Infinite magnetoresistance in superconducting spin switch with spin-orbit coupling

At a thin-film superconductor (S) interface with a ferromagnetic insulator (FI), the magnetic exchange field (MEF) of the FI can spin-split the density of states^1,2 and suppress the critical temperature (T_c) of S³. In a FI/S/FI spin switch, the suppression of T_c is reduced for antiparallel (AP) magnetizations between the two FI layer due to a net cancellation effect of MEF acting on the S layer. Conversely, for parallel (P) magnetizations, the exchange fields add enhancing the suppression of T_c i.e., ΔT_c=T_c(AP)–T_c(P)>0. For S materials with weak spin-orbit coupling (SOC) such as Al the spin splitting in S can exceed several Tesla^1,2 and ΔT_c can reach tens of mK³. It has also been shown that by dusting an atomic layer of Au at the Al/EuS interface, it is possible to quench the spin splitting in Al due to SOC in Au⁴. Here we report EuS/Nb/EuS superconducting spin switches in which the superconducting layer of Nb has strong SOC and a T_c down to a thickness of only 2 nm. By optimizing the Nb/EuS interface we obtain record-breaking values of ΔT_c that exceed 1.5 K with a ΔT_c/T_c(P) ratio of nearly 100%. The results indicate physics that goes beyond the standard quasiclassical picture of S/FI proximity effects in which superconducting spin-switch performance is boosted by the large values of T_c in the nearly nm-thick layer of Nb in conjunction with strong SOC.



1. X. Hao, et al., PRL 67, 1342–1345 (1991).

2. E. Strambini, et al., PR Mat. 1, 054402 (2017).

3. B. Li, et al., PRL 110, 097001 (2013).

4. P. Wei, et al., PRL 122, 247002 (2019).