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Title: 3D atom probe studies of thin film magnetic materials
Author: Pinitsoontorn, Supree
ISNI:       0000 0001 3490 9480
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2008
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A combination of TEM and 3D atom probe has been used to study thin film magnetic materials for read head applications. Three different types of magnetic thin films have been investigated: (1) Co-based films for bias magnets, (2) magnetic tunnel junction (MTJ) with . AIOx barrier and (3) MTJ with MgO barrier. Unlike in longitudinal recording media, the Co-based films showed no Cr segregation to grain boundaries. This difference is likely to be due to the addition of Pt, which reduces the miscibility gap of the Co-Cr system, and film deposition at room temperature which reduces . the kinetics of segregation. However, some compositional variations were observed within grains. Increasing the angle of film deposition resulted in lower stress in the film, presumably due to better lattice matching between the magnetic layer and the seed layer. The observed coercivity enhancement with the larger deposition angle is attributed to changes in the resultant microstructure, specifically a smaller average grain size and more in-plane orientation of the magnetic easy axis. In the as-deposited MTJ. with an AIOx barrier, the barrier was found to be a discontinuous layer which was wavy and non-uniform in thickness. The AIOx layer was seen to form an interconnecting network of oxide islands, consistent with the islands forming at the grain boundaries of the underlayer. After annealing, the AIOx layer had become thicker, smoother, more uniform in thickness and contained fewer pinholes. The AI:O ratio changed from approximately 4:3 before annealing to close to the stoichiometry of 2:3 after annealing. Three energy contributions were considered as possible driving force for the transformation of the separated islands into a continuous layer: pinhole geometry, metal/oxide interface energy and strain energy. The improved TMR with annealing was mainly attributed to the improvement in the quality of the barrier. For the MTJ with an MgO barrier, the MgO deposited on a flat amorphous surface showed a [001] texture. The amorphous CoFeB partially crystallised after annealing and formed the (001)[110]CoFeB II (001)[100]MgO II (001)[110]CoFeB orientation relationship in some areas. Variations were observed in the chemistry of the barrier, but this was not as extreme as in the case of AIOx• The Mg:O ratio within the oxide was measured to be approximately 1:1 and was not changed after annealing. B was found to diffuse from the CoFeB layers: this must have occurred during deposition, as its distribution was the same before and after annealing. In the region of high MgO concentration, B was found mostly at the CoFeB/MgO interface and the CoFeB/Ru interface whereas in the region of low MgO concentration, B was found throughout layers. Mn in the AF layer was seen to have intermixed with the CoFe synthetic AF layer. The MgO layer topography was anti-correlated to the Ru layer but the effect decreased during annealing.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available