Development and characterisation of magnetostrictive films

Abstract

Magnetostrictive materials play an important role for many years and have been employed in a wide use of applications, for example, electrical engineering, communication technology, computer science and spintronics arisen in decades. These developments of usage in turn accelerate the rise of magnetism and magnetic material research which depend on the high performance of materials. As is well known, the behaviour of magnetic materials is tightly related to the crystalline and magnetic microstructure. Therefore, it is of great significance to investigate their crystalline and magnetic structures and their correlation with their performance. Meanwhile, magnetic domain research is also useful to understand the magnetisation reversal behaviour and coercivity mechanism of magnetic materials. This sequentially ameliorate the manufacture techniques and improve the performance. Among all magnetic sensitive imaging techniques, magnetic force microscopy (MFM) is a very strong technique to investigate magnetic microstructures of magnetic materials due to its high lateral resolution, low sample preparation quality requirement, simple operation and wide range of materials applicable. MFM can capture the images of the magnetic domain structure and surface topography simultaneously. These advantages make MFM the mostly used magnetic domain sturcture observation instrument since its invention. In this thesis, topography and magnetic microstructure of bulk terfenol-d (Tb0.3Dy0.7Fe2), bulkgalfenol ( F e 8 1 G a 1 9 ) and galfenol thin film are investigated using MFM. The stereological method correlated to MFM images and MATLAB is discussed. Meanwhile, stereological method is also used to investigate magnetic domain width and magnetic domain wall energy density. To further understand the imaging principle of MFM and MFM image analysis of samples, the electrical noise, mechanical noise and suitable tip-to-sample distance of MFM are investigated as well. Besides, the correlation of magnetic microstructure, topography, magnetic domain and high temperature is also analyzed. The detailed content is givenbelow:1. Galfenol as a high magnetostriction alloy that attracts more attentions on its structure, properties and applications. The success of galfenol in sensor, actuator and energy harvesting devices is owing to the excellent properties (high magnetostriction, decent mechanical

properties and tractability) of galfenol itself. This thesis focuses on the effect of the grinding/polishing process on galfenol surface topography and magnetic domain structure, and the effect of DC magnetron sputter parameters on surface structure and magnetic domain structure of galfenol thin films. And the effect of high temperature on its surface microstructure. 2. The noise of atomic force microscopy operation is investigated to reduce its effect on measurements to improve the quality of measurements. The surface roughness of bulk galfenol shows a decreasing trend when polishing time increases with 0.25 μmgrade diamond paste polish. And the domain structure of galfenol is first studied in directions perpendicular and parallel to the sample magnetic alignment axis and at high temperature. The domain width and domain wall energy density of galfenol are first studied here as well. This thesis investigated when temperature increases, the calculated results of domain width increases and domain wall energy density decreases by using stereological method. 3. The results of experiments on galfenol thin film fabrication demonstrate that a rougher substrate surface results in a rougher galfenol film surface. Thicker galfenol films have a larger root mean square (RMS) surface roughness than thinner films. Increased sputter power from 50 W to 150 W would cause a decrease in galfenol thin film surface roughness. The same decreasing trend of galfenol thin film surface roughness as Ar working pressure increased from 9 mTorr to 14 mTorr. The thicker galfenol film also have a larger magnetic domain wall energy density and magnetic domain width than a thin film, as calculated by the stereological method. Further, galfenol thin film particle sizes, as well as domain widths, increase with temperature, whereas domain wall energy density decreases. 4. To further understand the galfenol itself. The galfenol-based rotation sensor is designed and fabricated to investigate the performance with different layer dimensions. Different layer dimensions would cause different rotations of the beams. Therefore, to understand the performance of layer dimensions, range of values are chosen to simulate the beam rotation angle.