Characterisation of selected magnetic elements

Crawford, Mhairi (2004) Characterisation of selected magnetic elements. PhD thesis, University of Glasgow.

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Abstract

The work presented in this thesis is a study of the reversal properties of selected magnetic materials. The reversal mechanisms of magnetic multilayer artificial anti-ferromagnetically biased spin valves (AAF SV), both in the form of continuous thin films and discrete elements have been investigated. SVs consist of two ferromagnetic layers separated by a non magnetic spacer layer. The magnetisation of one layer is pinned through exchange coupling to an adjacent anti-ferrornagnetic layer. The magnetisation of the other ferromagnetic layer (the free layer) can be reversed by applying a small external field. If the free layer aligned is parallel to the pinned layer, a low resistance state is measured. For an anti-parallel alignment, the resistance is considerably larger. This is a phenomenon known as Giant Magnetoresistance (GMR) and is highly desirable for applications such as read heads in disc drives. For AAF biased systems, the pinned layer consists of two ferromagnetic layers and a non magnetic spacer such that the two ferrornagnetic layers couple anti-ferromagnetically. This modifies the coupling within the entire multilayer stack giving a potential advantage for applications. A second area of investigation comprised hybrid ferromagnetic / semiconductor systems. Magnetic elements were formed on Hall bars enabling a noninvasive study of the interactions between the stray field from the elements and electrons in the two dimensional electron gas (2DEG) of the semiconductor. From this, the magnetisation state of the elements can be investigated giving further insight into the reversal properties of the material. The first Chapter introduces the basic concepts of ferromagnetism and reviews the energy contributions which govern the behaviour of magnetic materials. Following this is a discussion of domains, domain walls and some of the reversal mechanisms relevant to the work presented in later Chapters. Two types of magneto-resistive effects important to the two experimental techniques of this thesis are introduced: giant magnetoresistance with an emphasis on spin valve systems and quantum transport mechanisms for semiconductor heterostructures. Techniques necessary for the fabrication of all specimens investigated are discussed in Chapter 2. A discussion of electron beam lithography and photolithography, pattern design, transfer and resists with reference to the systems available precedes a description of some of the associated techniques. An investigation of the free layer magnetic reversal processes for a range of AAF biased spin valves is presented in Chapter 3. Chapter 4 continues the investigation of AAF biased spin valves with the study of a range of spin valve elements. The final experimental chapter discusses the fabrication and measurement of hybrid ferromagnet/semiconductor samples. General conclusions and a discussion of further work are presented in Chapter 6.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: John Chapman
Keywords: Electromagnetics
Date of Award: 2004
Depositing User: Enlighten Team
Unique ID: glathesis:2004-74049
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 23 Sep 2019 15:33
Last Modified: 23 Sep 2019 15:33
URI: http://theses.gla.ac.uk/id/eprint/74049

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