Magnetic Anisotropy And Magnetostriction In Functional Ferromagnets
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Type of WorkText
DepartmentElectrical and Computer Engineering
ProgramDoctor of Engineering
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Magnetic Anisotropy and magnetostriction are the most fascinating properties in magnetism that have enabled us in engineering powerful magnetic materials which play a significant role in data storage, permanent magnets and sensors. A large magnetic anisotropy and magnetization is the fundamental requirement of permanent magnets and materials for data storage, whereas small magnetic anisotropy and large magnetostriction is key requirement for materials used for sensors. Neodymium magnet (Nd2Fe14B) is the strongest permanent magnet available today due to its giant magentocrystalline anisotropy (4×106 J/m3) and large saturation magnetization (1.6 T). However, these magnets contain rare earth element, are expensive in comparison to other available magnets and are only suitable for applications for temperatures up to 200"°C" . There is a need for stronger and cheaper rare earth free permanent magnets, and research is ongoing to engineer new powerful permanent magnets that could replace the current ones. One of the promising materials to be investigated for this purpose is Cobalt Ferrite (CoFe2O4), which has a large cubic magnetocrystalline anisotropy (105 J/m3) and large magnetostriction(-590ppm). This work investigated the structural and magnetic characteristics of epitaxial Cobalt Ferrite heterostructures grown on single crystal MgO (100) and MgO (110). X-ray diffraction confirmed the tetragonal distortion in the films which is a precursor for large magnetic anisotropy and TEM measurements confirmed their epitaxial nature. Film grown on MgO (100) displayed biaxial behaviour with second order anisotropy of 2.7 × 104 J/m3 which was deemed to be unsuitable for permanent magnet applications whereas film grown on MgO (110) displayed uniaxial behaviour and a large uniaxial anisotropy of 1.33×106 J/m3. The BHmax and hardness parameter for (110) film was determined to be 9.5 MGOe and the 2.75 respectively, which surpassed the values of other ferrites available. The presence of such large anisotropy in these heterostructures have been attributed to the large magnetostriction of cobalt ferrite. These characteristics have determined cobalt ferrite as a suitable candidate for further investigation to engineer cheaper and stronger permanent magnets.