1.3 Magnetic Force Microscopy

Magnetic force microscopy (MFM) images the spatial variation of magnetic forces on a sample surface. For MFM, the tip is coated with a ferromagnetic thin film. The system operates in non-contact mode, detecting changes in the resonant frequency of the cantilever induced by the magnetic field's dependence on tip-to-sample separation (See Figure 1-7). MFM can be used to image naturally occurring and deliberately written domain structures in magnetic materials.

Figure 1-7. MFM maps the magnetic domains of the sample surface.

An image taken with a magnetic tip contains information about both the topography and the magnetic properties of a surface. Which effect dominates depends upon the distance of the tip from the surface, because the interatomic magnetic force persists for greater tip-to-sample separations than the van der Waals force. If the tip is close to the surface, in the region where standard non-contact AFM is operated, the image will be predominantly topographic. As you increase the separation between the tip and the sample, magnetic effects become apparent. Collecting a series of images at different tip heights is one way to separate magnetic from topographic effects.

An image of a hard disk acquired in MFM mode is shown in Figure 1-8.

Figure 1-8. MFM image showing the bits of a hard disk.
Field of view 30µm