Thin-film superconductor-ferromagnet hybrids: Competition between nucleation of superconductivity at domain walls and domains’ centers

Abstract

By using the Ginzburg-Landau theory we studied the order parameter nucleation in the planar superconductor-ferromagnet (S/F) hybrids of a finite thickness, when the magnetic field is inhomogeneous both in the lateral and transverse directions. We calculated the dependence of the critical temperature Tc on the external magnetic field H and studied the transformation of the phase transition line Tc(H), induced by the variation of the parameters of the S/F bilayers such as the period of the domain structure 2L and the thicknesses Ds and DF of the superconducting and ferromagnetic films. The actual shape of the Tc(H) line is demonstrated to be determined by the nucleating regime at H=0. For the thin-film S/F hybrids (L∕DF⪢1) a possible type of the phase boundary Tc(H) is predicted–the reentrant superconductivity with the positive slope dTc∕d∣H∣ at H=0, which appears as a consequence of the order parameter nucleation at the centers of the magnetic domains at H=0. The nucleation near the domain walls at H=0, resulting in the Tc(H) line with zero slope ∣dTc∕dH∣H=0, can be stimulated, e.g., by increasing the Ds value or decreasing the ratio L∕DF.

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