Magnetic anisotropy of magnetostrictive terbium-iron/iron-cobalt multilayer thin film and iron-platinum nanoparticles
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This dissertation focuses on the characterization of giant magnetostrictive multilayer thin film structured [TbFe/FeCo] n for application to highly sensitive stress sensors and magnetic random access memory (MRAM) incorporated with magnetic tunneling junction (MTJ). It also concerns the magnetic properties of RF sputtered L1 0 FePt nanoparticles using a porous alumina template for ultra-high density data storage. The effect of a stress-induced anisotropy on the magnetic properties of Tb 0.4 Fe 0.6 , Fe 0.5 Co 0.5 single layer films and [Tb 0.4 Fe 0.6 /Fe 0.5 Co 0.5 ] n multilayers deposited on various substrates under different thermal treatments was studied. The in-plane radial anisotropy observed in the as-deposited films was eliminated by optimizing the sputtering parameters such as sputtering gas pressure, power and substrate to target distance. The effect of various treatments on the sample is discussed, including forming field, thermal annealing treatment and induced stresses. External stress was deliberately introduced by mechanically inducing a small curvature into the substrate before sputtering and releasing it after the sputtering and post sputtering thermal annealing. This stress anneal method was found to achieve excellent control of both the direction and magnitude of magnetic anisotropy in the sample. Magnetic hysteresis (M-H) loops in both parallel- and perpendicular- to-the-stress directions were measured to analyze the direction and magnitude of this induced anisotropy. Beside stress anneal induced anisotropy, the exchange bias field attributed from exchange coupling effect was observed in the magnetic hysteresis (M-H) loops of strained FeCo/TbFe multilayer thin films. This exchange bias field was discussed theoretically from a model and investigated in detail relating to number of bilayers, thickness of each layer and field cool effect. This reproducible exchange bias field could be used in MTJ structured samples for stress sensors and MRAM applications. Scanning Electron Microscopy (SEM) and Auger electron spectroscopy were used to study the morphology of the samples. The magnetic properties were measured using Superconducting QUantum Interface Device (SQUID) and Vibrating Sample Magnetoscopy (VSM). The fabrication process of FePt nanoparticles using RF magnetron sputtered multilayers of Fe and Pt onto a porous alumina template was compared with a chemical synthesis method. The structural and magnetic characterization showed that magnetron sputtered FePt nanoparticles exhibited advantage in strong anisotropy energy and high thermal stability. In order to achieve FePt alloy nanoparticles with the face-centered tetragonal L 1 0 structure for high density data storage application, post sputtered thermal treatment and various atomic composition of FePt alloy were investigated. Electrical measurement of nanoparticles after e-beam nanolithography of 100 nm gap patterns is purposed. Preliminary research was done on stress sensors incorporated with magnetostrictive TbFe/FeCo multilayers and MTJ samples. This work can be continued by others as an extension of this dissertation.