Local fields and nuclear spin-lattice relaxation of 27Al in the heavy-fermion system YbNiAl

We report measurements of the Knight-shift, the linewidth and the spin-lattice (T1-1) and spin-spin (T2-1) relaxation rates of 27Al in the heavy fermion system YbNiAl at temperatures 1.3-70 K and in external fields between 0.3 and 8 T. From T2-1 the endpoints of the critical line of the transition to the magnetically ordered state are TN=2.6 K and Bcr=3.0 T. In the paramagnetic regime we find a linear dependence of the Knight-shift and the linewidth on the susceptibility χ with a small mean hyperfine coupling (0.011 mT/μB) but a large linewidth indicating the cancellation of stronger couplings. T1-1 is independent of T above 10 K, below it becomes strongly field and temperature dependent due to the approaching phase transition. While the high fi eld behaviour can be described consistently with χ within a simple two band model this model predicts a strong field dependence which is incompatible with the experiment. This indicates that many body effects in the Kondo-lattice can be identified much better from the field than from the temperature dependence of the physical properties. (orig.)