  Recent experimental observation of inertial spin dynamics calls upon holistic
reevaluation of the theoretical framework of magnetic resonance in
ferromagnets. Here, we derive the secular equation of an inertial spin system
in analogy to the ubiquitous Smit-Beljers formalism. We find that the frequency
of precessional ferromagnetic resonances is decreased as compared to
non-inertial case. We also find that the frequency of nutational resonances is
generally increased due to the presence of magnetic anisotropy and applied
magnetic field. We obtain exact solutions of the secular equation and
approximations that employ the terminology of non-inertial theory and thus
allow for convenient estimates of the inertial effects.
