Stray-fields-based magnetoresistance mechanism in Ni80Fe20-Nb-Ni80Fe20 trilayered hybrids

We report on the transport and magnetic properties of hybrid trilayers (TL's) and bilayers (BL's) that consist of low spin-polarized Ni80Fe20 exhibiting in-plane but no uniaxial anisotropy and low-T-c Nb. We reveal a magnetoresistance effect of magnitude identical to the ones that were reported in Pena [Phys. Rev. Lett. 94, 57002 (2005)] for TL's consisting of highly spin-polarized La0.7Ca0.3MnO3 and high-T-c YBa2Cu3O7. The presented effect is pronounced when compared to the one reported in Rusanov [Phys. Rev. B 73, 060505(R) (2006)] for Ni80Fe20-Nb-Ni80Fe20 TL's of strong in-plane uniaxial anisotropy. In our TL's the magnetoresistance exhibits an increase of two orders of magnitude when the superconducting state is reached: from the conventional normal-state values Delta R/R(nor)x100%=0.6% it goes up to Delta R/R(nor)x100%=45% (Delta R/R(min)x100%=1000%) for temperatures below T-c(SC). In contrast, in the BL's the effect is only minor since from Delta R/R(nor)x100%=3% in the normal state increases only to Delta R/R(nor)x100%=8% (Delta R/R(min)x100%=70%) for temperatures below T-c(SC). Magnetization data of both the longitudinal and transverse magnetic components are presented. Most importantly, we present data not only for the normal state of Nb but also in its superconducting state. Strikingly, these data show that below its T-c(SC) the Nb interlayer under the influence of the outer Ni80Fe20 layers attains a magnetization component transverse to the external field. By comparing the transport and magnetization data we propose a candidate mechanism that could motivate the pronounced magnetoresistance effect observed in the TL's. Adequate magnetostatic coupling of the outer Ni80Fe20 layers is motivated by stray fields that emerge naturally in their whole surface due to the multidomain magnetic structure that they attain near coercivity. Consequently, the stray fields penetrate the Nb interlayer and suppress its superconducting properties by primarily (secondarily) exceeding its lower (upper) critical field. Atomic force microscopy is employed in order to examine the possibility that such magnetostatic coupling could be promoted by interface roughness. Referring to the BL's, although out-of-plane rotation of the magnetization of the single Ni80Fe20 layer is still observed, in these structures magnetostatic coupling does not occur due to the absence of a second Ni80Fe20 one so that the observed magnetoresistance peaks are only modest.