Heusler alloys deposited by pulsed laser ablation for use in spin tunnelling applications

• Main Author: Leebody, Matthew James
• Published: Queen's University Belfast 2015
• Subjects: 530.4
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.679217

This thesis investigates the growth of Heusler alloys via pulsed laser deposition (PLO), and their use in tunnel magnetoresistance (TMR) devices. In doing so it investigates the capabilities of a novel beam scanning system with good potential uniformity characteristics over large areas. This thesis has presented a novel PLO system whose beam scanning system allows large diameter samples. Initially, the characteristics of this beam scanning system are analysed and shown to offer exceptional uniformity beyond typical PLO, such that variations of 2% thickness of films across a 3-inch wafer were realised. Thicknesses of deposited films were equally as reliable, with x-ray diffraction (XRD) analysis showing these results across both CF AS and MgO, used as a tunnel barrier material for TMR devices. Magnetic tunnel junctions were fabricated using a novel combination of photolithography phases, yielding a 4-point probe capable device. Data obtained from transmission electron microscopy of the deposited CFAS layers showed semi-qualitative results indicating levels of iron were lower than expected however broadly in line with that expected of a stoichiometric sample. The quality of the barrier interface was shown to be poor in large areas, with defects and poor crystal growth. Many devices were analysed to discover the TMR ratios and thus the spin polarisation of the CFAS junctions, and whilst the theoretical 100% spin polarisation was not observed, values as high as P=0.89 or 783% TMR ratios were observed at room temperature. The low iron levels combined with these structural problems may suggest why the observed TMR was lower than that predicted by theory. Despite this, the repeated observation of high spin polarisations of P=0.83 to 0.89 make these results amongst the highest such ratios to date for CFAS/MgO junctions