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Title: A combined density functional theory and Monte Carlo study of manganites for magnetic refrigeration
Author: Korotana, Romi Kaur
ISNI:       0000 0004 5994 169X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2014
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Perovskite oxides such as manganites are considered to be strong candidates for appli- cations in magnetic refrigeration technology, due to their remarkable magnetocaloric properties, in addition to low processing costs. Manganites with the general formula R1-x AxMnO3, particularly for A=Ca and 0.2 < x < 0.5, undergo a field driven tran- sition from a paramagnetic to ferromagnetic state, which is accompanied by changes in the lattice and electronic structure. Therefore, one may anticipate a large entropy change across the phase transition due to the first order nature. Despite many ex- perimental efforts to enhance the isothermal entropy change in manganites, the max- imum obtained value merely reaches a modest value in the field of a permanent mag- net. The present work aims to achieve an understanding of the relevant structural, magnetic, and electronic energy contributions to the stability of the doped compound La0.75Ca0.25MnO3 . A combination of thermodynamics and first principles theory is applied to determine individual contributions to the total entropy change of the system by treating the electronic, lattice and magnetic components independently. For this purpose, hybrid-exchange density functional (B3LYP) calculations are performed for LaMnO3, CaMnO3 and La0.75Ca0.25MnO3 . The most stable phases for the end-point compounds are described correctly. Computed results for the doped compound predict an anti-Jahn-Teller polaron in the localised hole state, which is influenced by long- range cooperative Jahn-Teller distortions. The analysis of the energy scales related to the magnetocaloric effect suggests that the charge, orbital, spin and lattice degrees of freedom are strongly coupled, since they are of a similar magnitude. Through the analysis of individual entropy contributions, it is identified that the electronic and lat- tice entropy changes oppose the magnetic entropy change. Therefore, the electronic and vibrational terms have a deleterious effect on the total entropy change. The results highlighted in the present work may provide a useful framework for the interpretation of experimental observations as well as valuable guidelines for tuning the magnetocaloric properties of oxides, such as manganites.
Supervisor: Harrison, Nicholas Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral