Magnetic coupling in superconducting spin valves with strong ferromagnets

Abstract

We investigate the magnetotransport behavior of ferromagnet ͑F͒/superconductor/ferromagnet trilayers made of ferromagnetic Ni 80 Fe 20 ͑Permalloy, Py͒ and superconducting Nb for temperatures both above and below the superconducting transition temperature T c . In such devices, and for weak ferromagnets, T c depends on the relative magnetization directions of the two F layers in such a way that T c P of the parallel ͑P͒ alignment is lower than T c AP of the antiparallel ͑AP͒ alignment ͑the so-called superconducting spin-valve effect͒. For strong magnets, the suppression of Andreev reflection may alter this picture, but also stray field effects become important, as is known from earlier work. We compare large-area samples with microstructured ones, and find blocklike switching in the latter. We show this not to be due to a switch between the P and AP states, but rather to dipolar coupling between domains which are forming in the two Py layers, making a stray-field scenario likely. We also present measurements of the depairing ͑critical͒ current I dp and show that a similar depression of superconductivity exists far below T c as is found around T c .

Evidence for spin selectivity of triplet pairs in superconducting spin valves

“…Before considering an explanation for this behaviour involving the generation and diffusion of spin-triplet pairs, we will first discuss possible effects arising from fringing fields [26][27][28][29] and spin imbalance 30 in Nb, factors that have been advanced as enhancing T C in the P state. In the fringing fields scenario, T C could be suppressed as a consequence of magnetic dipolar coupling between the two F (Py) layers, which introduces flux into the superconductor (Nb).…”

Memory-functionality superconductor/ferromagnet/superconductor junctions based on the high- Tc cuprate superconductors YBa2Cu3O7Prada1, Golod2, Kapran3 et al. 2019Phys. Rev. B8040View full textAdd to dashboardCite

“…This hysteresis is most pronounced at low temperature, and its magnitude decreases continuously with increasing temperature without a clear disruption at T c . This behavior is qualitatively different from the magnetoresistance of superconducting films induced by stray fields from ferromagnets in S/F hybrid films [58]. Therefore, the observed hysteresis on the MR loops is not related to vortices or superconductivity in YBCO but is solely associated with the CMR of LCMO and reflects the switching of magnetization in the F layer.…”

Evidence for competition between the superconductor proximity effect and quasiparticle spin-decay in superconducting spin-valves

“…We note that ∆T c values of several hundred mK have been reported with rare-earth ferromagnetic metals and insulators [19][20][21]. Negative ∆T c values have also been reported [14,[22][23][24][25][26][27][28][29][30] due to quasiparticle (QP) spin-accumulation [23,26,28] suppressing T c in the AP-state [31,32], or flux penetration in S from out-of-plane domain walls in the F layers [14,24,27,30].…”

“…The inset shows the equivalent trend of ∆T c versus d Nb [33]. For d Nb = 21 nm, the superconductor proximity effect reaches a positive ∆T c up to 314 mK (∆T c /T device = 0.26), which is unprecedented in transition metal F/S/F spin-valves where values are usually of the order tens of mK [9][10][11][12][13][14][15][16][17][18][22][23][24][25][26][27][28][29][30]. However, we note that these large values of ∆T c are linked to inflations in A∆R.…”