Multi-modal and quantitative magnetic force microscopy : application to thin film systems with interfacial dzyaloshinskii-moriya interaction

The applications of thin film magnetism play an important role in our everyday life. Magnetic data storage devices for example, rely on magnetic thin film structures for the magnetic recording media itself as well as for the read heads of those devices. Furthermore, different types of electronic sensors exploit the properties of magnetic thin film systems, engineered specifically for the desired function and often relying on processes taking place at the nanometer scale. Magnetic Force Microscopy (MFM) provides a tool for the microscopic magnetic characterization of these thin films and multilayers.
Two novel methods for controlling the tip-sample distance for MFM based on bimodal cantilever excitation modes were developed in this thesis.
With these, the topography and magnetic stray field emanating from the surface of a sample can be simultaneously measured in a single passage.
Moreover these modes are compatible with operation in vacuum and prevent the use of typical lift-mode operation relying on intermittent contact mode for mapping the topography of the sample.
The first method makes use of different decay lengths of magnetic and van der Waals forces acting between tip and sample.
A stable MFM operation at tip sample distances of only a few nanometers becomes possible resulting in highest lateral magnetic resolution that can be better than 10 nm.
However, this method is limited to small tip sample distances where the van der Waals forces become sufficiently large.
This limitation is overcome by the second distance control method presented in the thesis.
It uses the tip sample capacity as a proxy for the sample's topography and allows MFM-measurements performed with the tip tracing the local topography of the sample or with a tip sample distance that is kept constant in average.
Both operation modes are suitable for measurements in externally applied B-fields, as long as possible field induced changes of the cantilever quality factor are considered and the corresponding feedback setpoints are adjusted.
The operation at constant average height is preferred, when a later quantitative analysis of the MFM data is considered.
The theoretical background of quantitative MFM, and the tip calibration procedures used here are reviewed.
These data analysis and measurement techniques have been applied to study Pt/Co/Ir multi-layer ferromagnetic samples with perpendicular anisotropy and interfacially induced Dzyaloshinskii-Moriya (DM) interaction fabricated with a commercial ultra high vacuum, sput-ter deposition system.
A novel method for the determination of the average DM interaction from the near-equilibrium domain structure was developed.
The average DM interaction value of D=2.04 mJ/m² is higher than the D=1.6+- 0.2 mJ/m² found by Moreau-Luchaire et al.
To date it remains unclear whether this higher values arise from a better interface quality of the samples that consist of only five repeats of Pt/Co/Ir compared to the ten repeats used in earlier studies by Moreau-Luchaire et al., or results from the improved analysis method developed here.
MFM images of skyrmions performed after repeated saturation processes revealed that the skyrmions re-nucleate at a few specific locations of the film indicating that these may have distinct physical properties.
High-resolution images of these skyrmions revealed that their MFM contrast varies substantially between 1.1 Hz and 2.8 Hz, and that some skyrmions have an elliptical shape.
Local D-values could be obtained from fitting model calculations to the measured data.
These D-values varied between D=3.06 mJ/m² and D=3.48 mJ/m² and are thus considerably larger than the average D=2.04mJ/m² determined from an analysis of the equilibrium do-main size.
Apart from the skyrmions, a background contrast that remains constant in all applied fields was observed.
Using quantitative MFM methods the spatial variation of the areal magnetic moment density could be determined.