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Magnetic Force Microscopy (MFM) of a Magnetic Hard Disk

Kong-Gay Loh, Renee Jobe and Briggs Christie


INTRODUCTION

Previously, among the various existing methods of magnetic imaging, high resolution was only achievable using electron microscope based techniques. Magnetic imaging using the electron microscope however, often required that the sampe be thinned or, in the case of recording media, have the protective coating removed. Additionally, electron microscope based techniques are capable of only two-dimensional images.

The Magnetic Force Microscope (MFM) is a variation of the Atomic Force Microscope (AFM), capable of providing high resolution, 3-dimensional images of magnetic fields (1). The mode of operation is essentially non-contact imaging with a magnetic cantilever. As with normal non-contact imaging with the AFM, the technique is non-destructive and the samples can often be imaged in their original state.

[MFM of a Hard Disk] [MFM of a Hard Disk] click images for closer look (63k)

Simultaneous Topography Images (AFM left) and Magnetic Force Mapping (MFM right) on a 20Ám x 20Ám area using TopoMetrix' One Pass Imaging™


BASIS CONCEPTS

The AFM operated in the non-contact mode with a magnetic cantilever detects a force gradient (F') containing information from both the surface structure and surface magnetization.

F' = F'surface+F'magnetic

Signals from surface topography dominate at close distances to the surface while, at greater distances from the surface (typically beyond 1000┼), the magnetic signal dominates. Consequently, depending on the distance between the surface and the tip, normal MFM images may contain a combination of topography and magnetic signals. Though it is useful to topographically locate where the magnetic domains are, sometimes ti is more advantageous to completely separate the two signals from each other. This can be achieved by using TopoMetrix' One Pass Imaging™ Mode (Figure 1).

At each location (or data point) in the image, the cantilever is first brought down close to the surface where the magnetic force is negligible. Here the topography of the surface is measured. The cantilever is then raised to a preset height above the previous level for magnetic force detection. Depending on how high above the surface the first level is, one can either completely separate out the magnetic signals from topography or allow for some coupling to help towards locating the magnetic regions topographyically.

CONCLUSION

Magnetic Force Microscopy brings the many advantages of the AFM into the magnetic industry. As a magnetic imaging tool it is the only technique capable of mapping the magnetic stray field behavior within 300nm of the surface. In addition to its high resolution, as with other AFM techniques, it is non-destructive and requires minimal sample preparation. TopoMetrix' proprietary One Pass Imaging™ achieves the ability to directly correlate MFM signal to its surface topography without the drift separation caused by imaging on multiple passes.

REFERENCES

(1) - Y. Martin and H.K. Wickramasinghe, Appl. Phys. Lett. 50, 1455 (1987) (2) - H.J. Mamin, D. Rugar, J.E. Stern, R.E. Fontana, Jr., and P. Kasiraj, Appl. Phys. Lett. 55, 318 (1989)


Images collected on the [Explorer] Explorer™ - Tip Scanning SPM
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