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Title: Development of highfieldmagnetic Compton scattering andstudies of strongly correlated materials
Author: Shenton-Taylor, Caroline
Awarding Body: The University of Warwick
Current Institution: University of Warwick
Date of Award: 2007
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Abstract:
Magnetic Compton scattering (MCS) has been used to probe the spin momentum densities of ferromagnetic materials. A key focus of this work was the installation and commissioning of a new 9T cryomagnet, located at the European Synchrotron Radiation Facility in France. This generated a unique sample environment, allowing magnetic systems to be measured under applied fields ofup to 9T at temperatures from I.3K to 300K. This work reports on a series of materials studied using the new cryomagnet, alongside other interesting ferromagnetic systems measured at SPri~g-8 in Japan. A magnetic Compton profile (MCP) is produced by a difference measurement, obtained either by reversing the handedness of the incoming photon flux, or, reversal of an external magnetic field. As part ofthe cryomagnet installation tests, nickel MCPs were measured using both high and low magnetic field reversal and by using a helicity reversed approach. The helicity flipped approach was found to require large, relatively flat samples, for ease all the systems presented in this thesis were thereafter measured by magnetic field reversal. The hexaboride CeB6 was an ideal candidate for study using the new cryomagnet and allowed comparison with previous measurement. Approximately 10% of the spin moment was attributed to Ce 4felectrons with the remainder dominated by itinerant 5d and 2p contributions. Although difficult to distinguish between the 5d and 2p electrons, a boron contribution was essential in order to model the data at low momentum. The spin-dependent momentum density of the magnetocaloric material Gd7Pd3 was probed by magnetic Compton scattering using the cryomagnet facility at the ESRF. The total spin moment, at 2K, was determined as 50.8±0.7 JIB (f.u.rl . The Gd 4f contribution to the spin moment was determined K-as 43.4±l.8 JIB (f.u.r\ the Gd 5d moment as 4.4±0.7 JIB (f.u.rl and the Pd 4d spin moment contribution as 2.9±1.1 JIB (f.u.rl • At 280K the total spin moment was 27.3±0.9 JIB (f.u.rl with individual contributions determined as: Gd 4fspin moment of 23.8±l.1 JIB (f.u·rl , Gd 5d contribution of2.2±O.5 JlB(f.u.rl and a Pd 5dspin moment of 1.2±0.6J1B(f.u.rl . MCPs measurements on single crystal CO(1-x1exS2 samples were measured at the SPring-8 synchrotron. The experimental measurements were compared to linear muffm tin orbital (LMTO) predictions which successfully modelled the profile shape down to l.a.u. By altering the small level of Fe doping a small ·change in the profile shape was observed. Through application of band structure calculations the density of states was calculated and applied to estimate the spin polarisation. Fe304MCPs were measured at SPring-8 on beam line BL08W. A single crystal sample was measured crossing the Verwey transition at 120K in the [100], [110] and [111] directions by reversal ofa 2.5T external field. For each direction, crossing the Verwey transition did not result in a change in profile shape. Previous research suggested that geometrical frustration in the spinel caused a large magnetic anomaly around the Verwey transition temperature. The Warwick results provided no evidence to support this suggestion. MCS measurements, at SPring-8, allowed investigation of the NiO nanoparticle spin moment. For a particle size of 6.4±l.5 om the unexpected result of an enhanced spin moment was observed of 0.078 ± 0.005 JIB f.u.- l• This exceeded the total moment magnitude of 0.033 ± 0.002 JIB f.u: 1. This cannot be predicted by Ne6I's model of superparamagnetism. It suggests an appreciable orbital moment in an anti-parallel arrangement with the spin contribution. The magnitude ofthe spin moment was found to increase with decreasing particle size.
Supervisor: Not available Sponsor: Not available
Qualification Name: Not available Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.504860  DOI: Not available
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