X-ray detection of ultrashort spin current pulses in synthetic antiferromagnets

Abstract

We explore the ultrafast generation of spin currents in magnetic multilayer samples by applying fs laser pulses to one layer and measuring the magnetic response in the other layer by element-resolved x-ray spectroscopy. In Ni(5 nm)/Ru(2 nm)/Fe(4 nm), the Ni and Fe magnetization directions couple antiferromagnetically due to the Ruderman–Kittel–Kasuya–Yosida interaction but may be oriented parallel through an applied magnetic field. After exciting the top Ni layer with a fs laser pulse, we also find that the Fe layer underneath demagnetizes, with a 4.1±1.9% amplitude difference between parallel and antiparallel orientation of the Ni and Fe magnetizations. We attribute this difference to the influence of a spin current generated by the fs laser pulse that transfers angular momentum from the Ni into the Fe layer. Our results confirm that superdiffusive spin transport plays a role in determining the sub-ps demagnetization dynamics of synthetic antiferromagnetic layers, but also evidence large depolarization effects due to hot electron dynamics, which are independent of the relative alignment of the magnetization in Ni and Fe.

Time-Resolved XUV Absorption Spectroscopy and Magnetic Circular Dichroism at the Ni M2,3-Edges

“…On longer timescales, however, the MCD magnitude decreases and we observe a distortion of the spectra, especially for higher energies. We emphasize that these results are not at odds with data presented recently by Stamm et al [31]. In fact, their measurements on Ni are performed 0.7 ps after the excitation, which is well beyond the typical time where MCD energy shifts can be observed, as shown in the present study.…”

“…In fact, their measurements on Ni are performed 0.7 ps after the excitation, which is well beyond the typical time where MCD energy shifts can be observed, as shown in the present study. However, this recent work of Stamm et al [31] demonstrates that it is now possible to measure full MCD spectra for different delays. On longer timescales, as already mentioned, clear differences between the different demagnetization curves become apparent (Figure 4b).…”

“…Indeed, performing pure absorption measurements at X-ray free electron lasers (FELs) has been challenging, due to intrinsic fluctuations of the photon parameters and the lack of a reliable and linear incident intensity monitor [25]. However, motivated by time-resolved absorption experiments performed at femtoslicing facilities [17,22], as well as time-and spin-resolved photoelectron spectroscopy at high harmonic generation (HHG) sources [26,27], new technical advances were recently made to probe X-ray absorption dynamics using FEL [25,[28][29][30][31] and HHG [32,33], thereby laying the ground for a systematic analysis of charge and spin dynamics [30,34].…”

Simultaneous two-color snapshot view on ultrafast charge and spin dynamics in a Fe-Cu-Ni tri-layer

“…This could explain why in their recent study Stamm et al do not also observe an enhancement of the transient magnetization of the Fe layer in an Ni/Ru/Fe sandwich. 43 It also highlights the importance to perform more systematic time resolved studies in the pure absorption geometry where the measured intensities can be more easily related to physical concepts such as electronic density and magnetization.…”

Time-Resolved XUV absorption spectroscopy and magnetic circular dichroism at the Ni $M_{2,3}$-edges

“…Indeed, performing pure absorption measurements at x-ray free electron lasers (FEL) has been challenging which is due to intrinsic fluctuations of the photon parameters and the lack of a reliable and linear incident intensity monitor [23]. However, motivated by time-resolved absorption experiments performed at femtoslicing facilities [15,20], new technical advances were made recently to probe x-ray absorption dynamics using FEL [23][24][25][26][27] as well as high harmonic generation (HHG) sources [28,29], thereby laying the ground for a systematic analysis of charge and spin dynamics [26,30].…”