Laser induced ultrafast Co 3d and Ho 4f spin dynamics in CoHo ferrimagnetic alloys
The transition metal (TM) / rare-earth (RE) alloys have received renewed interest lately as model systems to decipher the origin of all-optical helicity-independent switching (AO-HIS) by femtosecond laser pulses. Recently, a distinct single-pulse magnetization reversal mechanism, based on domain-wall motion and coalescence, has been reported in CoDy and CoHo alloys, as well as in Co/Gd ultrathin bilayers. It has been claimed that this specific toggle switching is achieved when the Co sublattice is fully demagnetized and the characteristic demagnetization times {\tau}RE of the RE (Gd, Dy or Ho) sublattices is longer than that of the Co and the angular momentum transferred to Co is reduced. Element- and time-resolved X-ray spectroscopy studies of CoDy alloys have reported characteristic demagnetization times of {\tau}Co = 0.2 ps and {\tau}Dy = 0.6 ps at room temperature. Similarly, Ho is expected to exhibit a slower response, but its ultrafast 4f spin dynamics remain experimentally unexplored. Here, we report on element- and time-resolved investigations of femtosecond laser induced ultrafast dynamics of the Co 3d and Ho 4f spins in ferrimagnetic Co_{80}Ho_{20} alloys to verify this prediction. We observed characteristic demagnetization times {\tau}Co = 0.22 +/- 0.01 ps and {\tau}Ho = 0.87 +/- 0.15 ps at room temperature. These results show that Ho demagnetizes substantially more slowly than Co in Co_{80}Ho_{20}, supporting the proposed condition for domain-wall-mediated all-optical toggle switching