Infrared and terahertz studies of polar phonons and magnetodielectric effect in multiferroicBiFeO3ceramics

Abstract

BiFeO3 ceramics were investigated by means of infrared reflectivity and time domain THz 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 into 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 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 highfrequency permittivity increase on heating. The observed non-complete phonon softening seems to be the consequence of the first-order nature of the ferroelectric transition.

Fractal Dimension and Size Scaling of Domains in Thin Films of MultiferroicBiFeO3

“…This suggests a higher energy cost of the domain walls [21,22], consistent with a strong magnetoelectric coupling at the wall [6]. This contrasts with the apparently low intrinsic magnetoelectric coupling of the bulk material [23], and underlines the interest of domain walls as multiferroic entities in their own right [24].…”

Structural and magnetic properties of isovalently substituted multiferroic BiFeO3: Insights from Raman spectroscopy

“…42,57 The M3 mode at 618 cm −1 was found to not fit in the -point allowed modes for BiFeO 3 . 33 From previous mode assignments for BiFeO 3 it is therefore probable that the M3 mode is a Fröhlich activated mode, while the symmetry of the M2 mode is ambiguous.…”

Strong spin-phonon coupling in infrared and Raman spectra ofSrMnO3

“…This is caused by a nonhomogeneous conductivity of the sample in the ceramic grains and grain boundaries, which is responsible for the creation of internal barrier layer capacitors on the grain boundaries. This mechanism is responsible for a "giant" effective permittivity in many dielectric or multiferroic materials [25][26][27].…”