Infrared and terahertz studies of polar phonons and magnetodielectric effect in multiferroic Bi Fe O 3 ceramics

By S. Kamba, D. Nuzhnyy, M. Savinov, J. Sebek, J. Petzelt, J. Prokleška, R. Haumont, J. Kreisel. 2006

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$\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ ceramics were investigated by means of infrared reflectivity and time domain terahertz transmission spectroscopy at temperatures $20\char21{}950\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and the magnetodielectric effect was studied at $10\char21{}300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ with the magnetic field up to $9\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. Below $175\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the sum of polar phonon contributions to the permittivity corresponds to the value of measured permittivity below $1\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. At higher temperatures, a giant low-frequency permittivity was observed, obviously due to the enhanced conductivity and possible Maxwell-Wagner contribution. Above $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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 $175\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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.

Subfields: Electronic, Optical and Magnetic Materials, Materials Chemistry

View Paper: https://www.semanticscholar.org/paper/151ce5ebd3fcbbc212eb2da36f25a908650dd289