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The magnetocaloric effect at a first order phase transition

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Title: The magnetocaloric effect at a first order phase transition
Authors: Bratko, Milan
Item Type: Thesis or dissertation
Abstract: The magnetocaloric effect (MCE) can be defined as the isothermal entropy change (or adiabatic temperature change) of a material upon application/removal of an external magnetic field and is the key physics for a magnetic cooling device. A discontinuity of entropy at a first order phase transition (FOPT) allows for a large entropy change to be induced by a relatively small field. However, a hysteresis is necessarily associated with a FOPT. The effects of hysteresis, as measured in a sensitive microcalorimeter, are the focus of the thesis. The calorimetric setup used is unique in allowing a separate measurement of heat capacity and latent heat and thereby the possibility to clearly distinguish the first and higher order contributions to MCE. Due to the high measurement fidelity required, the experimental chapter is a core component of the thesis and includes a thorough analysis of the measurement errors associated with the microcalorimeter. Several improvements are proposed to improve precision and accuracy of the measurement in future studies. The first of the hysteresis effects is a spurious ‘colossal’ MCE. Its indirect observation was claimed in 2004 from magnetisation measurements analysed using a Maxwell relation and was widely disputed thereafter. It was shown that a different measurement protocol leads to non ‘colossal’ MCE. This thesis investigates whether the ‘colossal’ MCE can be achieved by a particular magnetisation history by reproducing the original measurement protocol in a more direct calorimetric measurement. It is shown that the ‘colossal’ MCE is just an artefact of the use of Maxwell relation in a non-equilibrium process. The final chapter discusses a second effect of hysteresis: a subtle difference between the indirect and calorimetric measurements of MCE that can be clearly observed when comparing measurements on field application and removal. Maxwell relation leads to an artefact related to temperature dependence of the hysteresis. In the calorimetric measurement the dissipation of magnetic work in a hysteretic magnetisation cycle is observed.
Content Version: Open Access
Issue Date: Sep-2013
Date Awarded: Jan-2014
URI: http://hdl.handle.net/10044/1/23653
DOI: https://doi.org/10.25560/23653
Supervisor: Sandeman, Karl G
Cohen, Lesley F
Sponsor/Funder: Engineering and Physical Sciences Research Council
Funder's Grant Number: DTA
Department: Physics
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Physics PhD theses

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