Thesis: Mutiferroic properties of rare-earth manganites (Download PhD Thesis, 17 Mb)
Supervisor: Geetha Balakrishnan
Single crystal samples of some multiferroicrare-earth manganites were prepared using IR image furnaces. Measurements of the heat capacity, magnetisation, and the electrical properties (dielectric constant and polarisation) of these samples were made in the laboratories of the Superconductivity and Magnetism Group. These data were complemented by neutron and X-ray scattering studies carried out at the ILL, the ESRF, and ISIS.
Well known multiferroic compounds TbMnO3and 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 GdMnO3system. 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.
After graduation Dan worked in India with the Raleigh International Trust. He then took up a Research Associate post at University College London in the Medical Physics and Bioengineering Department working with Professor Robert Speller.