LOCAL-FIELDS AND NUCLEAR-SPIN-LATTICE RELAXATION OF AL-27 IN THE HEAVY-FERMION SYSTEM YBNIAL

We report measurements of the Knight-shift, the linewidth and the spin -lattice (T-1(-1)) and spin-spin (T-2(-1)) relaxation rates of Al-27 i n the heavy fermion system YbNiAl at temperatures 1.3-70 K and in exte rnal fields between 0.3 and 8 T. From T-2(-1) the endpoints of the cri tical line of the transition to the magnetically ordered state are T-N = 2.6K and B-cr = 3.0T. In the paramagnetic regime we find a linear d ependence of the Knight-shift and the linewidth on the susceptibility X with a small mean hyperfine coupling (0.011mT/mu(B)) but a large lin ewidth indicating the cancellation of stronger couplings. T-1(-1) is i ndependent of T above 10K, below it becomes strongly field and tempera ture dependent due to the approaching phase transition. While the high field behaviour can be described consistently with X within a simple two band model this model predicts a strong field dependence which is incompatible with the experiment. This indicates that many body effect s in the Kondo-lattice can be identified much better from the field th an from the temperature dependence of the physical properties.