Origin of the Inverse Spin Switch Effect in Superconducting Spin Valves

Abstract

The resistance of a ferromagnet/superconductor/ferromagnet (F/S/F) spin valve near its superconducting transition temperature, Tc, depends on the state of magnetization of the F layers. This phenomenon, known as spin switch effect (SSE), manifests itself as a resistance difference between parallel (RP ) and antiparallel (RAP ) configurations of the F layers. Both standard (RP > RAP ) and inverse (RP < RAP ) SSE have been observed in different superconducting spin valve systems, but the origin of the inverse SSE was not understood. Here we report observation of a coexistence of the standard and inverse SSE in Ni81Fe19/Nb/Ni81Fe19/Ir25Mn75 spin valves. Our measurements reveal that the inverse SSE arises from a dissipative flow of vortices induced by stray magnetic fields from magnetostatically coupled Néel domain wall pairs in the F layers.

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).…”

“…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). This suppression should be greatest close to the coercive field of the F layers when the density of the fringing fields from Néel domain walls is maximized 28 . This explanation cannot, however, explain our results, as samples without Ho show a well-defined decrease in R at coercivity and in the AP state, which translates into an enhancement rather than a decrease of T C (similar results are reported in ref.…”

Magnetic coupling in superconducting spin valves with strong ferromagnets

“…This was explained as either due to enhanced reflection of spin-polarized quasiparticles ͑a mechanism not present in the calculations with equal spin subbands͒, 11 or to the effects of stray fields from domain walls. 10,[12][13][14] The difference between the observations of the standard and the inverse effects appears to be that those experiments finding standard behavior make use of an antiferromagnetic pinning layer in order to have one layer with fixed, and one layer with a freely rotating magnetization, while the reports on the inverse effect use a difference of d F or different materials for both layers in order to create a difference in coercive fields and thereby a field range where the magnetizations are AP. The pinning layer suppresses domain formation, and, in particular, the work of Zhu et al showed that if, in samples with a pinning layer, the free layer is brought in a domain state, the standard spin-valve effect is lost.…”

“…The pinning layer suppresses domain formation, and, in particular, the work of Zhu et al showed that if, in samples with a pinning layer, the free layer is brought in a domain state, the standard spin-valve effect is lost. 10 Focusing now on samples without pinning layer, almost all investigations were performed on ͑millimeter͒ large samples, where large amount of domains were present during the transition from the P to the AP state. In the work of Rusanov et al, samples were structured down to micrometer size, and the switching of the magnetization was instantaneous, giving blocklike variations in the resistance, and suggesting that a clear P to AP switch was obtained.…”

Giant triplet proximity effect in superconducting pseudo spin valves with engineered anisotropy

“…Before concluding that the trend of T C on θ can be explained on the basis of spin-triplet pair generation, we must first rule out the possibility that fringing fields from the PSV are suppressing T C (a particular issue in F-S-F structures [35,36]) or that the proximity effect is due to singlet pairs.…”