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Title: A TEM Study of Magnetic Tunnel Junctions and Magnetic Materials
Author: Kirk, Daniel James
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2007
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Since 1995, magnetic tunnel junction structures have been of great commercial interest because of their use in magnetic recording and data storage applications. In addition, the complex way in which the properties of the junction are influenced by the microstructure makes these structures of great scientific interest. The development of the capabilities of devices based on these structures has also lead to new applications for various materials. In particular, amorphous ferromagnets have become popular choices for use as ferromagnetic layers in these structures. Since the properties of magnetic tunnel junctions are determined by structural features less than 1 nm in size, a technique capable of studying these junctions with very high resolution is needed. In this thesis tunnel junction structures with two different types of barrier material are studied using a variety of transmission electron microscopy techniques. The effects of processing conditions were investigated for both structures. Electron microscopy was also used to investigate the origins ofuniaxial magnetic anisotropy in an important amorphous ferromagnet. The TiOx barrier of IrMnlCoFeffiOx/CoFe tunnel junctions formed by radical oxidation was found to be amorphous and its thickness highly dependent on oxidation time. Increased oxidation times led to the formation of oxides of Co and Fe from the lower ferromagnetic layer. Annealing was shown to have little effect on barrier thickness but does lead to diffusion of Mn to the barrier. Mn was observed to reduce oxides of Co and Fe and form MnOx in the barrier. In a study of PtMnlCoFe/AIOxfNiFe junctions, in which the barrier was formed by natural oxidation, the amorphous AIOx barrier width was found to be independent of oxidation time. The increase in the magnetoresistance of the higher oxidation time junctions was attributed to an increase in the barrier height of the oxide. In addition, evidence of oxidation of the ferromagnetic layers was found in a sample which had not been annealed. Magnetic anisotropy, induced by in-field annealing, was measured in amorphous CoFeB thin films and these films were then studied by electron diffraction to examine the short range order in this material. G(r) was obtained for directions parallel and perpendicular to the easy axis of magnetisation. No bond-length anisotropy was observed and an upper limit for the magnitude of the pair-ordering effect on the coordination of the transition metal and metalloid atoms was established.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available