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Ruth McKinnon


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.

Studies of New Multiferroics (Download MSc Thesis, 5 MB)

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 Gd0.9Lu0.1MnO3 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 LuFe2O4-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 LuFe2O4-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 Gd0.9Lu0.1MnO3. Due to the close proximity of Gd0.9Lu0.1MnO3 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 Gd0.9Lu0.1MnO3.



For Details of
Ruth's Work at
Warwick Please
Contact Geetha Balakrishnan


Contact Details

Department of Physics
University of Warwick
Coventry CV4 7AL
United Kingdom

+44 (0)2476573879
+44 (0)2476150897