Tuning Synthetic Antiferromagnetic and Ferromagnetic Multilayers for Hosting Skyrmions

We present ways to characterise and optimise multilayers for skyrmion-based devices using Co and CoB with Pt, Ir and Ta. Magnetic proximity effects at the interface of a ferromagnet and a non-magnet were studied to find agreements between literature - a topic which is often disputed. It was confirmed that Pt and Ir experience induced moments at a magnetic interface, particularly Pt, and Ta causes a magnetic dead layer. More moment was measured for CoB/Pt interfaces than Co/Pt, and an inhomogeneity in the layer was seen below 8 Å of ferromagnet. The Pt spin polarisation depth was shown to be ∼10 Å, and the top interface to contribute more moment than the bottom. These effects must be considered in the saturation magnetisation of a multilayer thin film. Attention was also given to the Dzyaloshinskii-Moriya interaction and its strength, a key parameter of a skyrmion. It was shown, via collaborative work, that this value could vary, even when the same sample was measured in different laboratories using both different and the same method. Proximity effects, and using an approximation of Bloch’s law specific for thin films could also change the strength of the Dzyaloshinskii-Moriya interaction. Synthetic antiferromagnets consisting of CoB, Ir and Pt were also designed to produce a device with capabilities more suited for skyrmion racetrack memories. Multiple parameters were characterised and trends were established as a function of repetitions, material thicknesses and temperature. An understanding of the hysteresis mechanisms was established via electrical measurements. A synthetic antiferromagnet was built with the ability to host magnetic field-nucleated skyrmions, however, the skyrmions were not coupled together antiferromagnetically across the two ferromagnetic layers. These samples were also probed with THz waves by depositing the material on a nanofabricated coplanar waveguide. Adaptations of the waveguide and the measurement set-up were successful, however, further work is necessary to confirm the observed trends.