Synergy of exchange bias with superconductivity in ferromagnetic–superconductinglayered hybrids: the influence of in-plane and out-of-plane magnetic order onsuperconductivity

Abstract

It is generally believed that superconductivity and magnetism are two antagonisticlong-range phenomena. However, as was preliminarily highlighted in Stamopoulos et al (2007 Phys. Rev. B 75 014501), and extensively studied in this work, under specificcircumstances these phenomena instead of being detrimental to each other may evenbecome cooperative so that their synergy may promote the superconducting properties of ahybrid structure.Here, we have studied systematically the magnetic and transport behaviorof such exchange biased hybrids that are comprised of ferromagnetic (FM)Ni80Fe20 andlow-Tc superconducting (SC) Nb for the case where the magnetic field is applied parallel to thespecimens. Two structures have been studied: FM–SC–FM trilayers (TLs) and FM–SCbilayers (BLs). Detailed magnetization data on the longitudinal and transversemagnetic components are presented for both the normal and superconductingstates. These data are compared to systematic transport measurements includingI–V characteristics. The comparison of the exchange biased BLs and TLs that are studied herewith the plain ones studied in Stamopoulos et al (2007 Phys. Rev. B 75 184504) enable usto reveal an underlying parameter that may falsify the interpretation of the transportproperties of relevant FM–SC–FM TLs and FM–SC BLs investigated in the recentliterature: the underlying mechanism motivating the extreme magnetoresistancepeaks in the TLs relates to the suppression of superconductivity mainly due tothe magnetic coupling of the two FM layers as the out-of-plane rotation of theirmagnetizations takes place across the coercive field where stray fields emerge in theirwhole surface owing to the multidomain magnetic state that they acquire. Therelative in-plane magnetization configuration of the outer FM layers exerts asecondary contribution on the SC interlayer. Since the exchange bias directlycontrols the in-plane magnetic order it also controls the out-of-plane rotation of theferromagnets’ magnetizations so that the magnetoresistance peaks may be tuned at will.All the contradictory experimental data reported in the recent literature are discussedfairly in the light of our results; based on a specific prerequisite we propose aphenomenological stray-field mechanism that efficiently explains the evolution of themagnetoresistance effect in TLs. Our experiments not only point out the need for a newtheoretical treatment of FM–SC hybrids but also direct us toward the design of efficientsupercurrent-switch elemental devices.