Thesis: Mutiferroic properties of rare-earth manganites (Download PhD Thesis, 17 Mb)

Supervisor: Geetha Balakrishnan

Research Summary

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.

Abstract

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 T_N(~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 Sm_1−xY_xMnO₃, were discovered as part of this work. The parent compound, SmMnO₃, is not ferroelectric and exhibits commensurate A-type antiferromagnetic order below T_N~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 Sm_0.5Y_0.5MnO₃) along the crystallographic c-axis.

The magnetoelectric coupling seen in Sm_0.6Y_0.4MnO₃ and Sm_0.5Y_0.5MnO₃ 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 Sm_1−xY_xMnO₃ (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 Sm_1−xY_xMnO₃ (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 T_N1~47 K, and cycloidal Mn order in the b-c plane below T_N2~24 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 Gd_1−xY_xMnO₃ (x = 0.1-0.4). Further research has been carried out on this group of compounds as a consequence of this work.

Current Role

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.

 

 

 

 

 

 

 

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