We investigate the static and ultrafast magneto-optical response of the hexagonal chiral helimagnet ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$ above and below the helimagnetic ordering temperature. The presence of a magnetic easy plane contained within the crystallographic $ab$ plane is confirmed, while degenerate optical pump-probe experiments reveal significant differences in the dynamic between the parent, $\mathrm{Nb}{\mathrm{S}}_2$, and Cr-intercalated compounds. Time-resolved magneto-optical Kerr effect measurements show a two-step demagnetization process, where an initial, subpicosecond relaxation and subsequent buildup $(\tau >50\mathrm{ps})$ in the demagnetization dynamic scale similarly with increasing pump fluence. Despite theoretical evidence for partial gapping of the minority spin channel, suggestive of possible half-metallicity in ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$, such a long demagnetization dynamic likely results from spin-lattice relaxation as opposed to minority state blocking. However, comparison of the two-step demagnetization process in ${\mathrm{Cr}}_{1/3}\mathrm{Nb}{\mathrm{S}}_2$ with other $3d$ intercalated transition metal dichalcogenides reveals a behavior that is unexpected from conventional spin-lattice relaxation, and may be attributed to the complicated interaction of local moments with itinerant electrons in this material system.

• Received 1 April 2021
• Revised 28 October 2021
• Accepted 2 November 2021

DOI:https://doi.org/10.1103/PhysRevB.104.174426