  Microwave absorption experiments employing a phase-sensitive external
resistive detection are performed for a topical van der Waals antiferromagnet
CrSBr. The field dependence of two resonance modes is measured in an applied
field parallel to the three principal crystallographic directions, revealing
anisotropies and magnetic transitions in this material. To account for the
observed results, we formulate a microscopic spin model with a bi-axial
single-ion anisotropy and inter-plane exchange. Theoretical calculations give
an excellent description of full magnon spectra enabling us to precisely
determine microscopic interaction parameters for CrSBr.
