Abstract:

In the present work we study extensively the manipulation of superconductivity through ferromagnetism in a new category of hybrids. The studied hybrids consist of antiferromagnetic/ ferromagnetic (AF/FM) [La0.33Ca0.67MnO3/La0.60Ca0.40MnO3](15) multilayers (MLs) in contact with a low-T-c Nb superconductor (SC). In these hybrids a relatively thick FM buffer layer was used as a reservoir for the generation of stray fields that influence the SC intensively. Our results show that in the parallel field configuration the SC becomes ferromagnetically coupled to the ML when field-cooled through its T-c(SC). Thus, although the SC should behave diamagnetically in respect to the externally applied magnetic field, its bulk magnetization behaves ferromagnetically and switches together with the magnetization of the ML when its coercive field is exceeded (switching effect). By employing specific experiments, where the ML was selectively exchange biased or not, we clearly demonstrate that the ML structure, separating the FM buffer and the SC layers, inflicts its magnetic properties on the whole hybrid. Thus, in such ML/SC hybrids the exchange bias mechanism can be used for regulating the switching of the SC magnetization. By employing specific experimental protocols for our magnetization measurements we directly uncover that the multidomain magnetic state of the ML strongly suppresses both the transition's height and the critical temperature of the SC. Simple FM/SC bilayers have also been studied. In these samples the switching effect is observed only for zero external field, while it is absent when a magnetic field is applied. This indicates that the ML structure is an essential ingredient for the generic observation of the switching effect. Our experimental results support recent theoretical studies referring to the mutual proximity effect (Kharitonov et al 2006 Phys. Rev. B 73 054511), to the possible formation of spin-triplet superconductivity (Bergeret et al 2001 Phys. Rev. Lett. 86 4096), and to the influence of an FM domain state on the properties of an SC (Buzdin and Mel'nikov 2003 Phys. Rev. B 67 020503( R)) in relevant hybrids. Apart from their importance for theory, our results are valuable for the design of spin-valve devices that recently have attracted great interest.