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

Motivated by the recent experiment (Vaidya et al 2020 Science 368 160) on spin pumping from sub-THz radiation-driven uniaxial antiferromagnetic insulator (AFI) MnF2 into heavy metal (HM) Pt hosting strong spin-orbit (SO) coupling, we compute and compare pumped spin currents in Cu/MnF2/Cu and Pt/MnF2/Cu heterostructures. Recent theories of spin pumping by AFI have relied on simplistic Hamiltonians (such as tight-binding) and the scattering approach to quantum transport yielding the so-called interfacial spin mixing conductance (SMC), but the concept of SMC ceases to be applicable when SO coupling is present directly at the interface. In contrast, we use a more general first-principles quantum transport approach which combines noncollinear density functional theory with Floquet-nonequilibrium Green's functions in order to take into account: SO-proximitized AFI as a new type of quantum material, different from isolated AFI and brought about by AFI hybridization with adjacent HM layer; strong SO coupling at the interface; and evanescent wavefunctions penetrating from Pt or Cu into AFI layer to make its interfacial region conducting rather than insulating as in the isolated AFI. The DC component of pumped spin current I-DC(Sz) vs. precession cone angle theta(l) of the Neel vector l of AFI does not follow putative I-DC(Sz) proportional to sin(2) theta(l) , except for very small angles theta(l) less than or similar to 10 degrees for which we define an effective SMC from the prefactor and find that it doubles from MnF2/Cu to MnF2/Pt interface. In addition, the angular dependence I-DC(Sz)(theta(l)) differs for opposite directions of precession of the Neel vector, leading to twice as large SMC for the right-handed than for the left-handed chirality of the precession mode.