Ultrafast demagnetization of Co2MnSi1–xAlx Heusler compounds using terahertz and infrared light

We investigate the ultrafast demagnetization in Co 2 MnSi 1-x Al x quaternary Heusler compounds induced by terahertz (THz) and infrared (IR) pulses. Adjusting the alloy's composition allows us to tailor the spin polarization at the Fermi energy from 97% ± 3% (Co 2 MnSi) due to its minority spin gap around 0.6 eV, down to 63% ± 3% (Co 2 MnAl) without a spin gap. Here we experimentally compare the cases of ultrafast demagnetization, when the material is excited with the help of laser pulses with photon energies below and above the minority spin gap, respectively. More particularly, the pump-photon energies were tuned from 1.02 eV (IR) down to 4.1 meV (THz). We found that the ultrafast demagnetization time decreases upon substitution of Si by Al and thus destroys the minority spin gap. Moreover, a decrease of the pump-photon energies in the near-infrared spectral range results in a slight increase of the demagnetization time. Nevertheless, further decrease of the photon energy by a factor of 250 hardly changes the characteristic time of ultrafast demagnetization. Both THz and IR pulses cause very similar ultrafast magnetization dynamics in our Co 2 MnSi 1-x Al x Heusler compounds.