  We report on the effects of spin fluctuations, magnetic ordering, and
external magnetic field on the dielectric constant of the ferromagnet SeCuO3,
and the antiferromagnet TeCuO3. A model based on the coupling between uniform
polarization and the q-dependent spin-spin correlation function is presented to
explain the different behaviors for these isostructural compounds. The large
magnetocapacitance near the transition temperature in the ferromagnet SeCuO3
suggests routes to enhancing the magnetodielectric response for practical
applications.
