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Simulation, preparation and characterisation of magnetocaloric thin films

Evans, Alex 2021. Simulation, preparation and characterisation of magnetocaloric thin films. PhD Thesis, Cardiff University.
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Abstract

Heisenberg and related models have previously been developed to investigate the magnetocaloric effect. Here an Ising model was developed to simulate the GdSiGe crystal system and used to compare the two dominant phases, monoclinic and orthorhombic, to show a dependence on the inter slab bonding that is present in the orthorhombic phase but not the monoclinic phase and the Tc. The model uniquely utilises 2 Ising models, representing each phase, to compare internal energies and derive a first-order phase change point. The Monte Carlo (MC) method with consideration for only nearest neighbour sites, magnetic coupling strength (J) calculated using the Ruderman-Kittel-Kasuya-Yosida (RKKY) model and an external field H was applied. The cohesive energy, under the assumption of the nearly free electron, was used to show indistinguishability in favour of individual phases. This indistinguishability leads to a first-order coupled magnetic and structural transition over a second-order magnetic transition usually expected from Ising models. Thin-film Magnetocaloric GdSiGe and LaFeCoSi were grown on SiO2 and AlN, SQUID and VSM magnetometry are used to investigate the phase transition and Curie temperature. X-ray, polarised neutron re ectometry and AFM measurements are used to investigate the structural and surface characteristics. For both materials, there exists a distinct change compared to their bulk counterpart. The Curie temperature of both materials moved from the bulk counterparts, with LaFeCoSi having an 8 K lower Tc and GdSiGe an up to 13 K higher Tc. A maximum entropy change for GdSiGe thin film was found to be 0:45 JkgK

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Physics and Astronomy
Subjects: Q Science > QC Physics
Uncontrolled Keywords: Magnetocaloric, Ising, Phase Transition, Thin Film, FOPT
Funders: EPSRC
Date of First Compliant Deposit: 23 June 2022
Last Modified: 23 Jun 2022 15:05
URI: https://orca.cardiff.ac.uk/id/eprint/150731

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