Magnetocapacitance effect in EuTiO$_{3}$ and related compounds

Perovskite titanates, EuTiO$_{3}$, contains Ti$^{4+}$ ions, similarly with BaTiO$_{3}$ and SrTiO$_{3}$, and is expected to show ferroelectric instability. In addition, this compound contains Eu$^{2+}$ ions with S=7/2 spin, which order antiferromagnetically at 5.5 K. We measured the dielectric constant of this compound [1], and found that large dielectric constants ($>400$) critically decrease with antiferromagnetic ordering of the Eu spins at 5.5 K. We also found a large change of the dielectric constant under magnetic field (magnetocapacitance) by 7 \% with 1.5 T at 2 K. From a comparison with a mean-field calculation, it was shown that the variation of dielectric constants scales with the pair correlation of the nearest-neighbor Eu spins. We also measured the magnetocapacitance of pyrochlore titanates, R$_{2}$Ti$_{2}$O$_{7}$ (R=rare earth) [2], having the same Ti$^{4+}$, but the magnetic moment is located on a pyrochlore lattice, and thus is dominated by geometrical frustration. By comparing the magnitude of mangetocapacitance with the square of magnetization, evidence of ferromagnetic (R=Ho) and antiferromagnetic (R=Gd) fluctuation was obtained. [1] T. Katsufuji et al., Phys. Rev. B 64, 054415 (2001). [2] T. Katsufuji et al., Phys. Rev. B 69 064422 (2004).