Magnetic Force Microscopy

In a Magnetic Force Microscope (MFM) a magnetic tip is used to probe the magnetic stray field above the sample surface. The magnetic tip is mounted on a small cantilever which translates the force into a deflection which can be measured.The Microscope can sense the deflection of the cantilever which will result in a force image (static mode) or the resonance frequency change of the cantilever which will result in a force gradient image.The sample is scanned under the tip which results in a mapping of the magnetic forces or force gradients above the surface.

Principle of MFM

The fact that no sample preparation is necessary and that a lateral resolution below 50 nm can be reached make it a powerful tool for investigation of submicron magnetisation patterns. Since it is possible to apply external magnetic fields during the measurement, the field dependence of domain structures and magnetic reversal processes can be observed. Methods to separate topography and magnetic features allow pure magnetic images to be achieved. Topographic and magnetic details from the same scan can be related to each other. We use an MFM for observation of intrinsic magnetic domains and structures of written bits on different recording media, two examples are given here:

Intrinsic domain studies:
(See picture of domain pattern of a Co-Ni/Pt Multilayer). The knowledge of the relation between preparation process parameters and magnetic parameters like domain size and shape can help to tailor better magneto-optic multilayer materials.
Observations of perpendicularly recorded bits on Co-Cr-Ta material:
For the judgement of the recording characteristics of magnetic media usually readback experiments are carried out.

Image: MFM Image (3.5 x 3.5 µm) of a domain pattern in a Co-Ni/Pt Multilayer

Using microscopic magnetic observation methods can give a much deeper insight into the magnetic processes during bit writing. Special attention is taken towards a correlation between the writing/reading characteristics of the media, measured in test stages and the observed magnetic microstructure. Our MFM is capable of visualising magnetic processes which limit the bit density in media.

The picture below shows the edge of a 30 µm wide bit track of 300 kfrpi (89nm bit length) on a Co-Cr-Ta hard disk for perpendicular magnetic recording. The bits are visible in the lower part of the image as vertical stripes. The side border of the track lies in the middle of the image, the upper part shows the domain structure next to the bit track. The readback noise for these bits will be determined by the two following effects:

Irregularities of the bit transitions :
In such high writing densities the bits approach the minimum stable domain size of the medium. The bits are broken up by reversed domains which are formed in the magnetic relaxation process immediately after the bits are written. Because of these irregularities thebits will differ from each other, which is reflected in a higher noise.
Irregularities of the track border:

Image: Border of a 30 µm wide bit track of 300 kfrpi (89 nm bit length)

At the track border the domains of the natural domain structure influence the bit formation. Also this will cause irregularities in the track width, which reflect in higher noise.

Electron beam fabrication of thin film MFM tips

One of the most crucial parts for the image formation process in a MFM is the force sensor formed by an tiny magnetic tip mounted on a flexible beam (cantilever). Artefacts because of the usually unknown magnetic state of the tip, its unknown behaviour in the sample's stray field and its influence on the sample magnetisation may lead to image perturbations and misinterpretations.

Image: A carbon contamination needle grown on top of the pyramidal tip of a commercial Si₃N₄ cantilever. The needle is covered afterwards from one side with a thermally evaporated 15 nm thick Co₈₀Ni₂₀ film.

In order to get quantitative information out of MFM measurements, tips with a well defined magnetic state are required. We use a new preparation technique for magnetic force microscope tips based on magnetic thin film evaporation on tiny, high aspect ratio contamination needles grown by electron beam induced deposition of carbon in a standard scanning electron microscope. The needles can be grown reproducibly on any type of cantilever whereby their orientation and position is easily controllable during the preparation process. A typical example is shown in the last picture. A carbon contamination needle is grown on top of the pyramidal tip of a commercial Si₃N₄ AFM cantilever. The needle is covered afterwards from one side with a thermally evaporated 15 nm thick Co₈₀Ni₂₀ film to make it sensitive for MFM measurements. The orientation of the needle is chosen to be approximately 10º with respect to the cantilever plane.

By varying the growth parameters of the needles and by using appropriate coating materials it is possible to tailor the magnetic properties of the tips reproducibly in a wide range.

Various MFM observations on different materials used in magnetic storage technology reveal a single pole behaviour of the tip. From the switching behaviour of the tips observed occasionally during scanning we conclude that the active magnetic volume of the tip is formed by a single domain element assuming a preferred magnetisation direction along the tip axis due to the shape anisotropy of the high aspect ratio thin film element. Observations of the emanating stray field as well as of the magnetisation within the needles using different imaging modes in the transmission electron microscope confirmed the conclusions drawn from the MFM experiments.

Last update 12 Januari 1996