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This study employs low-wavenumber magneto-Raman spectroscopy to characterize finite-momentum magnon dynamics in hematite ($\alpha$-Fe$_2$O$_3$) across its Morin transition, resolving both $k=0$ and finite-$k$ sub-terahertz magnons. The research reveals that while the limiting group velocity remains field-independent, the finite-$k$ group velocity is significantly altered near magnetic transitions, and quantifies temperature-dependent Dzyaloshinskii--Moriya and anisotropy fields. These findings attribute the Morin transition to the rapid decrease of $H_{\mathrm{K1}}$ and provide crucial material parameters for hematite-based magnonic devices.
The Morin transition in hematite is driven by a rapid decrease in uniaxial anisotropy, not the Dzyaloshinskii-Moriya field, as revealed by precise magnon velocity measurements.
Hematite ($\alpha$-Fe$_2$O$_3$) is a prototypical antiferromagnetic platform for high-speed spintronics and magnonics, yet its finite-momentum magnon dynamics and spin-reorientation mechanism remain incompletely understood. Here we use low-wavenumber magneto-Raman spectroscopy to resolve both $k=0$ and finite-$k$ sub-terahertz magnons in $\alpha$-Fe$_2$O$_3$ across the Morin transition. We find that the limiting group velocity $v_0$ remains nearly field independent, whereas the finite-$k$ group velocity $v_{\mathrm g}$ is strongly modified near the Morin and spin-flop transitions. By combining parallel- and transverse-field measurements with spin-wave modeling, we further extract the temperature-dependent Dzyaloshinskii--Moriya field $H_{\mathrm D}$ and uniaxial anisotropy fields $H_{\mathrm{K1}}$ and $H_{\mathrm{K2}}$. We find that $H_{\mathrm D}$ is weakly temperature dependent at approximately 2.0--2.3~T, while $H_{\mathrm{K1}}$ decreases rapidly and $H_{\mathrm{K2}}$ changes only weakly. This contrast drives the sign reversal of the effective anisotropy field and quantitatively accounts for the Morin transition. Our results establish low-wavenumber Raman spectroscopy as a quantitative probe of finite-momentum antiferromagnetic magnons and provide key material parameters for hematite-based magnonics.
