Magnetic Force Microscopy with High Resolution and High Sensitivity: Quantitative Approach and Applications to Thin Film Multilayer Systems Supporting Skyrmions

Nowadays, magnetic thin film multilayers and their applications constitute a very active research field. There are two major reasons why these materials keep stirring interest in the scientific community: first, multilayers coming with a naturally-tunable architecture provide a rich platform for fabricating appropriate samples for the observation of physical phenomena predicted theoretically; and second, understanding the novel magnetic states in such materials and controlling the technological processes within may lead to devices that could offer applications with very high impact.
One of the most interesting novel magnetic states in thin film multilayers are magnetic skyrmions. These are local spin textures possessing topological protection and particle-like nature. Due to their small size which may reach down to around 10 nm, and the possibilities of easily transporting them, room temperature magnetic skyrmions may serve as information carriers in very compact and also energetically-efficient storage such as racetrack memory.
The investigation of magnetic objects with such small dimensions presents a challenge on its own: a powerful imaging/microscopy tool with high spatial resolution and high sensitivity has to be implemented to study such tiny objects. Magnetic force microscopy (MFM) is for example an adequate technique for such studies: it is a versatile microscopy tool to image the micromagnetic state of a magnetic sample in close proximity of its surface by detecting the stray fields emanating from the sample. The state-of-the-art MFM - just like the one introduced in this dissertation - can reach a spatial resolution down to a length scale of about 10 nm, and is therefore suitable for imaging nanoscale magnetic objects such as skyrmions and magnetic nano-devices. Moreover, while the measured quantities in MFM are indirectly related to the quantity of interest (the magnetization of the sample), a quantitative approach would allow the extraction of the information on the latter; this makes MFM even more of value.
In this dissertation, the high-resolution (HR-) MFM instrument used for the measurements in this dissertation shall be briefly introduced in Chapter 1, and the potential of improving its sensitivity is explored from the aspect of the probes that are being used. More precisely, the influence of (magnetic) coating on the MFM cantilevers is investigated and discussed in Chapter 3. Then, the MFM sensitivity is explained in detail in Chapter 4, making emphasis on the high-quality data that one can obtain with an advanced MFM instrument and improved probes, which are consistently employed for the MFM data acquisition in this dissertation. The quantitative approach for extracting sample magnetization from raw MFM data via the transfer function method is explained thoroughly in Chapter 5; the subtleties of data handling and the limitations of the method are also addressed. Ultimately, with the HR-MFM instrument and the improved cantilever tips, various magnetic thin film multilayer systems which support skyrmions are investigated. In Chapter 6, a ferro-/ferri-/ferromagnetic trilayer system which hosts two distinct skyrmion states is studied; by varying the Fe-sublayer thickness in the ferromagnetic layer, we show that the coexistence of the two skyrmion states can be continuously tuned. In Chapter 7, a hybrid ferro-/ferrimagnetic multilayer system and an antiferromagnetically-coupled skyrmion system are studied. In the first system, by substituting the Co-sublayer with Co-rich ferrimagnetic material, dense, close-to $M=0$ skyrmions can be obtained. In the second system, by combining the well-studied skyrmion layers with a bias layer, room-temperature zero-field skyrmions could be achieved. The newly observed skyrmion states in the two latter chapters will potentially offer solutions to some of the problems that still remain for implementing magnetic skyrmions for technical applications.