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Title: Magnetocaloric properties of La(Fe,Si)13 compounds
Author: Lovell, Edmund
ISNI:       0000 0004 6348 4954
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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La(Fe,Si)13-based compounds are extremely promising for use in magnetocaloric cooling applications. This thesis aims to address multiple challenges to their use in cooling systems, as well as contribute to the fundamental understanding of their first-order magnetic phase transition. A significant field rate dependence in the field-induced paramagnetic (PM) to ferromagnetic (FM) transition in the first-order composition is determined to be a result of non-isothermal conditions from the inherent heating or cooling from the magnetocaloric effect (MCE). Reduction of the magnetic hysteresis has been shown with improved thermal linkage with the thermal bath. Metastability between the phases is demonstrated by magnetic temporal evolution (relaxation). The completion of this evolution is arrested by physical cracks and the sample shape (demagnetising field effects). The onset of the transition is shown to be dominated by the demagnetising fields, and as such a method to reduce magnetic hysteresis via sample shape is demonstrated. These effects are shown to not dominate for samples which show a second-order transition. Mn substitution on the Fe site decreases the strength of the first-order character which is correlated with the relaxation rate, the latent heat decrease and the heat capacity peak increase. Similar correlations are observed with increasing temperature and both the relaxation rate and latent heat go to zero at the critical point, defined by the temperature at which the first-order character (latent heat and hysteresis) disappears. The electronic and spin wave contributions to the low temperature heat capacity are investigated and significant enhancement of the former is shown. This is further enhanced with increased Mn-doping but to a much lesser extent when interstitial hydrogen is introduced to the compound. Suppression of this further enhancement in magnetic field suggest that there are two enhancement mechanisms: a base enhancement likely from electron-phonon interactions, and a second which is magnetic in origin.
Supervisor: Cohen, Lesley ; Caplin, David Sponsor: Engineering and Physical Sciences Research Council ; European Commission
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral