Fyzikální ústav Akademie věd ČR

We have investigated the magnetoelectric effect in BiFeO₃ and observed rather a large change of permittivity with magnetic field at high temperatures, where the sample becomes partially conducting due to defects. However, in this case the magnetoelectric effect is not intrinsic, i.e. due to coupling of polarization and magnetization, but due to combination of magnetoresistance and Maxwell-Wagner polarization effect (see S. Kamba et al., Phys. Rev. B 75, 024403 (2007)). It means that in BiFeO₃ one cannot expect switching of magnetization with an electric field and vice versa, although the electric control of antiferromagnetic domain structure was reported. Baettig and Spaldin predicted from ab initio calculations that the chemically ordered double perovskite Bi₂FeCrO₆ should have higher magnetization than BiFeO₃ and comparably high polarization. We have investigated Bi₂FeCrO₆ thin films and shown that if the B-site cations are ordered, this system actually belongs to the rare high-temperature multiferroics, but its magnetization and polarization is comparable to BiFeO₃ and critical temperatures are far above room temperature (T_N > 600 K and T_C > 900 K). For details see S. Kamba et al., Phys. Rev. B 77, 104111 (2008)).

Strong magnetodielectric effect was observed in EuTiO₃ crystal, which exhibits quantum paraelectric behavior similar to SrTiO₃: its permittivity ε′ increases on cooling and finally ε′ saturates below ~30 K. In contrast to SrTiO₃, EuTiO₃ undergoes an antiferromagnetic phase transition at T_N = 5,5 K, and the phase transition is accompanied by a sharp drop down of ε′ below T_N (without magnetic field). In a static magnetic field ε′ increases so that the drop down disappears for fields above 1 T. The magnetoelectric effect is huge - about 7%. We have shown that the temperature dependence of ε′ between 300 and 6 K is due to a soft optic phonon (vibration of magnetic Eu²⁺ cation) which reduces its frequency on cooling and finally the soft mode frequency saturates below 30 K (for details see Kamba et al. EPL, 80, 27002 (2007) and Goian et. al. Eur. Phys. J. B 71, 429 (2009)). Observed tuning of ε′ with magnetic field should be caused by tuning of the soft phonon frequency, which we experimentally confirmed.

Temperature dependence of the permittivity of magnetoelectric EuTiO₃ for various magnetic fields. The change of the permittivity with magnetic field is due to a magnetic field-induced frequency shift of the polar phonon.