Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.618525
Title: Investigating magnetism and superconductivity using high magnetic fields
Author: Ghannadzadeh, Saman
ISNI:       0000 0004 5354 5506
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2014
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Abstract:
This thesis investigates a number of transition-metal coordination polymers and iron-pnictide superconductors through the use of high magnetic fields, low temperatures, and on occasion, high pressures. The thesis will begin by describing my development of the proximity detector dynamic susceptometer, a novel technique that can be used for magnetometery and transport measurements in high magnetic fields. This technique is highly compact and has no moving parts, making it suitable for use in pressure cells, hence opening the way for a variety of new experiments. Through high-field magnetometery and other measurements, I will demonstrate that the pressure can be used to directly control the magnetic properties of the polymeric magnet CuF2(H2O)2(pyrazine). In particular, I observe a transition from quasi-two-dimensional to quasi-one-dimensional antiferromagnetism at 9~kbar, driven by the rotation of the Jahn-Teller axis. I will then present a series of measurements on two coordination polymers, showing how a small chemical difference can lead to drastically different magnetic properties. I show that [Cu(pyrazine)H2O(glycine)2]ClO4 is an excellent spin-chain, while the sister compound [Cu(pyrazine)(glycine)]ClO4 is a dimerised material that shows a spin-gap and is disordered down to very low temperatures, but then undergoes a field-induced phase transition to an ordered phase. I will also describe a series of pulsed-field measurements of the upper critical field of the iron-based superconductors NaFe1-xCoxAs across the whole of the doping phase diagram. It is shown that paramagnetic pair-breaking effects dominate the critical field when the field is parallel to the crystal planes. In the parent compound the paramagnetic limit is equal to that expected from BCS theory, but becomes significantly enhanced above the BCS limit upon doping. It is shown that the multi-band nature of the superconductivity leads to a convex curvature in the evolution of the critical field as the temperature is reduced.
Supervisor: Goddard, Paul A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.618525  DOI: Not available
Keywords: Condensed Matter Physics ; superconductivity ; pnictide ; molecular magnets ; proximity detector oscillator ; high magnetic fields ; high pressure
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