| Popis: |
One of the cornerstone effects in spintronics is spin pumping by dynamical magnetization that is steadily precessing (around, for example, the z -axis) with frequency ω _0 due to absorption of low-power microwaves of frequency ω _0 under the resonance conditions and in the absence of any applied bias voltage. The two-decades-old ‘standard model’ of this effect, based on the scattering theory of adiabatic quantum pumping, predicts that component $I^{S_z}$ of spin current vector $\big( I^{S_x}(t),I^{S_y}(t),I^{S_z} \big) \propto \omega_0$ is time-independent while $I^{S_x}(t)$ and $I^{S_y}(t)$ oscillate harmonically in time with a single frequency ω _0 whereas pumped charge current is zero $I \equiv 0$ in the same adiabatic $\propto \omega_0$ limit. Here we employ more general approaches than the ‘standard model’, namely the time-dependent nonequilibrium Green’s function (NEGF) and the Floquet NEGF, to predict unforeseen features of spin pumping: namely precessing localized magnetic moments within a ferromagnetic metal (FM) or antiferromagnetic metal (AFM), whose conduction electrons are exposed to spin–orbit coupling (SOC) of either intrinsic or proximity origin, will pump both spin $I^{S_\alpha}(t)$ and charge I ( t ) currents. All four of these functions harmonically oscillate in time at both even and odd integer multiples $N\omega_0$ of the driving frequency ω _0 . The cutoff order of such high harmonics increases with SOC strength, reaching $N_\mathrm{max} \simeq 11$ in the one-dimensional FM or AFM models chosen for demonstration. A higher cutoff $N_\mathrm{max} \simeq 25$ can be achieved in realistic two-dimensional (2D) FM models defined on a honeycomb lattice, and we provide a prescription of how to realize them using 2D magnets and their heterostructures. |
