Ultrafast Non-local Spin Dynamics in Metallic Bi-Layers by Linear and Non-linear Magneto-Optics

Abstract

We make a step towards the understanding of spin dynamics induced by spin-polarized hot carriers in metals. Exciting the Fe layer of Au/Fe/MgO(001) structures with femtosecond laser pulses, we demonstrate the ultrafast spin transport from Fe into Au using time-resolved MOKE and mSHG for depthsensitive detection of the transient magnetization.Ultrafast spin dynamics is the key for development of data storage and spintronics devices. Modern all-optical techniques provide ultimate time resolution of sub-10 fs for the related studies. However, recent findings on laser-induced magnetization dynamics [1] still challenge the understanding of ultrafast magnetism. With this contribution, we make a step towards the understanding of ultrafast spin dynamics in metallic multi-layers, which is (i) spatially non-uniform due to interfaces and strong absorption of the pump pulse and (ii) non-local due to highly mobile hot carriers (HC) that are spin-polarized if excited in a ferromagnet.The transient magnetization M was monitored by the magneto-optical Kerr effect (MOKE) and magneto-induced second harmonic (SH) generation (mSHG) using the 800 nm, 14 fs output of a cavity-dumped Ti:sapphire oscillator. The SH intensity where t is the pump-probe delay, Eeven(t) is independent of M and Eodd(t) ∝ M(t).The interface spin dynamics is analyzed by ∆odd(t) ≈ Eodd(t)/Eodd(t0) −1≈M(t)/M0 −1 defined from I 2ω (±M,t) and I 2ω (±M0,t0); t0 represents the absence of excitation [2]. The bulk M is probed by ∆MR, ML ≡ θ(t)/θ0 −1 ≈ M(t)/M0 −1, where θ is the MOKE rotation (MR) or ellipticity (ME). The results obtained in epitaxial Au/Fe/MgO(001) [3] serving as a model system, show pronounced dependence of ∆MR and ∆odd on the Fe thickness dFe (Fig.1 a, b). This is explained by an increase of the Fe/Au interface contribution with reducing dFe due to absorption (at 800 nm the light penetration depth in Fe ≈20 nm), which is corroborated by the transient reflectivity and Eeven(t) (not shown).

Interface-induced phenomena in magnetism

“…Ballistic Fe spins injected into a Au layer travelling close to the Au Fermi velocity arrive at the Au back interface within hundreds of femtoseconds while a diffusive component was detected at times up to 1 ps, in qualitative agreement with wave diffusion calculations [Kaltenborn et al ., 2012]. Variation of Fe layer thickness shows that the active injection region is an ≈1 nm thick Fe layer at the Fe/Au interface [Melnikov et al ., 2011; Alekhin et al ., 2015; Melnikov et al ., 2015], implying that superdiffusive transport is of limited importance for ultrafast demagnetization of significantly thicker ferromagnetic films, an observation supported by recent demagnetization experiments in Ni films [Schellekens et al ., 2013]. [Ando et al ., 2011; Hoffmann, 2013] used the inverse spin Hall effect to detect superdiffusive spin currents in non-magnetic layers.…”