Infrared and THz studies of polar phonons and improper magnetodielectric effect in multiferroic BFO3 ceramics

TL;DR

This study investigates the infrared and THz spectral properties of BFO3 ceramics across a wide temperature range, revealing an improper magnetodielectric effect driven by conductivity and Maxwell-Wagner contributions rather than direct polarization-magnetization coupling.

Contribution

It provides a detailed analysis of phonon behavior and magnetodielectric effects in BFO3 ceramics, highlighting the role of conductivity and Maxwell-Wagner effects in the observed phenomena.

Findings

Polar phonon contributions match low-frequency permittivity below 175 K.

Giant low-frequency permittivity observed above 200 K due to conductivity.

Magnetodielectric effect is caused by magnetoresistance and Maxwell-Wagner effect, not polarization-magnetization coupling.

Abstract

BFO3 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 high-frequency permittivity increase on heating. The observed non-complete phonon softening seems to be the consequence of the first-order nature of the ferroelectric transition.