Biography

Ruth graduated from Cambridge University with a B.Sc. in Natural Sciences. She joined the Superconductivity and Magnetism Group in October 2010 to study for an M.Sc. by research.

Ruth worked under the supervision of Prof. Geetha Balakrishnan. She submitted her M.Sc. thesis in September 2011 and then moved to begin work on a Ph.D. with Dr. Haixue Yan in the School of Engineering and Materials Science at Queen Mary, University of London.

Thesis

Supervisor

Geetha Balakrishnan

Thesis Abstract

Research into multiferroicity has increased dramatically in recent years. A number of new rare-earth oxides have been shown to exhibit a coupling between magnetism and ferroelectricity. Following this area of research, the project investigates the multiferroic properties of LuFe₂O₄ and Gd_0.9Lu_0.1MnO₃ in single crystal form.

Multiferroic LuFe₂O₄ is a charge ordered ferroelectric which shows a coupling between magnetisation and electric polarisation near room temperature. In order to investigate how sample preparation and crystal growth environment affect final stoichiometry in LuFe₂O₄, two single crystals of the rare-earth oxide were grown using the floating zone method. Single crystals of different oxygen stoichiometry were prepared by altering the ratio of the CO₂/CO gas atmosphere used during single crystal growth. The magnetic and electric properties of the oriented single crystals and polycrystalline sample were investigated by measuring the magnetisation, dielectric constant and polarisation in zero and applied magnetic fields. A paramagnetic to ferrimagnetic ordering of the Fe moments along the c-axis was observed below 250 K in both single crystals and the polycrystalline sample. The presence of this transition in the polarisation data confirms the multiferroic nature of the LuFe₂O_4-δ samples. A magnetostructural transition was also observed below 200 K in the polycrystalline sample and the single crystal grown in the CO2/CO=5 gas atmosphere. The absence of this transition for the single crystal grown in the CO2/CO=3 gas atmosphere is an indication of poor oxygen stoichiometry within this crystal. A ferroelectric state was observed in the polycrystalline and single crystal samples below 280 K. The temperature of ferroelectric transition observed for the LuFe₂O_4-δ samples is lower than the ferroelectric 2D to 3D charge ordering temperature reported in the literature.

Doping at the rare-earth site of the orthorhombic manganites is an effective method for tuning the average Mn-O-Mn bond angle. By adjusting the distortion of the MnO₆ lattice a multiferroic ground state can be induced in a series with a non-multiferroic parent compound. The effect of Lu-doping on the magnetic and electric properties of GdMnO₃ has been investigated. Heat capacity, magnetisation, dielectric constant and polarisation data have been collected from an oriented single crystal of Gd_0.9Lu_0.1MnO₃. Due to the close proximity of Gd_0.9Lu_0.1MnO₃ to multiferroic TbMnO₃ in the spin ordering phase diagram a series of features below the incommensurate (IC) antiferromagnetic (AF) ordering temperature, not present in GdMnO₃, have been observed in the zero and applied magnetic field experimental data. The absence of a canted A-type AF state below this transition, normally present for the parent compound, and the persistence of a ferroelectric state in magnetic fields applied along the a and b-axis allow for the suggestion of a new magnetically ordered ferroelectric state, influenced by the ordering of the Gd moments, to occur in Gd_0.9Lu_0.1MnO₃.

 

For Details of

Ruth's Work at

Warwick Please

Contact Geetha Balakrishnan

 

Contact Details

Telephone
+44 (0)2476573879
Fax
+44 (0)2476150897
E-Mail
G.Balakrishnan(at)warwick.ac.uk