Magnetic confinement of the superconducting condensate in superconductor-ferromagnet hybrid composites

The influence of an inhomogeneous magnetic field on the magnetoresistance of thin Al films, used in different superconductor-ferromagnet hybrids, has been investigated. Two contrasting magnetic textures with out-of-plane magnetization are explored: namely, (i) a plain film in a multidomain state and (ii) an array of microsized dots. The stray fields of the ferromagnetic structures confine the superconducting condensate and, accordingly, modify the condition for the nucleation of superconductivity. By switching between different magnetic states of the ferromagnet, this confinement can be tuned at will, thereby reversibly changing the dependence of the critical temperature T_c on an external magnetic field H. In particular, continuous evolution from a conventional linear T_c(H) dependence with a single maximum to a reentrant superconducting phase boundary with multiple T_c peaks has been demonstrated.