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Title: The effects of annealling on magnetic tunnel junctions
Author: Anderson, Graham Ian Robert
ISNI:       0000 0001 3420 8801
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2007
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Magnetic tunnel junctions (MTJs) have come to the technological fore in recent times due to their high applicability to recording media, and the wealth of information that can be obtained in understanding spin-polarised tunnelling phenomena. In 1997 it was first reported that performing a high-vacuum annealling step on MTJs significantly increased tunnelling magnetoresistance (TMR), the figure-of-merit in these devices. Since then research has continued apace into both understanding this increase and determining how to maximise TlvlR via annealling. Recently the annealling step has taken an even larger role in MgO based MTJs, as it is integral in creating the interfaces needed to obtain giant TMR. This thesis contains work based on annealling MTJs using two techniques that have previously not been used to study annealling, despite being highly applicable. The first technique is in situ transport measurements during annealling, which can be used to determine the barrier profile throughout the anneal process. The second technique is soft X-ray resonant magnetic scattering (SXRMS) which has two advantages; firstly, using the incoming X-rays tuned to an absorption edge, buried .interfaces can be probed and, secondly, by switching the photon helicity, magnetic properties of these interfaces can be monitored. The first study comprises the development of MgO plasma oxidised MTJs and uses in situ transport measurements to show that during annealling the Mn moves to the barrier interfaces, which affects the sample performance. The second and third studies use in situ transport in conjunction with SXRMS. One study investIgates the barrier evolution and interface sharpness during the anneal using AIO based MTJs. This shows that the improvement of the barrier quality appears to playa larger role than improving the magnetic interfaces. Lastly; CoFeBjMgOjCoFeB MTJs are measured by both techniques as they are annealed at various temperatures around the CoFeB crystallisation temperature. Upon annealling at 200°C the barrier quality is improved and TMR increases. Annealling at temperatures above this does not im;prove the barrier but causes part-crystallisation of the CoFeB causing a large increase in TMR. SXRMS results showed conflicting results depending upon the cumulative anneal process. These results were modelled qualitatively to understand the crystallisation/ diffusion behaviour seen in the MTJ.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available