NMR Studies on Magnetic Impurity Effect in Type II Superconductors

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Abstract
The nuclear spin-lattice relaxation time T 1 of Al 27 in a type II superconductor La 3- x Gd x Al has been measured as a function of concentration of Gd impurity, x , as well as temperature and magnetic field. The experimental results can be understood well on the basis of the current theory of magnetic field induced gapless superconductivity in the dirty limit, as in the case of V 3 Sn previously published by us. Relaxation rate in superconducting state in the case of x =0.2 at.% is a decreasing function of the external magnetic field. This anomalous relaxation behavior is contrary to the case of spin diffusion, in which case T 1 is a decreasing function of the external magnetic field. These facts suggest that this relaxation mechanism is due to the indirect coupling between nucleus and magnetic impurities. The experimental result for x =0.5 at.% is also discussed.