Optical inter-site spin transfer probed by energy and spin-resolved transient absorption spectroscopy

Abstract

Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization as it does not rely on secondary mechanisms to dissipate angular momentum. In the present work, we show that such an optical inter-site spin transfer (OISTR) from Pt to Co emerges as a dominant mechanism governing the ultrafast magnetization dynamics of a CoPt alloy. To demonstrate this, we perform a joint theoretical and experimental investigation to determine the transient changes of the helicity dependent absorption in the extreme ultraviolet spectral range. We show that the helicity dependent absorption is directly related to changes of the transient spin-split density of states, allowing us to link the origin of OISTR to the available minority states above the Fermi level. This makes OISTR a general phenomenon in optical manipulation of multi-component magnetic systems.

Element-Specific Magnetization Dynamics of Complex Magnetic Systems Probed by Ultrafast Magneto-Optical Spectroscopy

“…In elemental Co, this is not observed, again consistent with the transient occupations shown in Figure 6. The theoretical calculation of the exact same observable, µ/µ 0 , reproduces our experimental finding very well [18].…”

“…Second, a lot of research in resonant XUV spectroscopy has recently been devoted to exploring a potential influence of transient changes in the electro-optical functions β(t) and δ(t) to MCD signals [26,29,34,[47][48][49]. Transient changes in the transmission, T [18], or reflection [50] at a core-valence band transition have been shown to be on the order of a few percent, and hence of a similar magnitude compared to the changes in MCD signals in the XUV spectral range [38]. This is different for the Faraday rotation.…”

“…As outlined in the introduction, in multi-component magnetic systems with sites exhibiting a distinct number of available states above the Fermi energy, OISTR can become an important mechanism for ultrafast demagnetization. To this end, we have compared the ultrafast magnetization dynamics of an elemental Co film and Co 0.5 Pt 0.5 alloy via MCD in transmission geometry [18], as schematically depicted in Figure 2b. We generate high harmonic radiation by focusing intense laser pulses into a neon-filled gas cell, leading to discrete harmonic emission peaks in the spectral range between ≈40-72 eV with a width of ≈200 meV.…”

“…The authors of this theoretical study also predicted that the spin-selective charge transfer between distinct sites can lead to a metamagnetic phase transition, switching the magnetic order from an antiferromagnetic to a transient ferromagnetic alignment. In the last two years, a number of experiments demonstrated signatures of OISTR, both in the optical [13,14] as well as in the extreme ultraviolet (XUV) [15][16][17][18] spectral range. The latter experiments were carried out with high harmonic generation (HHG) sources and exploited the element specificity of resonant core-to-valence state transitions.…”

Ultrafast Magnetization Dynamics in Multilayered Films Down to the Few-cycle Regime

“…The initial femtoseconds of the coherent spin-dynamics can be explained by the optically induced spin transfer (OISTR) effect recently introduced by Dewhurst et al [182], whose the theoretical work predicted that few-cycle optical excitation can directly redistribute spins between different magnetic sublattices in a multicomponent magnetic material, without change the total magnetization of the material [183]. OISTR is the fastest and most efficient process to manipulate macroscopic magnetization since it does not rely on secondary mechanisms to dissipate angular momentum [184], and is particularly interesting from both a fundamental and an experimental point of view [183].…”