Spectroscopic studies of the ferroelectric and magnetic phase transitions in multiferroic Sr1-xBaxMnO3

摘要

Dielectric response of perovskite Sr1-xBaxMnO3 (x = 0.43 and 0.45) ceramics was investigated using microwave, THz and infrared spectroscopic techniques in order to study the ferroelectric and antiferromagnetic phase transitions with critical temperatures T-C approximate to 350 K and T-N approximate to 200 K, respectively. The two lowest-frequency polar phonons are overdamped above T-N and they exhibit pronounced softening on heating towards T-C. Nevertheless, permittivity epsilon' in the THz range shows only a small anomaly at T-C because the phonon contribution to epsilon' is rather small. The phonons are coupled with a central mode which provides the main contribution to the dielectric anomaly at T-C. Thus, the ferroelectric phase transition has characteristics of a crossover from displacive to order-disorder type. At the same time, the intrinsic THz central peak is partially screened by conductivity and related Maxwell-Wagner relaxation, which dominates the microwave and lower-frequency spectra. Below T-N, the ferroelectric distortion markedly decreases, which has an influence on the frequencies of both the central and soft modes. Therefore, epsilon' in the THz range increases at T-N on cooling. In spite of the strong spin-phonon coupling near T-N, surprisingly no magnetodielectric effect was observed in the THz spectra upon applying magnetic field of up to 7 T, which is in contradiction with the theoretically expected huge magnetoelectric coupling. We explain this fact as due to the insensitivity of T-N to magnetic field.