The Josephson effect applied to the study of superconducting magnetic alloys

The coexistence of superconducting and magnetic order in ternary rare earth compounds (Chevrell phases) is very important for the high field properties of new superconducting materials. By studying proximity sandwiches, where is a normal magnetic alloy and S is a pure superconductor, we can simulate such a ternary alloy in which the amount of superconductivity and magnetism is well defined. The most sensitive method to study the superconducting-magnetic interactions is the Josephson tunneling experiment on those sandwiches. The interpretation of the proximity effect data is difficult due to the spatial variation of the superconducting order and the existence of a finite barrier between the NX and S films. Both effects can be eliminated if the experiment is performed in the Cooper limit i.e. when the NX and S thicknesses are much smaller than the coherence length ξ. In this paper we report on calculations and first measurements performed on Nb-NbxOy- /Pb junctions where NX is the Kondo alloy CuCr or the spin glass AgMn. Even for low impurity concentrations the sandwich becomes highly gapless while the Josephson current can still be used as a sensitive probe for the interaction between superconductivity and magnetism.