Magnetic and nonmagnetic impurities in some A-15 structure systems. II. Magnetic properties

We present measurements of the susceptibility of the A-15 structure systems NbyPt_100–y–x M _x where M = Fe and Ru (y = 73–78). These measurements correlated with those of the critical temperature T _c permit us to conclude that there is a saturation of the effective number of magnetic impurity atoms and that these atoms are situated on the chains. We find also that the magnetic moment of the Fe atoms depend on the value of y. The high value of the Curie temperature and its variation is discussed with the aid of a model of pairs with antiferromagnetic interaction. In order to describe the formation of the localized moments we have taken into account the effect of the environment around each impurity. Depending on the number and the nature of the nearest neighboring atoms, we deduce a magnetic or nonmagnetic behavior of the impurity. The variation of the moment as a function of the nearest neighbors is explained by supposing electronic redistribution between the atoms, which is the same as supposing an interatomic charge transfer.     

Magnetic and nonmagnetic impurities in some A-15 structure systems. I. Superconducting properties

We present measurements of the critical temperature T _c and the critical field H _c2 of the A-15 structure systems Nb _y ,Pt_100–y–x M _x where M = Fe and Ru (y = 73–78). T _c is influenced by two effects: one due to the magnetic Fe atoms, which causes a strong decrease, and the other due to the nonmagnetic effect of the Fe atoms, which causes a smaller but non-negligible effect. For the study of the nonmagnetic effect we introduced Ru impurities instead of Fe. Anomalous high critical field values are found as T 0, and the curves H _c2 versus T do not follow the WHH theory generalized to include magnetic impurities. The introduction of an empirical parameter permits us to fit our experimental results. This abnormal behavior of H _c2 is discussed in terms of strong-coupling and anisotropy effects.