The formation of magnon polarons, quanta of magnetoelastic waves, was found to be able to stimulate an enhancement or suppression in the magnetic field dependence of the spin Seebeck effect when the dispersion curve of the magnon becomes tangential to those of acoustic phonons. In the present work, we systematically analyzed the properties of the magnon spectrum in body-centered cubic easy-axis antiferromagnets with varying strength of exchange interaction and magnetic anisotropy, which allowed us to classify the antiferromagnets according to the number of solutions for the tangential condition between the dispersion curves of magnons and acoustic phonons. The anomaly features were found to occur only in the relatively weak magnetic field regime before spin flop transition. The manifestation of magnon-polaron-induced anomaly on the longitudinal spin Seebeck coefficient was also calculated directly from which a triple-peak feature, never observed or proposed before, was predicted. Our analysis also works for antiferromagnets with other magnetic lattices.
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