Spectral tunability of laser-induced spin dynamics in the ferromagnetic semiconductor CdCr2Se4

Abstract

Here we report that femtosecond laser pulses are able to trigger oscillations of the magneto-optical Faraday rotation in the ferromagnetic semiconductor $\mathrm{CdC}{\mathrm{r}}_{2}\mathrm{S}{\mathrm{e}}_{4}$ in the presence of an applied magnetic field. The frequency of these oscillations is a linear function of the magnetic field and corresponds to the ferromagnetic resonance (FMR). Tuning the photon energy of the pump pulses we reveal two different mechanisms, which induce FMR precession in this material. In the case of pumping from the valence band deep into the conduction band (photon energy 3.1 eV), the phase of the spin oscillations is not sensitive to the polarization of the pump, but can be reversed over 180 deg by changing the polarity of the applied magnetic field. We assign these oscillations to the coherent spin precession triggered by ultrafast laser-induced heating. This mechanism requires a strong optical absorption in the material and becomes inactive if the pump photon energy is below the band gap. Tuning the photon energy in a wide range from 0.88 to 2.1 eV reveals the second mechanism of optical excitation of coherent spin oscillations with a maximum around 1.2 eV, i.e., very close to the energy of the band gap in the semiconductor. Contrary to the laser-induced heating, this excitation mechanism is pump polarization dependent, being the most efficient if the pump is circularly polarized. The phase of the spin oscillations is independent of the polarity of the applied magnetic field, but changes by 180 deg under reversing the helicity of light. We suggest that the effect can be interpreted in terms of spin transfer torque experienced by the network of the ordered $\mathrm{C}{\mathrm{r}}^{3+}$ spins as a result of excitation of electrons from the top of the $p$-type valence band to the bottom of the $s$-type conduction band. In particular, a strong spin-orbit interaction experienced by the carriers in the valence band is responsible for the coupling of the spins of the photogenerated carriers and the polarization of light. Due to strong $pd$- and $sd$-exchange interactions the spins of the photocarriers appear to be coupled to the network of ordered spins of the $\mathrm{C}{\mathrm{r}}^{3+}$ ions.