Altermagnets are promising candidates for next-generation low-dissipation magnonic devices due to their ultrafast spin dynamics and chiral splitting magnon bands. Hematite $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, a widely recognized altermagnet with an ultralow damping constant, subterahertz magnon frequencies, and remarkable magnon properties, provides an ideal platform for exploring magnonic functionalities. Here, we combine DFT-based first-principles calculations with atomistic spin-dynamics simulations to construct a four-sublattice atomistic altermagnetic model of $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ in both easy-axis and easy-plane phases. Linear spin-wave theory (LSWT) calculations confirm that the magnon bands exhibit chiral nondegeneracy along low-symmetry $K$ paths. In contrast to the easy-axis phase, the magnon bands in the easy-plane phase split into high-frequency and low-frequency branches near the Γ point. Atomistic magnon-resonance dynamical simulations further reveal the magnetic-moment precession modes with atomic resolution at the Γ point. We show that variations in the excitation field direction lead to distinct magnon-resonance modes, which give rise to the observed frequency gap in the easy-plane phase. Our work establishes a comprehensive framework for atomistic-scale magnon investigations in altermagnets, bridging first-principles calculations and atomistic spin-dynamics simulations, and provides microscopic insights into magnon dynamics in $\alpha \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$.

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• Altermagnetism
• Magnonics
• Spin dynamics
• Spin waves
• Density functional theory
• First-principles calculations
• Landau-Lifschitz-Gilbert equation
• Linear spin wave theory