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 analyze 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.
Alexey Melnikov, Ilya Razdolski, Alexandr Alekhin, Nikita Ilin, Jan Meyburg, Detlef Diesing, Vladimir Roddatis, Ivan Rungger, Maria Stamenova, Stefano Sanvito, Uwe Bovensiepen
Further development of spintronics requires miniaturization and reduction of characteristic timescales of spin dynamics combining the nanometer 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 spin transfer torque (STT) exerted by ultrashort SC pulses on the FM open the time domain for studying STT fingerprint on spatially non-uniform magnetization dynamics. Using the sensitivity of magneto-induced second harmonic generation to SC, we develop technique for SC monitoring. With 20 fs resolution, we demonstrate the generation of 250 fs-long SC pulses in Fe/Au/Fe/MgO(001) structures. Their temporal profile indicates (i) nearly-ballistic hot electron transport in Au and (ii) that the pulse duration is primarily determined by the thermalization time of laser-excited hot carriers in Fe. Together with strongly spin-dependent Fe/Au interface transmission calculated for these carriers, this suggests the non-thermal spin-dependent Seebeck effect dominating the generation of ultrashort SC pulses. The analysis of SC transmission/reflection at the Au/Fe interface shows that hot electron spins orthogonal to the Fe magnetization rotate gaining huge parallel (anti-parallel) projection in transmitted (reflected) SC. This is accompanied by a STT-induced perturbation of the magnetization localized at the interface, which excites the inhomogeneous high-frequency spin dynamics in the FM. Time-resolved magneto-optical studies reveal the excitation of several standing spin wave modes in the Fe film with their spectrum extending up to 0.6 THz and indicating the STT spatial confinement to 2 nm.
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