Defect-induced magnetic fluctuations in YbB12

Abstract We present the results of nuclear spin–lattice relaxation rate (1/ T 1 ) measurements in a typical Kondo insulator YbB 12 for 10,11 B and 171 Yb nuclei. Above 20 K, 1/ T 1 at the B sites shows thermally activated temperature dependence with a gap of about 100 K. However, it shows anomalous enhancement below 15 K, which is partially suppressed by magnetic field up to 16 T. No such anomaly was observed at the Yb sites. The ratio of 1/ T 1 for 11 B and 10 B nuclei indicates that the anomaly below 15 K is caused by dilute magnetic moments assisted by nuclear spin diffusion. The origin and the nature of the low temperature magnetic fluctuations are discussed.