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

TL;DR

This study investigates laser-induced spin currents in ferromagnetic/normal-metal stacks, revealing a transition from torque-mediated to conduction-electron-mediated transport, with distinct dynamics observed in different magnetic materials.

Contribution

It demonstrates a contact-free method to distinguish between torque- and conduction-electron-mediated spin currents using laser-driven experiments across various magnetic materials.

Findings

Identified two spin-current components in Fe3O4 with opposite directions.

Observed identical THz interfacial spin Seebeck effect in magnetic insulators.

Provided estimates of the spin-mixing conductance for GIG/Pt interface.

Abstract

Spin transport is crucial for future spintronic devices operating at bandwidths up to the terahertz (THz) range. In F|N thin-film stacks made of a ferro/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 cross-over 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 YIG, GIG and maghemite, identical dynamics is observed. It arises from the THz interfacial spin Seebeck effect (SSE), is fully determined by the relaxation of the electrons in the metal layer and provides an estimate of the spin-mixing conductance of the GIG/Pt interface. Remarkably, in the half-metallic ferrimagnet Fe3O4 (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 Fe3O4. The faster, negative component arises from the pyro-spintronic effect and can consistently be assigned to ultrafast demagnetization of e-sublattice minority-spin hopping electrons. This observation supports the magneto-electronic model of Fe3O4. In general, our results provide a new route to the contact-free separation of torque- and conduction-electron-mediated spin currents.