A Josephson current through a half-metallic ferromagnet between two conventional superconductors is theoretically studied. The spin dynamics such as magnon excitation plays a crucial role not only for the conversion between spin-singlet and spin-triplet pairs but also for the formation of the composite state of a triplet Cooper pair and magnon, by which the Josephson current flows between the superconductors. We propose the supercurrent pumping driven by the coherent precession of the magnetization by tuning the microwave frequency to the ferromagnetic resonance frequency in a ferromagnetic Josephson junction.

Based on the spin-pumping theory and first-principles calculations, the spin-mixing conductance (SMC) is theoretically studied for Pt/Permalloy (Ni 81 Fe 19 , Py) junctions. We evaluate the SMC for ideally clean Pt/Py junctions and examine the effects of interface randomness. We find that the SMC is generally enhanced in the presence of interface roughness as compared to the ideally clean junctions. Our estimated SMC is in good quantitative agreement with the recent experiment for Pt/Py junctions. We propose possible routes to increase the SMC in Pt/Py junctions by depositing a foreign magnetic metal layer in Pt, offering guidelines for designing the future spintronic devices.

We study the supercurrent resulting from coupling of the Josephson phase and the spin wave excited by microwave radiation in a ferromagnetic Josephson junction, in which two superconductors are separated by a ferromagnet. To explore how the spin-wave excitation affects the current-voltage curve, the resistively shunted junction model, which is an equation of motion for the Josephson phase, is extended by considering the gauge invariance including magnetization. When the magnetization is driven by the microwave adjusted to the ferromagnetic resonance frequency, the dc supercurrent is induced in the junction and the current-voltage curve shows step structures as a function of applied voltage. The position of each step in voltage is proportional to the microwave frequency multiplied by an even number. This means that the even number of magnons is necessary for the singlet Cooper pair to go through the ferromagnetic layer. The magnitudes of step height can be controlled by tuning the shape of the interface. Our results present a new route to observe the spin-wave excitation by the Josephson effect.

We propose the phase dynamics induced by spin waves in a superconductor/ferromagnet/superconductor (SC/FM/SC) Josephson junction. The resistively shunted junction (RSJ) model, which describes the dynamics of superconducting phase difference, is extended to include the spin wave excitation by ferromagnetic resonance (FMR) using the gauge invariant phase difference between two s-wave superconductors. The current-voltage characteristics show step structures when the magnetization in FM is driven by tuning the microwave frequency to FMR in the SC/FM/SC junction. The result presents a new route to observe the spin wave excitation using the Josephson effect.

We theoretically study spin current through ferromagnet (F) in a Josephson junction composed of s-wave superconductors and two layers of ferromagnets. Using quasiclassical theory, we show that the long-range spin current can be driven by the superconducting phase difference without a voltage drop. The origin of this spin current is due to spin-triplet Cooper pairs (STCs) formed by electrons of equal spin, which are induced by the proximity effect inside the F. We find that the spin current carried by the STCs exhibits long-range propagation in the F even where the Josephson charge current is practically zero. We also show that this spin current persists over a remarkably longer distance than the ordinary spin current carried by spin polarized conduction electrons in the F. Our results thus indicate the promising potential of Josephson junctions based on multilayer ferromagnets for spintronics applications with long-range propagating spin current.