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.

Spintronics had a widespread impact over the past decades due to transferring information by spin rather than electric currents. Its further development requires miniaturization and reduction of characteristic timescales of spin dynamics combining the sub-nanometre spatial and femtosecond temporal ranges. These demands shift the focus of interest towards the fundamental open question of the interaction of femtosecond spin current (SC) pulses with a ferromagnet (FM). The spatio-temporal properties of the impulsive spin transfer torque exerted by ultrashort SC pulses on the FM open the time domain for probing non-uniform magnetization dynamics. Here we employ laser-generated ultrashort SC pulses for driving ultrafast spin dynamics in FM and analysing its transient local source. Transverse spins injected into FM excite inhomogeneous high-frequency spin dynamics up to 0.6 THz, indicating that the perturbation of the FM magnetization is confined to 2 nm.

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Non-equilibrium magnetic effects at interfaces for ultrafast dynamics Representing the future of spintronics, femtosecond spin current (SC) pulses constitute a versatile tool to transfer spin and control magnetization on the ultrafast timescale. It is therefore of paramount importance to understand the kinetics of these pulses and the fundamentals of their interaction with magnetized media. In our work, we demonstrate the key role of interfaces for the SC dynamics in Fe/Au/Fe multilayers. In particular, we argue that both (i) demagnetization caused by a pulse of hot electrons and (ii) spin transfer torque exerted by the orthogonal to the Fe magnetization projection of magnetic moment delivered by SC pulse are localized in the vicinity of the Fe/Au interface. We analyse both processes in details, showing that the SC-driven excitation of the sub-THz spin wave dynamics in Fe film is enabled by the spatial confinement of the exerted spin transfer torque. Moreover, a pulse of hot electrons leads to the efficient demagnetization of the Fe film. By disentangling the magneto-optical Kerr effect (MOKE) transients we demonstrate the strong spatial non-uniformity of this demagnetization. We argue that simultaneous recording of transient MOKE rotation and ellipticity is crucial for drawing such conclusions. Our findings have a twofold impact: firstly, they illustrate rich opportunities of utilizing SC pulses for manipulation of magnetization in ferromagnets and, secondly, they highlight the importance of spatial localization for understanding the ultrafast spin dynamics in multilayers.

An experimental setup, which combines direct heating and temperature measurement of metal nanofilms allowing temperature programmed desorption experiments is described. This setup enables the simultaneous monitoring of the thermal desorption flux from the surface of chemi-electric devices and detection of chemically induced hot charge carriers under UHV conditions. This method is demonstrated for the case of water desorption from a Pt/SiO2-n-Si metal-oxide-semiconductor nanostructure.