the YbCu5-xAlx series, down to the 50 mK range, using Yb-170 Mossbauer absorption spectroscopy and magnetisation measuremen ts. In this series, the hybridisation between the Yb 4f electrons and the c onduction electrons is known to decrease as the Al content x increases. We apply the variational solution of the impurity Kondo problem to the interpr etation of our data. We show that the Kondo temperature can be derived from the measured 4f quadrupole moment and, for the magnetically ordered compou nds (x greater than or equal to 1.6); we obtain the exchange energy as a fu nction of the Al content. Our findings are in general agreement with Doniac h's model describing the onset of magnetic ordering according to the relati ve values of the Kondo and exchange energy scales.