Multiferroic properties of rare earth manganites

Preview

PDF WRAP_THESIS_OFlynn_2010.pdf - Requires a PDF viewer. Download (15MB)

Request Changes to record.

Abstract

Well known multiferroic compounds TbMnO3 and DyMnO3 show a strong coupling
between antiferromagnetism and ferroelectricity. Magnetic susceptibility
data for both compounds have been taken, showing large magnetic anisotropy,
and providing strong evidence for the existence of crystal field effects up to 150 K
above TN (∼40 K). Further evidence for crystal field effects is given by a measurement
of the magnetic heat capacity of TbMnO3.
A new group of multiferroic compounds, of the form Sm1−xYxMnO3, were discovered
as part of this work. The parent compound, SmMnO3, is not ferroelectric
and exhibits commensurate A-type antiferromagnetic order below TN∼58 K. By
doping with Y on the Sm site, significant changes are seen in the magnetic properties,
with a feature seen in the susceptibility data at ∼24 K for x = 0.4, 0.5
corresponding with a peak in the dielectric constant and the onset of an electric
polarisation (∼275 μC m−2 for Sm0.5Y0.5MnO3) along the crystallographic c-axis.
The magnetoelectric coupling seen in Sm0.6Y0.4MnO3 and Sm0.5Y0.5MnO3 is linked
to alterations in the local structure of the Mn-O octahedra, and in particular to
the Mn-O-Mn bond angle, which can be tuned by Y doping.
The zero-field electric properties of Sm1−xYxMnO3 (x = 0.4, 0.5) are very similar
to those of TbMnO3, but show markedly different magnetic field dependence.
Neutron diffraction and X-ray resonant scattering experiments were performed
on Sm1−xYxMnO3 (x = 0, 0.4, 0.5) in order to attempt to determine the magnetic
structure of the Y doped compounds. Evidence was found for a sinusoidally
modulated collinear order of the Mn moments along the b-axis below TN1∼47 K,
and cycloidal Mn order in the b-c plane below TN2∼24 K. This magnetic model is
similar to that of TbMnO3, and it is proposed that the difference in the magnetic
field dependence of the electric properties seen in TbMnO3 is due to coupling of
the Mn moments with the strongly anisotropic Tb moments.
The studies of Y doping were extended to the GdMnO3 system. Preliminary
measurements show a similar magnetoelectric coupling in polycrystalline
Gd1−xYxMnO3 (x = 0.1-0.4). Further research has been carried out on this group
of compounds as a consequence of this work.

Request changes or add full text files to a record

Repository staff actions (login required)

View Item