Anisotropic Coherent Propagation of Sub‐Terahertz Magnons in Altermagnetic α ‐Fe ₂ O ₃

Magnon‐mediated spin transport in antiferromagnets (AFMs) holds great promise for energy‐efficient and high‐speed spintronic applications. However, the direction‐dependent anisotropy of terahertz (THz)‐frequency coherent spin wave propagation in AFMs remains elusive due to the lack of effective experimental approaches. The recent discovery of altermagnets (AMs), which combine antiferromagnetic spin order with ferromagnet‐like strong magneto‐optical responses, has emerged as a promising platform for exploring propagating THz coherent magnons. In this study, we employ femtosecond time‐resolved magneto‐optical spectroscopy to successfully excite and detect propagating sub‐THz coherent exchange magnons in AM α ‐Fe 2 O 3 , demonstrating remarkable group velocities (17.31–24.43 km/s) and micrometer‐scale propagation lengths. By selecting three samples with different crystallographic cut planes to tune the relative orientation between magnon wavevector and crystal axes, we identify crystal orientation‐dependent characteristics in both group velocities and lifetimes, attributed to anisotropic exchange stiffness and magnon‐magnon scattering, respectively. Furthermore, in combination with atomistic spin dynamics simulations, we reveal that the Dzyaloshinskii‐Moriya torque could significantly enhance propagating magnon amplitudes at low potential curvature, while external magnetic fields provide effective control over their frequency and polarization. These findings not only elucidate the dynamics of ultrafast laser‐driven coherent magnon propagation in AMs but also deliver valuable guidance for advancing next‐generation magnonic devices.