Optimising MFM Tips for the Investigation of Magnetic Microstructures

- Optimierung von MFM Spitzen zur Untersuchung von magnetischen Mikrostrukturen -

Author: Thomas Pfaffelhuber
Supervisor: Prof. Dr. H. Hoffmann
Submission Date: 2nd March 1998

Abstract

Magnetic force microscopy (MFM) has now become an established technique in the micromagnetic characterisation of materials. Although capable of routinely providing magnetic contrast on a scale of < 50nm image interpretation is limited by the usually complex nature of the interaction between the sensing probe and the sample. In particular the magnetic state of the tip may not be well defined and may be perturbed by the stray field from the sample under investigation thus providing a non-uniform response. This point is particularly true for soft magnetic tips.

The tip-sample interaction in MFM may be viewed alternatively as: (i) the interaction between the tip magnetisation and sample stray field or (ii) the interaction between the sample magnetisation and tip stray field. Therefore whereas MFM is thought to provide a picture of the sample stray field interacting with the tip magnetisation, the images may be thought of as essentially a convolution between the tip stray field and the sample magnetisation. If the tip stray field is known the reconstruction of the sample magnetisation is then a deconvolution process from the information in the image. Studying the stray field from the tip therefore provides extremely useful information regarding the imaging properties of a tip. Lorentz microscopy, practised on an electron microscope, has demonstrated the possibility of imaging the stray field distribution of MFM tips as the film comprising the tip itself is coated on an AFM tip which is electron opaque. In this thesis results from various MFM tip coatings together with some results from in-situ experiments on the stability of the magnetic state of the tips in external fields are described.

Consequently using the results obtained by Lorentz microscopy, MFM tips have systematically been optimised for MFM investigations of magnetically soft and hard structures on a micron- and sub-micron scale. Not only the influence of MFM tip coating material but also coating thickness was studied; also being compared with the properties of a commercial MFM tip. For this optimisation process Cobalt MFM tip coatings proved to be extremely useful.

Further more a 2D deconvolution algorithm based on inverse Fourier transform is suggested. This procedure can be used for a deconvolution of the imaging system response function and the MFM image, therefore improving the quality of a MFM image. The algorithm was succesfully tested on a MFM linescan of hard disc material. Within the scale of the track width this system offers planar symmetry.