Critical Magnetic Fields of a Kondo Superconductor: ZnMn

The superconducting critical fields of pure Zn and of a series of ZnMn alloys have been measured as a function of temperature T down to 0.06 K. The critical fields have been used to calculate the entropy difference between the normal and superconducting states. For all the alloys studied, the entropy in the superconducting state is proportional to T as T -> 0, indicating a finite density of states at the Fermi energy. We interpret this result as evidence for the existence of localized bound states within the energy gap centered at the Mn impurities. Results obtained for the depression of To, for the critical field at T = 0, for the jump in specific heat at T-c, and for the law of corresponding states are also presented and compared with the predictions of the Abrikosov-Gor'kov theory. A simple expression relating the critical fields of these alloys to the Mn concentration has been found to be in very good agreement with experiment.