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Title: Metallic magnetic heterostructures
Author: Leung, Chi Wah
ISNI:       0000 0004 2707 400X
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2002
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This work studied sputter deposited conventional spin valves (SV) and related structures. In SV layered structures, two ferromagnetic layers are separated by a non-magnetic spacer. Under an external magnetic field, the relative orientation of the magnetization changes in the ferromagnets, exhibiting the giant magnetoresistive effect. The controlled switching of ferromagnets in convention SV is facilitated by the exchange bias (EB) effect, which is achieved by depositing an antiferromagnetic layer next to one of the ferromagnetic layers in a magnetic field. Two highly related investigations were performed in this work. In the first part the exchange bias effect in the Ni80Fe20/Fe50Mn50/Co trilayer structure was studied. Samples were deposited in a low field condition that permitted EB to be established in NiFe/FeMn but not in FeMn/Co bilayer structures. Temperature-dependent magnetic measurements were performed on the trilayer sample, as well as the corresponding NiFe/FeMn and FeMn/Co bilayer samples. Recent literature on similar system showed that an AF spiral could be formed in the trilayer, which was probed by relative EB directions of the NiFe and Co layers. In this work, no exchange bias was found to propagate from the NiFe/FeMn system into the FeMn/Co system, showing that the AF spiral was induced by the specific magnetic treatment and was not the cause of EB effect. Besides, exchange bias field and coercivity of the samples indicated the influence of the EB system in the presence of an adjacent EB system. Explanations of the effect were made with some existing EB models. In the second part of the work, conventional SV of target structure Nb/NiFe/Cu/Co/FeMn/Nb was studied in a 'built-up samples' strategy. A batch of these built-up samples, which corresponded to the different stages of the deposition of the target top conventional SV structure, were prepared by terminating the sputtering process after a certain number of layers were deposited. These samples were thoroughly characterized by structural, magnetic and electrical measurements. In terms of structural characterization by x-ray techniques, more reliable information concerning the morphology and microstructure of the layers was obtained by probing the built-up samples, instead of relying solely on the information of the full SV structure. For the electric and magnetic measurements, a number of unexpected observations were made in the built-up samples, although the final performance of the full SV structure was of comparable quality to the literature. These results showed the ability of the 'built-up samples' strategy in critical characterization and optimization of magnetic multilayered structures.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Heterostructures ; Magnetic Devices