Spin pumping from antiferromagnetic insulator spin-orbit-proximitized by adjacent heavy metal: A first-principles Floquet-nonequilibrium Green function study

We compute and compare pumped spin currents from sub-THz-microwave-driven uniaxial antiferromagnetic insulator (AFI) MnF2 into heavy metal (HM) Pt with strong spin-orbit (SO) coupling. In contrast to using the concept of interfacial spin mixing conductance (SMC) which fails in the presence of SO coupling at the interface, we use noncollinear DFT Hamiltonian with Floquet-nonequilibrium Green functions to take into account SO-proximitized AFI due to adjacent HM; SO coupling at interfaces, and evanescent wavefunctions penetrating from Pt or Cu into AFI layer to make its interfacial region conducting. The DC component of pumped spin current vs. precession cone angle (theta) of the Neel vector of AFI does not follow putative relation proportional to sin^2 theta, except for very small angles theta less than 10 deg for which we define an effective SMC from the prefactor and find that it doubles from MnF2/Cu to MnF2/Pt interface. The angular dependence spin current is twice as large as SMC for the right-handed as for the left-handed chirality of the precession modes of localized magnetic moments within AFI at resonance.