  We investigate collective excitations in magnetic topological insulators
(TIs) and their impact on particle axion detection. In the three-dimensional TI
model with the Hubbard term, the effective action of magnons and amplitude
modes is formulated by dynamical susceptibility under the antiferromagnetic and
ferromagnetic states. One of the amplitude modes is identified as ``axionic''
quasi-particle and its effective coupling to the electromagnetic fields turns
out to be enhanced by about four orders of magnitude larger than the previous
estimate, which may drastically change the sensitivity of the particle axion
search using ``axion'' in magnetic TIs.
