NanoWiki

In MFM, a tapping cantilever equipped with a special coated tip is first scanned over the surface of the sample to obtain topographic information. The tip is then raised just above the sample surface. The surface is scanned while being monitored for the influence of magnetic forces. These influences are measured using the principle of force gradient detection. In the absence of magnetic forces, the cantilever has a resonant frequency. This frequency is shifted by a small amount proportional to vertical gradients in the magnetic forces on the tip. The shifts in resonant frequency tend to be very small, typically in the range 1-40 Hz for cantilevers having a resonant frequency ~75 kHz.

(version july 2007)

The cantilever should be mounted as described in the contact mode manual “multimode-manual-revb.pdf”. The cantilever holder should be suited for tapping mode. For adjustment of the laser, settings of the lockin amplifier etc. look into the tapping mode manual. The tip should have a magnetic coating and will have to be magnetized in the proper direction before use. Tip magnetization is done in the following way: Instead of a sample, a NdFeB magnet disc with the field perpendicular to the surface is mounted on the scanner. Approach the tip in the “contact mode” manner. After touching the surface retract the tip, remove the NdFeB magnet and mount your sample. With tapping mode find the structure on your sample of which you want to measure the magnetic structure. Zoom into that structure.

The next step describes how the system switches from tapping mode to MFM
Switch off the feedback (on hold), change the Z-position on the front of the RHK spm100 box, so that the tip is lifted from the surface. (potmeter 0-10.0 means -130 V to +130 V on the Z-piezo, and 1 V on the Z-piezo is 7.9 nm). Scan again. If the tip is not touching the surface one should obtain a magnetic signal now. If not, you may have to decrease the drive amplitude of the cantilever (switch on feedback, approach, scan, switch off feedback, and lift the tip). With lower drive amplitude on average the tip is closer to the surface, however, the signal to noise ratio also decreases.

There is also a more advanced way of doing MFM. This described in the XPMPro manual (page 69 -71) and is called dynamic lithography. When using this, correct cabling is imperative. Change the cable at the rear of the spm100 box from Z-position to DAC-2A. When you stop the MFM measurement change back the cable!!