Experimental observation of the triplet spin-valve effect in a superconductor-ferromagnet heterostructure

Abstract

The theory of superconductor-ferromagnet (S-F) heterostructures with two ferromagnetic layers predicts the generation of a long-range, odd-in-frequency triplet pairing at non-collinear alignment (NCA) of the magnetizations of the F-layers. This triplet pairing has been detected in a Nb/Cu 41 Ni 59 /nc-Nb/Co/CoO x spin-valve type proximity effect heterostructure, in which a very thin Nb film between the F-layers serves as a normal conducting (nc) spacer. The resistance of the sample as a function of an external magnetic field shows that for not too high fields the system is superconducting at a collinear alignment of the Cu 41 Ni 59 and Co layer magnetic moments, but switches to the normal conducting state at a NCA configuration. This indicates that the superconducting transition temperature T c for NCA is lower than the fixed measuring temperature. The existence of a minimum T c , at the NCA regime below that one for parallel or antiparallel alignments of the F-layer magnetic moments, is consistent with the theoretical prediction of a singlet superconductivity suppression by the long-range triplet pairing generation.An odd-in-frequency triplet pairing generation in singlet superconductor/ferromagnet thin-film heterostructures was predicted theoretically [1][2][3]. At least two ferromagnetic layers (F 1 ,F 2 ) with a non-collinear alignment of their magnetizations, are required to couple the conventional opposite-spin singlet s-wave pairing channel with the unconventional, odd-triplet s-wave pairing channel. The latter one is of extraordinary long range in F layers [1,2,4], because the magnetized conduction band of a ferromagnetic metal serves as an eigenmedia supporting the equal-spin pairing.Intense activities followed to formulate optimal conditions and realize experimental schemes for the generation and detection of this odd-triplet pairing utilizing the Josephson effect [5][6][7][8][9][10][11][12][13][14].

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.…”

Evidence for spin selectivity of triplet pairs in superconducting spin valves

“…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 . A number of experiments have been performed to test these predictions: for example, in S-F 0 -F structures, both Leksin et al 13 and Zdravkov et al 14 recently demonstrated a minimum in T C when the F and F 0 layers were orthogonal-the configuration that theoretically maximizes singlet-triplet pair conversion.…”

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.…”