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Title: Macroscopic and microscopic investigation of spin reorientation of iron tin
Author: Heritage, Kevin
ISNI:       0000 0004 5367 6637
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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The work reported in this thesis examines the spin reorientation transition in Fe3Sn2 using a combination of microscopic and macroscopic techniques. Macroscopic measurement of single crystal Fe3Sn2 was undertaken using a SQUID (superconducting quantum interference device) magnetometer to measure the change in magnetisation along the Kagome plane. The domain structure on the surface of the crystal was measured using magnetic force microscopy (MFM) over a range of temperatures. The combination of techniques used for measuring the bulk and microscopic properties increases the understanding of first order spin reorientation transitions. The bulk magnetisation measurement of single crystal Fe3Sn2 displayed features that are typical of a first order phase transition. The magnetisation measured along the Kagome plane on warming and cooling differed in magnitude and a first order jump in magnetisation was also observed on cooling. This highlighted the hysteretic nature of the phase transition. The hysteresis and first order jump arises from the mechanism of the spin reorientation. On cooling through the spin reorientation temperature the sample is supercooled until the low temperature phase nucleates, after the nucleation the low temperature phase grew rapidly leading to a cascade effect and a first order jump in magnetisation. No evidence of a first order jump on warming was observed and therefore no significant superheating occurred. The lack of superheating is due to remanent high temperature phase at 2 K from an incomplete phase transition. The remanent phase provided 'seeds' for the growth of the high temperature phase, reducing the activation energy. The variable temperature magnetic force microscopy (MFM) measurements allowed the changes in the domain structure to be examined as the crystal underwent a spin reorientation transition. The evolution of the domain structure during the phase transition allowed the mechanism of the phase transition to be observed. On cooling from the high temperature phase, a fine structure associated with the low temperature phase formed within the branches of the domains associated with the high temperature structure. The fine structure grew with further cooling as a greater portion of the sample underwent the spin reorientation transition. Evidence of a thermal hysteresis was also observed in the domain images on warming and cooling, with images at a given temperature having different domain structures. The development of a fine structure was only observed on cooling, the absence of a fine structure on warming confirms that remanent high temperature phase prevents superheating on warming. The combination of macroscopic and microscopic results has allowed the phase diagram for the spin reorientation transition to be determined and the phase coexistence region to be determined.
Supervisor: Skinner, Stephen Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral