Magnetic force microscope

A magnetic force microscope (MFM) is a scanning probe microscope (SPM) that can map the spatial distribution of magnetism by measuring the magnetic interaction between a sample and a tip. As magnetic devices have become smaller and smaller, an evaluation technique with nanoscale spatial resolution has become necessary. To meet this need, the MFM was developed.

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The principle of MFM measurement is based on noncontact atomic force microscopy (NC-AFM). Unlike NC-AFM, magnetic materials are used for the sample and tip, so that not only the atomic force but also the magnetic interaction are detected. Many kinds of magnetic interactions are measured by MFM, including magnetic dipolar interaction.

In MFM measurements, the magnetic force between the sample and tip is given by:

F = (m .∇)H

m: magnetic moment of the tip

H: magnetic stray field from the sample

Because the magnetic stray field from the sample will affect the magnetized state and vice versa, in most cases it is difficult to obtain quantitative information from the MFM measurement. To interpret the information quantitatively, the configuration of the tip must be known. With this measurement, a typical resolution of 30 nm can be achieved (Abelmann, 1998). The best resolution obtained so far is 18 nm (SwissProbe, August 2005).

A potential method of increasing the resolution would involve using an electromagnet on the tip instead of a permanent magnet. By only enabling the magnetic tip when placed over the pixel being sampled could increase the resolution.

References

• L. Abelmann, S. Porthun, M. A. M. Haast, J. C. Lodder, A. Moser, M. E. Best, P. J. A. Schendel, B. Stiefel, H. J. Hug, G. P. Heydon, A. Farley, S. R. Hoon, T. Pfaffelhuber, R. Proksch, K. Babcock (1998). "Comparing the resolution ofƏÊ magnetic force microscopes using the CAMST reference samples". J Magn Magn Mater 190: 135–147.
• SwissProbe. Quantum leap in hard disk technology. Retrieved on August 1, 2005.