Infrared and terahertz studies of polar phonons and magnetodielectric effect in multiferroic BiFeO3 ceramics

BiFeO3 ceramics were investigated by means of infrared reflectivity and time domain terahertz transmission spectroscopy at temperatures 20-950 K, and the magnetodielectric effect was studied at 10-300 K with the magnetic field up to 9 T. Below 175 K, the sum of polar phonon contributions to the permittivity corresponds to the value of measured permittivity below 1 MHz. At higher temperatures, a giant low-frequency permittivity was observed, obviously due to the enhanced conductivity and possible Maxwell-Wagner contribution. Above 200 K the observed magnetodielectric effect is caused essentially through the combination of magnetoresistance and the Maxwell-Wagner effect, as recently predicted by Catalan [Appl. Phys. Lett. 88, 102902 (2006)]. Since the magnetodielectric effect does not occur due to a coupling of polarization and magnetization as expected in magnetoferroelectrics, we call it an improper magnetodielectric effect. Below 175 K the magnetodielectric effect is by several orders of magnitude lower due to the decreased conductivity. Several phonons exhibit gradual softening with increasing temperature, which explains the previously observed high-frequency permittivity increase on heating. The observed noncomplete phonon softening seems to be the consequence of the first-order nature of the ferroelectric transition.