Ultrafast non-local spin dynamics in metallic bi- layers by linear and non-linear magneto-optics

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 I2(M,t) ∝ |Eeven+Eodd|, 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 I2(±M,t) and I2(±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 λω Fe≈20 nm), which is corroborated by the transient reflectivity and Eeven(t) (not shown).