Microscopic theory of spin transport at the interface between a superconductor and a ferromagnetic insulator

Abstract

We theoretically investigate spin transport at the interface between a ferromagnetic insulator (FI) and a superconductor (SC). Considering a simple FI-SC interface model, we derive formulas for the spin current and spin-current noise induced by microwave irradiation (spin pumping) or the temperature gradient (the spin Seebeck effect). We show how the superconducting coherence factor affects the temperature dependence of the spin current. We also calculate the spin-current noise in thermal equilibrium and in non-equilibrium states induced by the spin pumping, and compare them quantitatively for an yttrium-iron-garnet-NbN interface. arXiv:1901.02440v2 [cond-mat.mes-hall]

Giant Transition-State Quasiparticle Spin-Hall Effect in an Exchange-Spin-Split Superconductor Detected by Nonlocal Magnon Spin Transport

“…Notably, ⌈Δ V nl th ⌉ Pt,no Al 2 O 3 /⌈Δ V nl th ⌉ Pt,with Al 2 O 3 drops abruptly right below T c (extracted from the Nb resistance R Nb versus T base plot of Figure 2 c), and then it rises progressively as the Nb enters deep into the superconducting state, resulting in a downturn at T base / T c ≈ 0.95 (inset of Figure 2 f). Such a nontrivial behavior is compatible with recent theoretical predictions 29 , 30 and experimental reports 31 , 32 on ferromagnetic insulator (FMI)/SC structures, where (spin-singlet) Cooper pairs from the SC cannot leak into the FMI even if the exchange spin-splitting can still penetrate the SC. 4 So rather well-developed coherence peaks of the QP density-of-states (DOS) at the FMI/SC interface 5 are accessible to the transporting spin current.…”

“…The calculated J s0 qp / J s0 increases largely near T c (0.8 T c – 0.9 T c ), and it decreases exponentially when T < 0.8 T c , reflecting the singularity behavior in a nonequilibrium population of spin-polarized QPs. 29 , 30 , 42 In addition, the peak amplitude of J s0 qp / J s0 is inversely proportional to Δμ m /2Δ 0 SC (inset of Figure 5 a and b), explaining qualitatively the heating power dependence of the transition-state enhancement ( Figure 3 f). Nonetheless, this analysis based on the superconducting coherence factor does not capture the mechanism behind the nontrivial t Nb dependence ( Figure 4 j).…”

“…The spin-to-charge conversion mediated by QPs is substantially enhanced in the normal-to-superconducting transition regime, where the interface superconducting gap matches the magnon spin accumulation. The conversion efficiency and characteristics depend crucially on the driving/heating power and the SC thickness, which is understood based on the two competing effects: the superconducting coherence 29 , 30 , 42 and the exchange-field-modified QP relaxation. 4 − 6 , 48 The validity of these competing mechanisms is experimentally confirmed by spatially resolved measurements with varying the separation of electrical contacts on the spin-split Nb layer.…”

Nonlinear spin torque, pumping, and cooling in superconductor/ferromagnet systems

“…However, spin splitting of the superconductor enables the precession to pump energy current at linear response, and as its Onsager counterpart, there is nonzero thermal spin torque. This torque arises via the mechanism of [18], in contrast to magnon spin-Seebeck effects [3,16,25] or the in our case small normal-state Mott thermopower [3,29,34], which are not included above.…”

“…In such systems charge and (non-collinear) spin transport are closely linked and need to be treated on the same footing. Recently there has also been increased interest to couple superconductors to magnets and find out how superconductivity affects the magnetization dynamics, [5][6][7][8][9][10][11][12][13][14][15] including thermally driven effects [16]. On the other hand, recent work has shown that a combination of magnetic and superconducting systems results to giant thermoelectric effects [17][18][19][20] coupling charge and heat currents.…”