The role of the stray field on superconducting properties of hybrid ferromagnetic/superconducting heterostructures

A set of hybrid ferromagnetic/superconductor heterostructures was prepared by a confocal DC magnetron sputtering setup. The samples were systematically studied by measuring the magnetization and magneto-transport properties up to 2.5T. The anisotropic behavior of the critical temperature, under applied magnetic fields, was investigated. For the Nb/Cu/Co samples, the nucleation of the superconductivity is determined by the formation of a large distribution of out-of-plane magnetic domains in the ferromagnetic Co layer. In the regions between the Co magnetic domains, the results suggested that the effective magnetic field can be drastically reduced, favoring the nucleation of Cooper pairs responsible for the superconducting properties of the Nb films. Oppositely, in the Nb/Cu/Py samples, the superconducting properties were more severely depleted due to the increase of the effective magnetic field, caused by the combined effects of the applied and ferromagnetic Py stray field. Values of spin-valve effects at the interval 60-300mK were obtained for the different hybrid Nb/Cu/Py (or Nb/Cu/Co) and Nb/Cu/Py/Cu/Co systems. The results were mainly explained considering the effective stray fields derived from the changes on the magnetic domains of the ferromagnetic layers caused by the external field.