Enhancement of superconductivity by external magnetic field in magnetic alloys

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An asymptotic solution of the thermodynamics of a BCS superconductor containing spin 1/2 and 7/2 magnetic impurities, obtained recently in Ref. 1 is exploited to derive the expressions for critical magnetic field Hc(T). The credibility of the resulting theoretical equations, which depend on the magnetic coupling constant g and impurity concentration c, is verified on the experimental data for the following superconducting alloys: LaCe, ThGd and SmRh₄B₄. Good quantitative agreement with experimental data is found for sufficiently small values of c. The discrepancies between theoretical and experimental values of Hc(T) for larger values of c in case of LaCe and ThGd are reduced by introducing the concept of the effective temperature T, which accounts for the Coulomb interactions between the electron gas and impurity ions. At low temperatures, the critical magnetic field is found to increase with decreasing temperature T. This enhancement of the critical magnetic field provides evidence of the Jaccarino-Peter effect, which was experimentally observed in the Kondo systems like LaCe, (La₁₋ₓCeₓ)Al₂ and also in the pseudoternary compounds, including Sn₁₋ₓEuₓMo₆S₈, Pb₁₋ₓEuₓMo₆S₈ and La₁.₂₋ₓEuₓMo₆S₈. The effect of an external magnetic field H on a BCS superconductor perturbed by magnetic impurities was also studied. On this grounds, by analyzing the dependence of superconducting transition temperature Tc on H of (La₁₋ₓCeₓ)Al₂, we have shown, that for certain parameter values, external magnetic field compensates the destructive effect of magnetic impurities.