Low-Field Superconducting Spin Switch Based on a Superconductor/Ferromagnet Multilayer

Abstract

The principle of a novel device, which is called a superconducting spin switch or a spin valve for supercurrent, is proposed and theoretically justified. It is based on a four-layer antiferromagnet͞ferromagnet͞superconductor͞ferromagnet spin-valve-like structure. Calculations show that this structure has either zero value or lower superconducting transition temperature for the parallel alignment of magnetizations in the ferromagnetic layers as compared with an antiparallel alignment of magnetizations. Thus, the supercurrent flowing through the superconducting layer can be switched by rotating the magnetization of the top free ferromagnetic layer by a weak external magnetic field. PACS numbers: 74.80.Dm, 74.50. + r, 74.62.Bf, The superconductor͞ferromagnet (SC͞FM) artificial superlattices provide the possibility of controlled studies of interplay between superconductivity and ferromagnetism. The work by Wong et al. [1], where the indications of nonmonotonic dependence of superconducting transition temperature T c on the thickness of ferromagnetic layers had been observed in SC͞FM multilayers, triggered the investigations of the proximity effect between superconductor and ferromagnet. Buzdin et al. [2][3][4][5] and Radović et al. [6,7] have suggested that superconductivity in SC͞FM multilayers is suppressed because of the large conduction-band exchange splitting in FM layers, which extends into a superconductor via the proximity effect. At the same time, the Cooper pairs leaking from SC into FM acquire a spatially dependent phase, because the partners in a Cooper pair belong to the different spin subbands of a ferromagnet, split by the exchange energy 2I ¿ k B T c0 (T c0 is the superconducting transition temperature of an isolated SC layer). The quantum interference of the incident on the SC͞FM interface pairing wave function with the wave reflected from the opposite side of a FM layer leads to the oscillating and reentrant behaviors of superconducting transition temperature T c in SC͞FM bi-, tri-, and multilayers upon changing the thickness of FM layers, as can be seen from a careful reading of Refs. [3][4][5][6][7]. Shortly afterwards the oscillations of T c were observed experimentally in Nb͞Gd multilayers [8,9], Nb͞CuMn multilayers [10], Nb͞Gd͞Nb trilayers [11], and Fe͞Nb͞Fe trilayers [12,13].The above physical picture proposes the idea that, if the interference of the spin-dependent pairing function is influenced by an external perturbation, the transition temperature will change in a controlled way. At the proper choice of operation temperature the SC͞FM structure can be switched from the superconducting state to the normal one and vice versa, thus valving the supercurrent flow through the superconducting layer.The proposed device structure is a four-layer thin-film structure on a relevant substrate: the first layer is an insulating antiferromagnet (AFM), whose role is to pin the (in-plane) direction of magnetization of the second layer (FM1), made of a metallic ferromagnet of the thickness d F . The third la...

Theory of proximity effect in superconductor/ferromagnet heterostructures

“…The study of the influence of the mutual orientation of magnetizations on T c (Refs. 19,20,21) in the framework of the given model requires to consider ∆(z) as a complex valued function. These question is beyond the present study and will be discussed in the forthcoming publication.…”

Resonant peak in the density of states in normal-metal/diffusive-ferromagnet/superconductor junctions

“…The occurrence of the π-phase shift makes it possible to realize the SFS π -junctions 1 , as was confirmed experimentally 4,5,6,7,8 . The order parameter oscillations also lead to nonmonotonous dependence of T c in SF bilayers on the F-layer thickness 9,10,11,12,13 . Effects of resonant transmission in conductivity of SF structures were discussed in Ref 14,15,16 .…”

Evidence for spin selectivity of triplet pairs in superconducting spin valves

“…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] . 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] . 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.…”