By extension, the realization of superconducting spintronics requires a selectivity between spin up–up and spin down–down triplet pairs however, experiments performed to date are not spin sensitive and the polarization of a triplet supercurrent is so far unknown. As spin-one triplet pairs, unlike singlet pairs, can carry spin, these results mean that combining superconducting and spin electronics (superconducting spintronics) opens up real potential for practical low-temperature applications 24.Ĭonventional spintronics relies on the spin selectivity of ferromagnets, which originates from the difference between the spin-up and spin-down density of states at the Fermi level.
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More direct evidence for the generation of triplet pairing was obtained from S–F–S Josephson junctions in which, if interfacial spin-mixer layers were present, supercurrents could be measured through F-layer thicknesses much larger than the singlet coherence length 15, 16, 17, 18, 19, 20, 21, 22, 23. 14 recently demonstrated a minimum in T C when the F and F′ layers were orthogonal-the configuration that theoretically maximizes singlet–triplet pair conversion.
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A number of experiments have been performed to test these predictions: for example, in S–F′–F structures, both Leksin et al. As with the conventional superconductor–normal metal proximity effect, the increased ‘leakage’ of pairs from the S layer should reduce the singlet pair amplitude within it and hence decrease the T C of the structure 12. As spin-one triplet pairs are immune to pair breaking by the exchange field in F, the proximity effect coupling between S and F layers is enhanced. This behaviour should be substantially modified in S–F systems in which conversion between singlet pairs and odd-frequency spin-one triplet pairs is possible, for example, by introducing magnetic non-collinearity (that is, a spin-mixer layer) at the S–F interface 10, 11. This (standard) spin-switch effect has been experimentally demonstrated 8, 9 and the difference in T C between P and AP configurations of an F–S–F spin valve can be understood as follows: in the AP state, the pair-breaking effect of the F layers is reduced, as the net ferromagnetic exchange field of the structure is partially compensated, meaning T C is maximized 1 for a P state, the pair-breaking effect is maximized, thus T C is reduced. In an F–S–F spin valve, the reversal of one F layer modifies the spatial properties of the decaying oscillation, resulting in a spin-switch effect in which the T C is greater when the F layer moments are antiparallel (AP) than when they are parallel (P) 5, 6, 7.
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The exchange field introduces a momentum mismatch between the spin-up and spin-down electrons that form a singlet pair, and this results in a weak oscillatory dependence superimposed on the decrease of T C with increasing thickness of the F layer 2, 3, 4. The dependence of the critical temperature ( T C) of ferromagnet–superconductor (F–S) bilayers has its origin in the spatially oscillating components of the wave function of the singlet Cooper pairs induced by the exchange field in the F layers 1.