Probing laser-induced spin-current generation in synthetic ferrimagnets using spin waves

Several rare-earth transition-metal ferrimagnetic systems exhibit all-optical magnetization switching upon excitation with a femtosecond laser pulse. Although this phenomenon is very promising for future optomagnetic data storage applications, the role of nonlocal spin transport in these systems is scarcely understood. Using Co/Gd and Co/Tb bilayers, we isolate the contribution of the rare-earth materials to the generated spin currents by using the precessional dynamics they excite in an adjacent ferromagnetic layer as a probe. By measuring terahertz (THz) standing spin-waves as well as GHz homogeneous precessional modes, we probe both the high- and low-frequency components of these spin currents. The low-frequency homogeneous mode indicates a significant contribution of Gd to the spin current but not from Tb, consistent with the difficulty in achieving all-optical switching in Tb-containing systems. Measurements on the THz frequency spin waves reveal the inability of the rare-earth generated spin currents to excite dynamics at the subpicosecond timescale. We present modeling efforts using the $s-d$ model, which effectively reproduces our results and allows us to explain the behavior in terms of the temporal profile of the spin current.

Figure 1

(a) Schematic overview of the experiments, where the effect of the generation layer composition on the response of the absorption layer is sketched for each of the three studied configurations (Co, Co/Gd, and Co/Tb). (b) Example magnetization response calculated using the s-d model for Co and for Gd in a Co/Gd bilayer. [(c) and (d)] Response of the absorption layer for each studied configuration. Both (c) the homogeneous ($H_{\text{ext}}=100$ mT) and (d) the first order inhomogeneous mode ($H_{\text{ext}}=0$ mT) are plotted, where the latter is measured using complex MOKE.

Figure 2

(a) Measured FMR mode ($H_{\text{ext}}=100$ mT) for several Gd thicknesses, fitted with a damped cosine function. The orange dashed line indicates the FMR phase shift and the thickness calibration is discussed in Sec. I of Ref. [34]. [(b) and (c)] FMR phase as a function of Gd (b) and Tb (c) thickness. [(d) and (e)] FMR amplitude as a function of Gd (d) and Tb (e) thickness, normalized to the amplitude without RE layer for both datasets. The orange solid line in (b) and (d) represents the modeled behavior using Eq. (1), and the black solid line in (d) is a fit with an exponential function.

Figure 3

(a) Measured THz mode for several Gd thicknesses, fitted with a damped cosine function. The orange dashed line indicates the spin-wave phase shift. (b) Spin-wave amplitude, orange line indicates a solution of Eq. 1 using $A_{\text{Gd}}=0$ and ${\lambda }_{\text{diff,Gd}}=1.9$ nm and (c) frequency as a function of Gd thickness.

Figure 4

(a) Calculated FMR mode excited by the spin current from a Co(1) (black) and Co(1)/Gd(2) (blue) generation layer. (b) Absolute phase of the excited FMR mode as a function of the effective thickness $t_{\text{eff,Gd}}$ of the (generation) Gd layer. (c) Absorbed spin current as function of time generated by a Co (black) and a Co/Gd (blue) generation layers.