Coupling between magnetism and dielectric properties in quantum paraelectric ${\mathrm{EuTiO}}_3$

The dielectric constant of quantum paraelectric ${\mathrm{EuTiO}}_3,$ which contains ${\mathrm{Eu}}^{2+}$ with $S=7/2\mathrm{}$ spin and ${\mathrm{Ti}}^{4+},$ has been measured under a magnetic field. The dielectric constant shows a critical decrease at the antiferromagnetic ordering of the Eu spins at 5.5 K, as well as a substantial change under a magnetic field (by $\sim 7\%$ with 1.5 T), indicating a strong coupling between the Eu spins and dielectric properties. We show that the variation of the dielectric constant is dominated by the pair correlation of the nearest-neighbor Eu spins, likely via the variation of the soft-phonon-mode frequency.