Seemingly anisotropic magnetodielectric effect in isotropic EuTiO3 ceramics

In the antiferromagnetic phase of $\mathrm{Eu}\mathrm{Ti}{\mathrm{O}}_{3}$ ceramics, a seemingly anisotropic magnetodielectric effect (up to 2.5%) was observed via the low-frequency dielectric permittivity measured in external magnetic field below 1.9 T. We explain the effect theoretically by taking into account the demagnetizing field which effectively reduces the internal magnetic flux density by 0.6 T when the external magnetic field is applied perpendicular to the plane of the disk-shaped sample. This finding is also confirmed experimentally by magnetization measurements. The refractive index between 0.2 and 0.5 THz exhibits a large anisotropy in an external magnetic field of up to 7 T. This anisotropy is due to a shift of the ferromagnetic resonance from the microwave to the sub-THz region with external magnetic field applied perpendicular to the magnetic component of the THz radiation. This magnon observation is performed in the strong external magnetic field (above 2 T) and in both paramagnetic and antiferromagnetic phases.