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

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∕dH 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∕dHH=0, can be stimulated, e.g., by increasing the Dₛ value or decreasing the ratio L∕DF.