Magnetoresistance is a multifaceted effect reflecting the diverse transport mechanisms exhibited by different kinds of plain materials and hybrid nanostructures; among other, giant, colossal, and extraordinary magnetoresistance versions exist, with the notation indicative of the intensity. Here we report on the superconducting magnetoresistance observed in ferromagnet/superconductor/ ferromagnet trilayers, namely Co/Nb/Co trilayers, subjected to a parallel external magnetic field equal to the coercive field. By manipulating the transverse stray dipolar fields that originate from the out-of-plane magnetic domains of the outer layers that develop at coercivity, we can suppress the supercurrent of the interlayer. We experimentally demonstrate a scaling of the magnetoresistance magnitude that we reproduce with a closed-form phenomenological formula that incorporates relevant macroscopic parameters and microscopic length scales of the superconducting and ferromagnetic structural units. The generic approach introduced here can be used to design novel cryogenic devices that completely switch the supercurrent 'on' and 'off', thus exhibiting the ultimate magnetoresistance magnitude 100% on a regular basis.

Magnetoresistance effects observed in ferromagnet/superconductor (FM/SC) hybrids, FM/SC bilayers (BLs) and FM/SC/FM trilayers (TLs), have attracted much interest. Here, we focus on the stray-fields-based superconducting magnetoresistance effect (sMRE) observed in Co(d(Co))/Nb(d(Nb))/Co(d(Co)) TLs with sufficiently thick Co outer layers so that out-of-plane magnetic domains (MDs) and MDs walls (MDWs) emerge all over their surface when subjected to a parallel external magnetic field, H-ex, equal to the coercive field, H-c. To explore the conditions necessary for maximization of the sMRE, we focus on the different kinds of the stray dipolar fields, H-dip, that emerge at the interior of the out-of-plane MDs and at the boundaries of MDWs; these have a different inherent tendency to create straight and semi-loop vortices, respectively. In the recent literature, the creation of straight and semi-loop vortices has been addressed at some extent both theoretically [Laiho et al., Phys. Rev. B 67, 144522 (2003)] and experimentally [Bobba et al., Phys. Rev. B 89, 214502 (2014)] for the case of FM/SC BLs. Here, we address these issues in FM/SC/FM TLs in connection to the sMRE. Specifically, we focus on an experimental finding reported recently [D. Stamopoulos and E. Aristomenopoulou, J. Appl. Phys. 116, 233908 (2014)]; strong magnetostatic coupling of the FM outer layers is accompanied by an intense sMRE in TLs in which the thickness of the SC interlayer, d(SC), matches the width of MDWs, D-MDWs. To investigate this finding, we employ simulations-modeling and energy-considerations and propose two quantitative criteria that facilitate the creation of straight vortices over semi-loop ones. The first focuses on the maximization of the stray H-dip that occur at the interior of the out-of-plane MDs. The second enables the estimation of a crossover between the preferable creation of one kind of vortices over the other. Both criteria respond well, when tested against experimental results. These generic criteria on the interference between d(SC) and D-MDWs can assist the design of cryogenic devices based on FM/SC/FM TLs. (C) 2015 AIP Publishing LLC.

In the last decades, many superconductor-based devices have been utilized in practical applications that refer to the production/sensing of ultra-high/low magnetic fields, resistive storage of data, etc. The superconducting magnetoresistance effect (sMRE) observed in ferromagnetic/superconducting/ferromagnetic trilayers (TLs) has been widely studied in the literature. Here, we investigate Ni80Fe20/Nb/Ni80Fe20 and Co/Nb/Co TLs on a comparative basis and we provide technical guidelines that empower us to design magnetic field sensors and magnetic field-controlled supercurrent switches for cryogenic applications. The performance of these TLs has been studied in great detail. It turned out that Ni80Fe20-based TLs are applicable in the regime of low magnetic fields (on the order of a few hundred Oe) due to their magnetically soft character, while Co-based ones are effective at relatively higher magnetic fields (on the order of a few thousand Oe) owing to their magnetically harder behavior. The Ni80Fe20-based TLs exhibit a sMRE magnitude, (R-max - R-min)/R-nor x 100 %, on the order of 30-50 % and a field sensitivity, dR(H)/dH, on the order of 4.0-6.0 mOhm/Oe. The Co-based TLs have higher sMRE magnitude on the order of 30-100 %; however, they exhibit lower field sensitivity on the order of 0.3-0.6 mOhm/Oe. Finally, the Co-based TLs exhibit an abnormal behavior of the upper critical field line, H-c2(T), at low magnetic fields. This feature is absent in the Ni80Fe20-based TLs. Due to the differences outlined above, the Ni80Fe20-based and Co-based TLs can be possibly utilized in two distinct categories of devices, namely magnetic field sensors and magnetic field-controlled supercurrent switches, respectively.