Superconductivity tuned through magnetic irreversibility in two-dimensional Co/Nb/Co trilayers under a parallel magnetic field

Abstract

In ferromagnetic/superconducting (FM/SC) planar hybrids, a reentrance of the upper-critical field line Hc2(T) is observed close to Tc. The effect is generally ascribed to the so-called domain-wall superconductivity and has been theoretically explored in great detail. Experimental investigations are limited mostly to FM/SC bilayers in which the FM layers host out-of-plane magnetic domains of strong anisotropy and large width (>300 nm), the SC layer is quite thick (>30 nm), and the external magnetic field was applied normal. To expand our knowledge to until now unexplored conditions, we study a series of Co(dCo)/Nb(dNb)/Co(dCo) trilayers under a parallel magnetic field; Co outer layers exhibit out-of-plane magnetic domains of weak anisotropy and small width (<150 nm) and the Nb interlayer is very thin (<20 nm). We demonstrate a strong reentrance of Hc2(T) that can be tuned through the irreversible magnetization processes of the FM outer layers, the two-dimensional character of the SC interlayer, and the matching between the interfering SC and FM length scales. These results refine the nature of the reentrance of Hc2(T) in FM/SC hybrids and can motivate new theoretical and experimental investigations.

Spin texture on top of flux avalanches in Nb/Al2O3/Co thin film heterostructures

“…However, most of the works on these hybrids are dedicated to the study of the effects of the magnetic layers on the superconducting films [15]. A relatively smaller number of investigations are focused in the effects of superconductivity on the spin configuration of the magnetic layers [16,17,18].…”

Superconducting magnetoresistance in ferromagnet/superconductor/ferromagnet trilayers

“…Superconducting MR (SMR) is observed at cryogenic-environment conditions in ferromagnet/superconductor/ferromagnet trilayers (FM/SC/FM TLs) when subjected to a parallel external magnetic field, H ex 12 13 14 15 16 17 18 19 20 21 22 23 24 . Regarding the phenomenological behavior and potential for applications, SMR has much in common with some of the above mentioned MR effects.…”

“…Specifically, FM/SC/FM TLs exhibit SMR when they are driven to the coercive field, H C by a parallel H ex 15 16 . The out-of-plane magnetic domains (MDs) and MDs walls (MDWs) that develop at H C are accompanied by transverse stray dipolar fields that magnetostatically couple the FM outer layers through the SC interlayer 12 13 24 . Thus, the SC interlayer experiences local magnetic fields that exceed either its lower-critical field (SMR relates to vortex-motion dissipation processes) or its upper-critical field (SMR relates to the charge-dependent orbital effect) 12 15 .…”

“…In this work we employ Co/Nb/Co TLs as a model system. We present experimental data and phenomenological modeling of the SMR effect that, in these TLs, is accompanied by an intense reentrance of the upper-critical field line, H c2 (T) close to the critical temperature 12 13 24 . We investigate many sets of Co(d Co )/Nb(d Nb )/Co(d Co ) TLs with relatively thick Co outer layers, d Co = 60 and 100 nm > 40–50 nm, so that out-of-plane MDs do emerge at H C , and relatively thin Nb interlayer, 15 nm < d Nb < 23 nm, to enhance the SMR magnitude.…”

“…Third, the SMR effect is restricted to the regime below the saturation field, H sat ( Fig. 1b ; gray area) where irreversible magnetization processes exist 24 .…”