Transition of laser-induced terahertz spin currents from torque- to conduction-electron-mediated transport

Abstract

Spin transport is crucial for future spintronic devices operating at bandwidths up to the terahertz range. In F|N thin-film stacks made of a ferromagnetic/ferrimagnetic layer $F$ and a normal-metal layer $N$, spin transport is mediated by (1) spin-polarized conduction electrons and/or (2) torque between electron spins. To identify a crossover from (1) to (2), we study laser-driven spin currents in $F$|Pt stacks where $F$ consists of model materials with different degrees of electrical conductivity. For the magnetic insulators yttrium iron garnet, gadolinium iron garnet (GIG) and $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$, identical dynamics is observed. It arises from the terahertz interfacial spin Seebeck effect (SSE), is fully determined by the relaxation of the electrons in the metal layer, and provides a rough estimate of the spin-mixing conductance of the GIG/Pt and $\ensuremath{\gamma}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}/\mathrm{Pt}$ interfaces. Remarkably, in the half-metallic ferrimagnet ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ (magnetite), our measurements reveal two spin-current components with opposite direction. The slower, positive component exhibits SSE dynamics and is assigned to torque-type magnon excitation of the A- and B-spin sublattices of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$. The faster, negative component arises from the pyrospintronic effect and can consistently be assigned to ultrafast demagnetization of minority-spin hopping electrons. This observation supports the magneto-electronic model of ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$. In general, our results provide a route to the contact-free separation of torque- and conduction-electron-mediated spin currents.