Angular Momentum Flow During Ultrafast Demagnetization of a Ferrimagnet

One of the key processes setting the speed of the ultrafast magnetization phenomena is the angular momentum transfer from and into the spin system. However, the way the angular momentum flows during ultrafast demagnetization and magnetization switching phenomena remains elusive so far. We report on time-resolved soft x-ray magnetic circular dichroism measurements of the ferrimagnetic GdFeCo alloy allowing us to record the dynamics of elemental spin and orbital moments at the Fe and Gd sites during femtosecond laser-induced demagnetization. We observe a complete transfer of spin and orbital angular momentum to the lattice during the first hundreds of femtoseconds of the demagnetization process.

Figure 1

Static soft x-ray absorption (XAS) and XMCD spectra of the studied GdFeCo sample measured in the energy range of (a) Fe $L_{3,2}$ and (b) Gd $M_{5,4}$ absorption edges. The XAS spectra (red and black) were recorded with circularly polarized x-ray light by using an out-of-plane magnetic field $B$ to saturate the sample in opposite directions. Taking the difference of both spectra leads to the XMCD spectrum (blue). The measurements were carried out with an energy resolution of 0.1 eV using the ALICE reflectometer [14] at the PM3 beam line of BESSY II [15].

Figure 2

Time-resolved XMCD at the (a) Fe $L_{3,2}$ and (b) Gd $M_{5,4}$ absorption edges of GdFeCo measured as a function of pump-probe delay. The temporal resolution of the measurement is $\approx 130\mathrm{fs}$. The data are normalized to the unpumped XMCD signal and fitted using a single exponential function (black solid lines). The error bars are calculated as the standard error of the mean.

Figure 3

Time-resolved evolution of spin and orbital moment at Fe and Gd sites during demagnetization extracted by application of sum rules from the XMCD data shown in Fig. 2. (a) At Fe sites the demagnetization occurs due to a decrease of both spin and orbital moments, equal in magnitude and speed, leading to a constant $L/S$ ratio (green) during the demagnetization process. (b) At Gd sites, the demagnetization is purely driven by a decrease of spin moment, while the orbital moment stays zero at all times; implicitly, the Gd $L/S$ ratio remains also zero and is not shown in the figure. The error bars and shaded areas correspond to the error propagation of the standard error of the mean.