Controlled suppression of superconductivity by the generation of polarized Cooper pairs in spin-valve structures

Abstract

Transport measurements are presented on thin-film superconducting spin-valve systems, where the controlled noncollinear arrangement of two ferromagnetic Co layers can be used to influence the superconducting state of Nb. We observe a very clear oscillation of the superconducting transition temperature with the relative orientation of the two ferromagnetic layers. Our measurements allow us to distinguish between the competing influences of domain averaging, stray dipolar fields, and the formation of superconducting spin triplets. Domain averaging is shown to lead to a weak enhancement of transition temperature for the antiparallel configuration of exchange fields, while much larger changes are observed for other configurations, which can be attributed to drainage currents due to spin triplet formation. The normally antagonistic ground states of conventional superconductivity and ferromagnetism give rise to a variety of intriguing phenomena when brought into close proximity, a subject that has gained much attention both theoretically [1][2][3][4][5][6][7][8][9] and experimentally [10][11][12][13][14][15][16][17][18] over recent years. The underlying proximity effect of singlet Cooper pairs penetrating a ferromagnetic (F ) layer is nonmonotonic in nature, which is very different from the monotonic decay found for the case of proximity coupling into a normal (N ) metal. This unconventional proximity effect leads, for example, to oscillations in the critical temperature (T c ) of the superconductor as function of the thickness of the F layer [19][20][21].In 2002 the superconducting spin valve was proposed theoretically [22,23], comprising a superconducting (S) spacer layer separating two F layers. For ideal operation, the supercurrent in the S layer can be controlled by switching the relative orientation of the exchange fields (H ex ) of the F layers from a parallel (P) to an antiparallel (AP) alignment. The underlying physical mechanism involves the interaction of the singlet Cooper pair with both exchange fields, whereby it experiences an additional pair dephasing if the device is in the P state, due to a potential energy mismatch between the spin up and spin down electron of the penetrated pair, thus lowering T c . Such an effect does not occur in the AP case, since both electrons find themselves in equivalent bands. This mechanism can be generalized as a relative enhancement of T c by domain averaging and has been observed in a variety of experiments [24][25][26][27][28], where, with the exception of Ref.[25], a pinned magnetic layer is used to create the AP arrangement. However, several seemingly anomalous results with precisely the opposite behavior have also been reported [29][30][31][32][33]. One plausible explanation proposed for these results, in systems * Corresponding author: mgf@st-andrews.ac.uk where no pinning layer was used, is the dominance of a suppression of superconductivity by dipolar fields generated by the domains [25]. In experimental work caution therefore needs to be exercised to avoid a dom...

Amplitude Control of the Spin-Triplet Supercurrent inS/F/SJosephson Junctions

“…Those groups found evidence for spin-triplet generation occurring during the magnetization reversal process while sweeping an external magnetic field. Better control of the magnetic states has been achieved recently by several groups measuring the critical temperature T c of S/F/F trilayers, where generation of spin-triplet correlations results in lowering of T c [21][22][23][24][25][26]. Controlling T c , however, is less likely to be useful for future device applications than controlling supercurrent.…”

A Boost for Superconducting Logic

“…We find TSV effects over a wide range of magnetic fields. This was not the case for previous TSVs measured in an in-plane configuration, where the maximum field of operation was limited to 0.2-0.3 T [18]. Robust proximity effects were also observed in CrO 2 -based Josephson junctions [12,19,20], where critical currents in various configurations were observed up to the Tesla range.…”

“…There are several recent experimental results on TSVs, which use standard ferromagnets (Fe, Co, Ni) and their alloys as spin mixers and drainage layers [14][15][16][17][18]. In all cases, magnetic anisotropy or an antiferromagnetic pinning layer was used to reliably control the relative magnetization directions, always in the plane of the films.…”

Isolation of proximity-induced triplet pairing channel in a superconductor/ferromagnet spin valve

“…The observed angular dependence of the superconducting transition temperature T c is caused by a combination of the singlet and triplet components of the superconducting condensate. The existence of the triplet contribution to the superconducting spin-valve effect has also been shown in [11][12][13][14][15]. An important question for basic and applied research in the field of spintronics is whether there is the possibility of observing the "isolated" triplet contribution to the superconducting spin-valve effect.…”