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Title: X-ray Scattering and Absorption and High Pressure Magnetisation Studies of 3d Transition Metal Magnetic Compounds and the Development of High Pressure Magnetic Compton Scattering
Author: Blaauw , Leopold V.
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2007
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Abstract:
Magnetic Compton scattering was performed on the weak itinerant ferromagnet MnSi providing a direct measurement of the one dimensional spin-dependent electron momentum density of the sample. These results were then compared to band structure calculations performed by the Bristol positron group. The complementary technique of X-ray magnetic circular dichroism was utilised to measure the spin to orbital magnetic moment ratio of NiO nanoparticles. High pressure magnetisation studies were performed on the rare Earth perovskite EUO.5SSr0.42Mn03 and led to the construction of a new ambient and high pressure magnetic phase diagram. The technique of magnetic Compton scattering has been extended into the high pressure regime via the use of a diamond anvil cell capable ofpressures ofup to 20 GPa. The rare Earth perovskite EU0.5SSr0.42Mn03 exhibits a large magnetic moment in the ferromagnetic metal phase. The phase diagram proposed by previous authors was reanalysed in the light of new magnetometry data. A SQUID clamp cell was used to extend the study of the new phase diagram into the high pressure regime. Behaviour which is indicative of. the formation of ferromagnetic clusters within an antiferromagnetic matrix is observed at low temperatures and fields. The application of pressure radically suppresses antiferromagnetism with the sample and favours ferromagnetic ordering via the double exchange mechanism. A new high pressure phase diagram is proposed. The intermetallic compound MnSi exhibits weak ferromagnetism below a temperature of 29K under an applied field of 0.6T. Band structure calculations, when compared to experimentally obtained magnetic Compton profiles fail to adequately explain· the observed momentum densities for pz < 2 a.u. However, the features observed in the profiles at higher momentum are described well by the LMTO theory. The discrepancies at low momentum are the result of the incorrect weighting of the d-like bands. The spin moment was measured as 0.36 ± 0.005 J.lB f.u.-1 indicating a 10% orbital contribution to the total observed moment of 0.4 J.lB f.u.-I A study ofNiO nanoparticles revealed a critical surface to volume ratio which results in a large enhancement of the ferromagnetisation. X-ray absorption spectra indicate that this enhancement is due to the formation of a thicker surface layer of Ni2+ spin symmetry and that the contribution to the spectra from Ni2+ low spin and Ni3+ symmetries does not change as particle size is reduced. X-ray magnetic circular dichroism studies show that the spin and orbital moments are parallel and that as particle size is decreased the orbital to spin moment ratio is increased from the bulk value of 17% to 25%. The development of the equipment and experimental technique of high pressure magnetic Compton scattering utilising a diamond anvil cell is discussed in detail. It is shown that this technique can be successfully applied to measure the one dimensional electron momentum densities of samples situated within the anvil cell. The development and adaptation of the cell for several complementary laboratory techniques is also discussed.
Supervisor: Not available Sponsor: Not available
Qualification Name: University of Warwick, 2007 Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.490685  DOI: Not available
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