Spin-polarized Shapiro steps and spin-precession-assisted multiple Andreev reflection

Abstract

We investigate the charge and spin transport of a voltage-biased superconducting point contact coupled to a nanomagnet. The magnetization of the nanomagnet is assumed to precess with the Larmor frequency ω_L when exposed to ferromagnetic resonance conditions. The Larmor precession locally breaks the spin-rotation symmetry of the quasiparticle scattering and generates spin-polarized Shapiro steps for commensurate Josephson and Larmor frequencies that lead to magnetization reversal. This interplay between the ac Josephson current and the magnetization dynamics occurs at voltages |V| = \hbar ω_L/2en for n = 1,2, . . ., and the subharmonic steps with n > 1 are a consequence of multiple Andreev reflection (MAR). Moreover, the spin-precession-assisted MAR generates quasiparticle scattering amplitudes that, due to interference, lead to current-voltage characteristics of the dc charge and spin currents with subharmonic gap structures displaying an even-odd effect.