AC MFM

Magnetic force microscopy (MFM) [1, 2] is an effective tool for magnetic investigations on submicron scale. Image obtained by MFM is the space distribution of some parameter characterizing magnetic probe-sample interaction, i.e. interaction force, amplitude of vibrating magnetic probe etc. The magnetic probe is standard silicon cantilever (or silicon nitride cantilever) coated by magnetic thin film. MFM measurements enable the high resolution investigation of magnetic domain structure, reading and recording information in magnetic media, magnetization reversal processes etc.

In magnetic investigations on submicron scale first of all one must separate the magnetic image from the topography. To solve this problem the magnetic measurements are executed by means of two-pass method. In the first pass the topography is determined Contact or Semicontact mode. In the second pass the cantilever is lifted to a selected height for each scan line (or after topography measurement), and scanned using the stored topography (without the feedback). As a result the tip-sample separation during second pass is kept constant. This tip-sample separation must be large enough to eliminate the Van der Waals’ force. During second pass the short-range Van der Waals’ force vanishes and the cantilever is affected by long-range magnetic force. Both the height-image and the magnetic image are obtained simultaneously with this method.

In the AC MFM during second pass the cantilever resonance oscillations are used to detect the magnetic force data (just as in the non-contact or semiconact modes). In AC MFM the microscope detects the force derivative: the force gradient in the point dipole approximation can be written in the form:

F' = n grad(n F),  F = (m grad) H

n is unit vector normal to the cantilever plane. It is seen that AC MFM signal is proportional to the stray field second derivative.

References

1. Appl. Phys. Lett. 50, 1455 (1987).
2. J. Appl. Phys. 62, 4293 (1987).