Origin of spontaneous currents in a superconductor–ferromagnetic proximity system

Abstract

We have previously shown that a ferromagnet-superconductor heterostructure may possess a spontaneous current circulation parallel to the interface. This current is caused by Andreev bound states in the thin ferromagnetic layer, and can be fully spin-polarized. Here we investigate the total energy of the system in cases where the current either does or does not flow. We show that the current is a true quantum ground state effect, and examine the effect of the current on the different contributions to the total energy.Key words: proximity effect,, superconductor-ferromagnet interface PACS: 72.25.-b, 74.50.+r,, 75.75.+a The proximity effect between a superconductor and a ferromagnet has been a subject of great interest in recent years. Unlike the usual proximity effect between a superconductor (S) and a normal metal, a superconductor in contact with a ferromagnet (F) is subject to time reversal symmetry breaking which splits the up and down spin components of the spin-singlet Cooper pairs. In bulk superconducting materials a ferromagnetic exchange splitting leads to the FFLO (or LOFF) state in which the superconducting order parameter oscillates in space [1,2]. In hybrid S-F structures the same physical effect also leads to spatial oscillations [3]. This gives rise to a number of new phenomena, including: Josephson π-junction behaviour in S-F-S heterostructures [4,5], a giant mutual proximity effect in S-F nanostructures [6], and spin valve [7]

Spin-polarized supercurrents for spintronics: a review of current progress

“…The presence of spin-mixing angles has many consequences. For example, it leads to Andreev bound states at the interface, as predicted theoretically [151][152][153][154][155][156], and verified experimentally [157]. It also is responsible for giant thermoelectric effects in non-local setups [158,159] and is the main ingredient for creating triplet supercurrents in strongly spin-polarized ferromagnets [133,152].…”

Theory of Andreev bound states in S-F-S junctions and S-F proximity devices

“…The central quantity of the theory is the scattering matrix S, the eigenvalues of which are given for conserved k by e iϑ ↑ (k ) and e iϑ ↓ (k ) , and the eigenvectors of which determine for each k the quantization axis along which the scattering matrix is diagonal, and around which the spin precession takes place. An important consequence of spin-mixing phases is the appearance of Andreev bound states at magnetically active interfaces, predicted theoretically [82][83][84][85][86][87][88] and verified experimentally [89].…”

“…An important consequence of spin-mixing phases is the appearence of Andreev bound states at magnetically active interfaces, predicted theoretically [67][68][69][70][71][72]77], and verified experimentally [78]. Consider a superconductor near an interface with a ferromagnetic insulator.…”

Spin-dependent proximity effects in ferromagnetic semiconductor/superconductor structures

“…AR process plays an important role in the proximity effect. Proximity effects in ferromagnetic (FM)/SC structures have recently attracted much attention in experimental and theoretical investigations [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. The electron (hole) and the AR hole (electron) in FM have opposite spins and different Fermi momentum, interference between their wave functions causes the induced superconducting order parameter F(x) [32]to have a damped oscillation [33,34].…”