Open Access Version

Abstract:
The generation of spin currents is a key ingredient in the spintronics based memory devices such as spin transfer torques - magnetic random-access memory. Advances in ultrafast magnetism [1] in the last decade have demonstrated that the switching time can be reduced to the sub-picosecond time scales with low power consumption [1]. Ever since this pioneer works, several experiments on ferromagnetic 3d transition metals have studied the ultrafast spin dynamics induced by such fs spin currents in spin-valve structures [2–7], while recent studies suggest that antiferromagnetic materials are more promising for ultrafast spintronics [8]. Among these AFM materials, the rare-earth/ transition metal (RE-TM) alloys represent model systems [8,9]. Here, I investigate the sub-picosecond and picosecond spin current-induced demagnetization dynamics in RE-TM alloys. To distinguish the element- specific dynamics, I used the element-selective technique of Time-resolved X- Ray Magnetic Circular Dichroism available at the large-scale infrastructures HZB-BESSY II - Berlin. In this thesis work, the focus is on studying the ultrafast demagnetization dynamics of thin buried ferrimagnetic layer (FeGd and CoGd alloys), using the ultrashort spin-polarized hot-electron (SPHE) pulses as a pump and ultrashort X rays as a probe. I have explored the impact of SPHE induced effect on two different spin valve structures consisting of out-of-plane Fe74Gd26/Cu/{Co/Pt}*3 , and of an in-plane Co61Gd39/Cu/CoPt. The key findings are summarized as follows: In Fe74Gd26 based spin valves: • In the case of out-of-plane Fe74Gd26 based spin-valves, we investigated the spin-polarized hot electron induced dynamics of Fe 3d and Gd 4f moments, and through fitting, crucial parameters such as demagnetization time, hot electron pulse width, and recovery were obtained. The estimation of hot electron pulses highlighted their elongation as they traversed different capping layers. • I observed that ultrafast spin current accelerated the Fe3d dynamics in the case of P configuration, which shows that antiparallel spins between Fe3d and SPHE spins help in the demagnetization of the Fe3d sublattice. • Theoretical calculations based on atomistic spin dynamics with STT successfully reproduced the spin dependent demagnetization dynamics at Fe and Gd edges. The calculations approximated that the thin polarizer film Co/Pt could generate 100 % polarization of the spin current and show that the spin polarization of the SPHE current out of Co/Pt multilayers is opposite to the Co magnetization. 2 In Co61Gd39 based spin valves: • A slow demagnetization of the hot-electron induced dynamics at Gd4f sublattice could be related to the concentration of such alloys irrespective of the excitation source (laser light or hot electron pulses). • I observed that ultrafast spin current accelerated the Gd4f dynamics in the case of AP configuration, which shows that parallel spins between Gd4f and SPHE spins help the demagnetization of Gd4f sublattice. • The surface-sensitive soft X-ray photoelectron spectroscopy (XPS) technique has evidenced a concentration profile of Gd at the surface of the GdCo alloy. The profile shows an enhanced Gd content in the first 2 nm in Co65Gd35, whereas, for a Co80Gd20 alloy, a single segregated Gd monolayer on the top of an almost homogenous alloy was defined.