After graduating with an MSci degree from Durham University in 2007, I moved to Warwick to study for a PhD as part of the Superconductivity and Magnetism group beginning October 2007 under the supervision of Dr Martin Lees. I submitted my PhD thesis on 17th August 2011 and my viva took place on 15thSeptember 2011. After graduation I enrolled on the Inspire PGCE course at Imperial College London and I am now teaching in London.
My research was focused on studying the structural and magnetic behaviour of the cobaltate compounds SrxY1-xCoO3-d, which shows an interesting metal-insulator and ferromagnetic to antiferromagnetic transition at room-temperature, and Ca3Co2O6, which is a quasi-1D magnetic material exhibiting metamagnetism and geometrical frustration. I used both polycrystalline and single-crystal samples grown using the floating-zone method, and used a variety of equipment here at Warwick to characterise and measure the samples produced. I also used both neutron and resonant x-ray techniques at large-scale facilities such as ISIS-Rutherford Appleton Laboratories, LLB at Saclay, France and the ILL/ESRF in Grenoble, France.
Thesis Title: Magnetism in the complex cobaltates Y1−xSrxCoO3−d(0.7 ≤ x ≤ 0.95) and Ca3Co2O6 (Download PhD Thesis, 13.6 MB)
Supervisor: Martin Lees
The magnetic phases in the complex cobaltates Y1−xSrxCoO3−d (0.7 ≤ x ≤ 0.95) and Ca3Co2O6 have been investigated by susceptibility, heat capacity, X-ray and neutron scattering techniques. These measurements have shown that the super-structure ordering in the perovskite cobaltate Y1−xSrxCoO3−dwhich evolves as afunction of temperature heavily influences the ferrimagnetic behaviour of this material. Neutron scattering has also been used to probe the unusual time and magnetic field dependent behaviour of the spin-chain compound Ca3Co2O6, and to further our understanding of the magnetic phase diagram of this system.Both polycrystalline and single crystal samples have been used in this study. High quality single crystals of the A-site (Sr/Y) and oxygen vacancy ordered form of the perovskite Y1−xSrxCoO3−dhave been produced using the floating zone technique and characterised using EDAX and TGA. The single crystals produced were large enough to perform polarised and inelastic neutron scattering experiments on this compound for the first time, revealing anisotropic quasi-elastic scattering above the magnetic transition temperature. In addition, diffraction experiments on these samples found evidence of coincident structural and magnetic transitions in Y1−xSrxCoO3−d at both 370 and 280 K.
Neutron diffraction measurements were also performed on the geometrically frustrated compound Ca3Co2O6. The low temperature magnetisation process was found to be accompanied by clearly visible steps in the intensity of the ferromagnetic and antiferromagnetic Bragg peaks. Detailed measurements have shown that the presence of short-range correlations cannot account for the reduction in intensity of the antiferromagnetic Bragg peaks at low temperatures. Instead, the origin of this drop in intensity was found to be a slow time-dependent magnetic transition from one long-range ordered antiferromagnetic state to another. This transition occurs over a timescale of hours and is never complete. The experimental work detailed in this thesis provides new information about the phase diagrams of Y1−xSrxCoO3−d and Ca3Co2O6and contributes to our overall understanding of the physics of these complex cobaltate compounds.
Publications resulting from Ph.D. work
Slow magnetic order-order transition in the spin chain antiferromagnet Ca3Co2O6, S. Agrestini, C.L. Fleck, L.C. Chapon, C. Mazzoli, A. Bombardi, M.R. Lees, and O.A. Petrenko, Physical Review Letters 106, 197204, (2011).
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Field-driven magnetisation steps in Ca3Co2O6: A single-crystal neutron-diffraction study, C. L. Fleck, M. R. Lees, S. Agrestini, G. J. McIntyre, and O. A. Petrenko, EPL 90, 67006 (2010).
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On the growth and properties of high quality single crystals of the yttrium doped strontium cobaltates, Y1−xSrxCoO3-d(0.7<x<0.95), C. L. Fleck, G. Balakrishnan, and M.R. Lees, Journal of Materials Chemistry 21, 1212 (2011).
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