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The temperature dependence of magnetostriction : an ab initio theory
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Marchant, George A. (2020) The temperature dependence of magnetostriction : an ab initio theory. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3253100~S15
Abstract
In this thesis we present an ab initio theory for the calculation of magnetoelasticity at finite temperature. Magnetostriction, the spontaneous deformation of magnetic materials under the influence of an applied magnetic field, originates from the balance between elasticity and magnetoelasticity - the response of magnetic torque to structural distortions of the lattice. The influence of temperature is modelled through thermal fluctuations of magnetic moments within the Disordered Local Moment picture. The orders of magnitude-separation between the timescales of different processes in the electron dynamics allows for the consideration of a magnetic system as a “frozen” ensemble of magnetic moments, from which a mean-field statistical model can be built to describe their disorder. The explicit dependence of the local moments’ free energy on their direction immediately provides an expression for their magnetic torque. Measuring the linear response of this torque to small, specially chosen strains of the lattice provides magnetoelastic constants. Green’s function-based Multiple Scattering Theory is used within Density Functional Theory to model the electronic structure of the disordered local moments. The use of the Coherent Potential Approximation to construct an effective medium for the orientational degrees of freedom means that the method is also equipped to describe arbitrary chemical disorder, allowing for the study of highly-magnetostrictive alloys.
The method has been used to calculate the cubic magnetoelastic constant B1 in bcc Fe across its ferromagnetic temperature range. Our results reproduce bcc Fe’s anomalous, non-monotonic magnetoelastic temperature dependence and provide a theory for its origin based on the competition between the effects of thermal expansion and magnetic disorder-induced homogenisation on the electronic band structure. Finite-temperature magnetoelasticity in the A2, B2 and D03 phases of Fe1−xGax alloys has also been calculated. Our results show no evidence of the alloy’s characteristic magnetostrictive enhancement in the fully-disordered A2 phase. In contrast, the selective-doping of the partially-ordered phases exhibit significant enhancement of their magnetelasticity, especially the D03 phase which was found in previous studies to have a detrimental influence. A preliminary study is also carried out on the magnetic properties of highly-magnetostrictive rare earth-transition metal magnets belonging to the Laves phase REFe2 class (RE=Y,La,Gd-Lu). Calculated values of temperature-dependent magnetisation show good qualitative agreement with experiment, including excellent agreement with zero-temperature magnetisation measurements and impressive evaluation of compensation temperatures. Calculations of magnetoelasticity and magnetocrystalline anisotropy as a function of temperature in GdFe2 provide a model for itinerant anisotropic phenomena in the REFe2 series.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QC Physics | ||||
Library of Congress Subject Headings (LCSH): | Magnetostriction -- Mathematical models, Transition metals -- Magnetic properties, Iron alloys -- Magnetic properties, Rare earth metal compounds -- Magnetic properties | ||||
Official Date: | August 2020 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Department of Physics | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Staunton, Julie B. | ||||
Format of File: | |||||
Extent: | xix, 152 leaves : illustrations (chiefly colour) | ||||
Language: | eng |
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