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Introduction to MFM

Plane Subtraction Mode

To separate topographic effects from the MFM-signal the tip is scanned at a constant tip-sample distance relatively far away from the surface where the magnetostatic forces dominate. However, in general there is a small tilt between tip and sample, which has to be compensated to achieve a constant height scan. For sufficiently flat surfaces this is accomplished by adding a proportion of the xy-scan voltages to the z-scan voltage. The correct voltages can be obtained during a topographic image of the sample.

Contact Potential Difference

To separate the long-range electrostatic force, from the MFM-Signal the contact potential difference between tip and sample has to be compensated. The physical origin of the contact potential are their different work functions between tip and sample.

For a metallic tip-sample system the electrostatic part of the total interaction is always attractive. Since the charges follow the surface contour, electrostatic forces are related to the sample topography. This can be seen in the upper quarter of the image above, when the grain structure at the surface is clearly visible. At the bias voltage with the smallest tip-sample interaction the contact potential is compensated and the topographic-like contrast vanishes, as in the lower quarter of the image.

Tip Preparation

To produce magnetic sensitive tips, the native oxide layer of commercially available silicon tips is removed by argon ion sputtering. Subsequently, iron is evaporated on that side of the tip pyramid, which faces the cantilever substrate. This results in a thin film of prolonged triangular shape with a magnetization along its symmetry axis.

Such tips are nearly exclusively sensitive to the out-of-plane component of the sample stray field and exhibit a negligible in-plane component. The appropriate thickness of the iron film depends on the magnetic characteristics of the sample and should be a compromise between signal strength and minimized disturbance of the sample magnetization.

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