Efficient terahertz emission from R-plane $\alpha {\text{-Fe}}_2{O}_3/\mathrm{Pt}$

We report on the generation of ultrafast spin currents through femtosecond laser excitation of epitaxial bilayer thin films comprising $\alpha \text{−}{\mathrm{Fe}}_2{O}_3$ (an insulating antiferromagnet) and Pt (a heavy metal). The epitaxial thin films of $\alpha \text{−}{\mathrm{Fe}}_2{O}_3$ were simultaneously grown in three distinct orientations, namely, R, A, and C planes, employing pulsed laser deposition. By using the R plane $\alpha \text{−}{\mathrm{Fe}}_2{O}_3$, we demonstrate a substantial enhancement of the THz signal, nearly one order of magnitude greater compared to the A and C planes $\alpha \text{−}{\mathrm{Fe}}_2{O}_3/\mathrm{Pt}$. We perform a detailed investigation into the sample azimuthal and laser polarization dependence of the THz emission. Our investigations establish that the sample azimuthal dependence of THz emission in the $\alpha \text{−}{\mathrm{Fe}}_2{O}_3/\mathrm{Pt}$ system is linked to the spin domain distribution as well as the crystalline symmetry of the orientation of $\alpha \text{−}{\mathrm{Fe}}_2{O}_3$. These results contribute significantly to understanding the ultrafast dynamics of spin currents in antiferromagnets.