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

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 γ−Fe2O3, 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 γ−Fe2O3/Pt interfaces. 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 pyrospintronic effect and can consistently be assigned to ultrafast demagnetization of minority-spin hopping electrons. This observation supports the magneto-electronic model of Fe3O4. In general, our results provide a route to the contact-free separation of torque- and conduction-electron-mediated spin currents.
The authors acknowledge funding by the German Research Foundation through the collaborative research centers SFB TRR 227 “Ultrafast spin dynamics” (Project ID 328545488, projects A05, B02, and B03), SFB TRR 173 “Spin + X” (Project No. 358671374, projects A01 and B02), the European Union H2020 program through the project CoG TERAMAG(GrantNo.681917), the Spanish Ministry of Science and Innovation (Project No. PID2020-112914RB-I00), the National Natural Science Foundation of China (Grants No. 61988102 and No. 61975110), and the Czech Science Foundation through Project GA CR/Grant No. 21-28876J. P.J.-C. acknowledges the Spanish MECD for support through the FPU program (References No. FPU014/02546 and No. EST17/00382.)