Optimising the material properties of CoFeB/MgO/CoFeB based magnetic tunnel junctions (MTJs) is a key step for enabling the further development of data storage and processing technologies based on spin electronics (spintronics). A promising area of current research is the use of thin films with antiferromagnetic (AF) ordering, which has led to a focus on AF materials for spintronic memories. The first order AF to ferromagnetic (FM) metamagnetic phase transition found in FeRh at technologically useful temperatures offers additional degrees of freedom for developing AF spintronics. The work presented in this thesis explores the characterisation and optimisation of thin layers for spintronic and AF spintronic memories. An in-depth study of the sputter deposition conditions required to fabricate MTJs with uniform layers and pinhole free MgO tunnel barriers was undertaken. Kelvin Probe Force Microscopy was used to determine the electrical properties of the barrier, demonstrating that a low sputtering power produced smooth barriers without pinholes, critical for fabricating reliable MTJs. The sputtering parameters necessary to fabricate Ta/CoFeB/MgO/CoFeB/Ta/Pt MTJ films with perpendicular magnetic anisotropy (PMA) were optimised through a study of annealing conditions on their structural and magnetic properties. Annealing films to 340C enhanced the PMA, which was attributed to the intermixing of Ta and CoFeB at the bottom layer Ta/CoFeB interface, whereas annealing to 360C was shown to degrade magnetic properties, attributed to the diffusion of oxygen from the MgO barrier. To electrically characterise films, a new time-saving fabrication process utilising PMMA resist was developed for point contact current-in-plane-tunnelling measurements. Initial results demonstrated the successful fabrication of point contacts onto the film surface with the required spatial resolution of 1.5 +/- 0.05 um, without causing structural damage. The nucleation of the AF to FM phase transition in FeRh thin films grown onto MgO(001) was investigated using polarised neutron reflectivity. The FM phase was found to initiate at the MgO/FeRh interface in the form of a 5 nm strained FeRh layer. The strained layer was observed to be ferromagnetically ordered at room temperature. These results demonstrated the impact of topography and strain on the magnetic properties of FeRh as thicknesses are reduced to the order of the exchange length, highlighting the challenges associated with material optimisation for AF spintronic memories.
|Date of Award||1 Aug 2020|
- The University of Manchester
|Supervisor||Paul Nutter (Supervisor) & Thomas Thomson (Supervisor)|
- Kelvin Probe Force Microscopy
- tunnel barrier
- current-in-plane tunnelling
- magnetic tunnel junction