Central peak position in magnetization hysteresis loops of ferromagnet/superconductor/ferromagnet trilayered films

We report on the magnetization hysteresis loops of rf-sputtered Co/Nb/Co trilayered films under applied fields parallel to the film surface. Below superconducting transition temperature Tc, the central peak around zero field in the superconducting magnetization is found to be anomalously shifted toward a positive field on the descending branch of the hysteresis loop, in contrast to a negative peak position of a single superconducting film observed in this geometry. The central magnetization peak shifts toward a lower field with decreasing temperature and increasing the thickness of superconducting Nb layer. These results are explained as due to the stray field of the ferromagnetic Co layer with in-plane magnetization, which effectively reduces the internal field of the superconductor.