Terahertz frequency magneto-dielectric effect in Ni-doped CaBaCo$_4$O$_7$

We explore the behavior of spin and lattice excitations in high magnetic field in Ni doped magneto-electric material CaBaCo$_4$O$_7$. This material crystallizes in a polar non-centrosymmetric crystal structure and exhibits a non-collinear ferrimagnetic Co spin ordering. Additional ferroelectricity was reported due to the onset of magnetic order, as well as a strong magneto-dielectric effect of 16\%. Low-energy excitations of the material have been studied using THz spectroscopy and include spin excitations and strong lattice vibrational modes at the 1.2 and 1.6 THz frequency. Our THz magnet-spectroscopy measurements in magnetic field up to 17 T addresses the question of the origin of the magnet-dielectric effect. We find that the magneto-dielectric effect results from a significant loss of spectral weight by the lattice vibrational resonances near 1.2 and 1.6 THz frequency. We also find that the vibrational frequencies soften upon doping with Ni.