MAGNETIC INSTABILITY OF THE GAP STATE IN CENISN AT VERY LOW-TEMPERATURE

The low-temperature magnetic property of the gap-type Kondo lattice compound CeNiSn has been investigated down to 0.08 K by the Sn-119 nuclear magnetic resonance (NMR) technique. The temperature dependence of Sn-119 nuclear-spin-lattice relaxation rate (1/T1) down to 0.4 K was well interpreted by a model that the quasi-particle density of states has a V-shaped structure of the pseudogap proportional to the energy in the vicinity of the Fermi level. Below 0.13 K, however, (1/T1) is found to decrease exponentially as exp (-DELTA/k(B)T) due to the apperance of the energy gap of DELTA-0.25 K. The NMR spectrum starts to broaden below 0.13 K, signifying that there develops a static magnetic correlation with a magnetic moment estimated to be as small as 10(-3) mu(s)/Ce atom. It is suggested that the pseudogap state is changed into the "spin gap" state induced by the development of the static magnetic correlation.