Transition of laser-induced terahertz spin currents from torque- to conduction-electron-mediated transport
Pilar Jiménez-Cavero, Oliver Gueckstock, Lukáš Nádvorník, I. Lucas, Tom S. Seifert, Martin Wolf, Reza Rouzegar, Piet W. Brouwer, Sven Becker, G. Jakob, Mathias Kläui, Chenyang Guo, Caihua Wan, Xiufeng Han, Zuanming Jin, Hui Zhao, Di Wu, L. Morellón, Tobias Kampfrath
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.