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Title: Advanced magnetoelastic and magnetocaloric materials for device applications
Author: Hadimani, Ravi L.
ISNI:       0000 0004 2748 6397
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2009
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Magnetocaloric and magnetoelastic materials can be utilised in various device applications and have a potential to increase their efficiency by a considerable amount. In this thesis, Gd5(SixGei_x)4 is extensively researched on its magnetic properties such as magnetic phase transition temperature, magnetostriction, magnetoresistance and anisotropy. Field induced phase transition in Gd5(SixGei_x)4 was observed in several compositions and the rate of change of the first order phase transition temperature was determined to be approximately 5 K/Tesla. Various methods of transition temperature measurements were compared and the Arrott plot technique was determined to be accurate method for magnetocaloric materials. An advanced technique based on Arrott plots was developed to estimate the second order phase transition temperature when it is suppressed by the first order phase transition. This technique was also extended to estimate the transition temperature of mixed phase alloys. Field induced phase transition at high temperature using high magnetic field measurements up to 9 Tesla were carried out on two compositions of Gd5(SixGei-x)4 for x=0.5 and x=0.475 to validate the Arrott plot technique. Magnetostriction measurements were carried out on Gd5(SixGei_x)4 for various compositions. Fine structure was observed in the magnetostriction measurement in single crystal and polycrystalline Gd5Si1.95Ge2.05 samples but not on other compositions, which might be due to the presence of a secondary phase. It was demonstrated that a giant magnetostriction of the order of 1813 ppm could be obtained by varying the temperature using a Peltier cell and removing the requirement of bulky equipment such as Physical Properties Measurement System (PPMS). Magnetoresistance was measured for various compositions and an irreversible increase in resistivity was observed which depended linearly on the number of thermal cycles passing through the first order phase transition temperature. The irreversibly increased resistivity was recovered by holding the samples at high temperature for a long period of time of up to 3 days. A theoretical model was developed to explain the recovery in the resistance and was experimentally verified. First order magnetocrystalline anisotropy constant Kj, easy and hard axes of the single crystal Gd5Si2.7Gei j sample were determined using magnetic moment as a function of angle of rotation of the sample at room temperature. Dependence of the first order phase transition temperature on the angle of rotation of the single crystal Gd5Si2Ge2 sample was determined to be negligible. Additionally polycrystalline samples of Gd5Sii.8Ge2.2 and Gd5Sii.9Ge2.i were prepared by arc- melting and heat treatment was carried out on these samples in accordance with the literature to remove residual secondary phases in the sample at the Materials and Metallurgy Department of the Birmingham University. XRD measurements were carried out on these samples to confirm the crystal structure.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available