Ultrafast Interaction of the Angular Momentum of Photons with Spins in the Metallic Amorphous Alloy GdFeCo

Ultrashort laser pulses have been used to study the effect of circularly polarized light on spins in the ferrimagnetic metal GdFeCo. By turning the sample into a multidomain state and thereby suppressing the observation of the heating effect of light, we have been able to demonstrate an ultrafast nonthermal excitation of spin waves with a phase that depends on the angular momentum of the photons. These results demonstrate the possibility of ultrafast coherent control of the magnetization in this metallic system.

Figure 1

The initial magnetic state of the GdFeCo sample is a single domain state (left image). Certain values of an external field applied at an angle from the normal to the sample turn the sample into a multidomain state (right image) with magnetic domains much smaller than the probe spot diameter. The images where captured by means of a polarization microscope.

Figure 2

(a) At $\Delta t<0$, the magnetizations ${\mathbf{M}}_1$ and ${\mathbf{M}}_2$ of oppositely magnetized domains are tilted due to the balance between magnetocrystalline anisotropy field ${\mathbf{H}}_a$, shape anisotropy field ${\mathbf{H}}_s$, and external field ${\mathbf{H}}_e$. (b) After photoexcitation, $\Delta t>0$, heating induces out of phase oscillations of the $z$ components of ${\mathbf{M}}_1$ and ${\mathbf{M}}_2$.

Figure 3

Precession of the magnetization excited by circularly polarized pump pulses in GdFeCo, probed via the magneto-optical Faraday effect. For certain values of the external field ${\mathbf{H}}_e$, the two helicities ${\sigma }^{+}$ and ${\sigma }^{-}$ give rise to precession with different phase. The lines are guides to the eye.

Figure 4

The difference (a) and the sum (b) of the experimental data resulting from excitations by ${\sigma }^{+}$ and ${\sigma }^{-}$ pump pulses, representing the optomagnetic and the heat-driven excitation, respectively. The curves are vertically displaced for clarity. The insets of the figure are schematic representations of the optomagnetic and the heat-driven excitation mechanisms.