Resonance Line Shapes of Weak Ferromagnets of the $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$ and Ni${\mathrm{F}}_2$ Type

Based on a two sublattice model the resonance line shapes of the low- and high-frequency branch of weak ferromagnets of the $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$ and Ni${\mathrm{F}}_2$ type were calculated by solving the equations of motion with a damping term of the Landau-and-Lifshitz type. When the rf driving field is applied perpendicular to the ferromagnetic component and in the easy plane of the $\alpha -{\mathrm{Fe}}_2{\mathrm{O}}_3$ type crystal, an enhancement factor appears in the susceptibility. The frequency linewidth is proportional to the exchange frequency and approximately independent of an applied field. The linewidths are compared with experiments and good agreement is found for MnC${\mathrm{O}}_3$. For MnC${\mathrm{O}}_3$ the damping of the high-frequency branch is by a factor of about 2.6 more effective than that of the low-frequency branch.