Fleck, C. L. (Catherine Louise) (2011) Magnetism in the complex cobaltates Y1−xSrxCoO3−δ (0.7 ≤ x ≤ 0.95) and Ca3Co2O6. PhD thesis, University of Warwick.

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Abstract

The magnetic phases in the complex cobaltates Y1−xSrxCoO3−δ (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−δ which evolves as a
function of temperature heavily influences the ferrimagnetic behaviour of this mate-
rial. 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−δ have 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−δ 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−δ and Ca3Co2O6 and contributes to our overall
understanding of the physics of these complex cobaltate compounds.

Item Type:	Thesis or Dissertation (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Crystalline polymers -- Magnetic properties, Ferromagnetism
Official Date:	October 2011
Dates:	Date Event October 2011 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Lees, Martin
Extent:	xvi, 184 leaves : ill., charts
Language:	eng
URI:	http://wrap.warwick.ac.uk/id/eprint/45164

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