In-situ magnetoresistance measurements during patterning of spin valve devices

摘要

This dissertation describes an experimental study on the patterning of thin films and spinvalve devices. Initially the change in the magnetisation reversal of ferromagneticNi80Fe15Mo5 thin films was investigated as the shape anisotropy was increased using opticallithography to pattern wire arrays. These structures show a progressive increase in coercivityand a transition between single and two-stage reversal with increasing milling depth. Asimilar patterning technique was applied to unpinned (Ni80Fe20/Cu/Ni80Fe20) pseudo spinvalve (PSV) structures in order to enhance the coercivity of one of the ferromagnetic layers.The increased coercivity induced by micropatterning changed the natural similarity of themagnetic layers and the structure exhibited a small spin valve response. These initialmeasurements were carried out with separate milling and electrical characterisation steps.However, it was decided that it would be ideal to design a technique to do in-situmagnetoresistance measurements during milling. This meant that the samples could be milledand characterised in the same step, leading to a much cleaner and more efficient process.In-situ magnetoresistance measurements were carried out during micropatterning of PSVdevices, and the measurements showed the evolution in the electrical response as wirestructures were gradually milled through the thickness. Contrary to what was expected, thestructures showed a maximum spin valve response when fully milled through. The effect offurther increasing the shape anisotropy by reducing the wire width, and changing the materialproperties in the PSV structure has also been investigated. MR measurements were taken asthe temperature was increased from 291K to 493K, and the results show that the patternedPSV structures have a better thermal stability than exchange biased spin valves with an IrMnpinning layer.The experiment was extended to the nanoscale, and the results show that a significant increasein MR is not observed despite the fact that the magnetic configuration tends more towardssingle domain. This is thought to be due to an increase in the initial resistance of thestructures. A small increase in MR was observed as the wire width was decreased from 730 to470nm, although the spin valve response is heavily dependent on the gallium dosage densityduring patterning in the Focused Ion Beam (FIB). Micromagnetic simulations were carriedout, which agree with the experimental results and showed the change in the magnetisationreversal from rotation to switching as the dimensions were reduced on the nanoscale.