Femtosecond Spin Current Pulses Generated by the Nonthermal Spin-Dependent Seebeck Effect and Interacting with Ferromagnets in Spin Valves

Abstract

Using the sensitivity of optical second harmonic generation to currents, we demonstrate the generation of 250-fs long spin current pulses in Fe=Au=Fe=MgOð001Þ spin valves. The temporal profile of these pulses indicates ballistic transport of hot electrons across a sub-100 nm Au layer. The pulse duration is primarily determined by the thermalization time of laser-excited hot carriers in Fe. Considering the calculated spin-dependent Fe=Au interface transmittance we conclude that a nonthermal spin-dependent Seebeck effect is responsible for the generation of ultrashort spin current pulses. The demonstrated rotation of spin polarization of hot electrons upon interaction with noncollinear magnetization at Au=Fe interfaces holds high potential for future spintronic devices. DOI: 10.1103/PhysRevLett.119.017202 Optimization and control of spin currents (SC) and their interaction with magnetic constituents in heterostructures on a femtosecond time scale is key for future terahertz spintronics applications. Although electronic transport through a ferromagnet (FM), as described by Mott's two current model [1], generates a spin-polarized current, its density is intrinsically limited by Joule losses. The discovery of the spin-dependent Seebeck effect (SdSE), where thermal gradients over a bulk FM [2] or across an interface to a normal metal [3] generate SCs, opened a path towards overcoming such limitations. Indeed, shortlived thermal gradients can produce short (∼100 ps) SC pulses at densities exceeding the static Joule limit, as recently demonstrated upon laser excitation of spin-valve structures [4].Creating highly energetic electrons [5][6][7][8][9], femtosecond laser excitation is promising for SC pulse generation on subpicosecond time scales, before the electron-electron [9] and electron-lattice [8] equilibration is reached.

Micromagnetic view on ultrafast magnon generation by femtosecond spin current pulses

“…In the outlook section we discussed a way to overcome the diffraction limit when exciting propagating inplane spin waves. The basic idea relies on the fact in Fe/Au/Fe trilayer spin valve trilayers 8,24 , nanostructuring of the top, laser-excited layer enables spatial tailor-ing of the spin current profile. In the simplest case, spin currents can only be excited where the top Fe layer exists, thus introducing an in-plane inhomogeneity into the STT stimulus.…”

Interface reflectivity of a superdiffusive spin current in ultrafast demagnetization and terahertz emission

“…4 more easily through the interface from Fe to Au than the spin-down electrons, which is qualitatively consistent with recent calculations. 48 The difference in the reflectivity between the two spin channels results in spin filtering for the electrons passing through the Fe|Au(001) interface. The calculated reflectivity from Au to Fe is plotted in Fig.…”

Long-range transverse spin Seebeck effect in permalloy stripes using Sagnac interferometer microscopy

“…Recently, the magneto-optic Kerr effect (MOKE) has been successfully employed for measuring SSE via the detection of the underlying generated spin accumulation [25] and SDSE [23,26,27], which are not affected by thermoelectric and magneto-thermoelectric artefacts. Moreover, the MOKE technique can directly probe the spin accumulation in conducting FM slabs without involving additional top metal layers required to measure spin current via ISHE, so that the possible unintentional out-of-plane thermal gradient can be greatly suppressed.…”