Band-Structure-Dependent Demagnetization in the Heusler Alloy ${\mathrm{Co}}_2{\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{Si}$

We investigate the ultrafast demagnetization for two Heusler alloys (${\mathrm{Co}}_2{\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\mathrm{Si}$) with a different lineup of the minority band gap and the Fermi level. Even though electronic spin-flip transitions are partially blocked by the band gap in one compound, the respective magnetization dynamics, as measured by the time-resolved Kerr effect, are remarkably similar. Based on a dynamical model that includes momentum and spin-dependent carrier scattering, we show that the magnetization dynamics are dominated by hole spin-flip processes, which are not influenced by the gap.

Figure 1

Schematic density of states for the Heusler alloys CMS and CFS. Note that the band gap is above $E_F$ for CMS and below $E_F$ for CFS [17]. Transitions induced by the optical pump pulse and spin-flip scattering pathways are also shown.

Figure 2

Magnetization dynamics of CFS (black circles) and CMS (red squares) measured by TR-MOKE with a pump fluence of $4.5\mathrm{mJ}/{\mathrm{cm}}^2$. Lines are exponential fits (see text).

Figure 3

Majority $(\uparrow )$ and minority $(\downarrow )$ bands for (a) CFS and (b) CMS used for calculations. Optical transitions are shown as black (red) arrows for majority (minority) bands. Regions of dominant spin-flip transitions are marked by an ellipse. Degeneracies are included in gray italics in the legends.

Figure 4

(a) Calculated transient magnetization for CFS and CMS including calculations for CMS with 5% and 100% Co AD. Electronic occupation of bands for (b) CFS and (c) CMS. Displayed is an effective occupation number density, which it not normalized, because it includes several, almost degenerate bands.

Figure 5

Momentum- and time-resolved occupation of the CMS majority band $1(\uparrow )$. Relevant signatures of optical excitation and spin-flip scattering are marked by arrows.