MFM is a two-pass technique: the magnetic tip first records the topography in resonant mode, then retraces it at a lift height of 10–200 nm and records the phase shift of the cantilever, which follows the magnetic force gradient. Lateral resolution is below 20 nm, depending on the tip. MFM with the Variable Magnetic Field Module shows magnetic domains while the applied field changes — in plane up to 3000 G or out of plane — without heating the sample. Field steps can be applied within a single scan, domain and domain-wall rotation can be followed across a field range, and EFC keeps the image free of electrostatic contributions. Combined with HD-KFM on the same sample, magnetic and electrical domains can be compared directly. How does MFM image magnetic domains? MFM is a two-pass technique: a magnetically coated tip first records the topography, then retraces it at a set lift height (10 to 200 nm) and records the phase shift caused by the magnetic force gradient. Lateral resolution is below 20 nm, depending on the tip. How can a magnetic field be applied during MFM imaging? With the Variable Magnetic Field Module (VMFM): two unbalanced magnetic rods, rotated by a stepper motor, apply a field to the sample while it is imaged. Depending on how the rods are placed, the field is in the plane of the sample (0 to 3000 G) or out of its plane. Can the field be changed during a single scan? Yes. In the results shown here the field was changed during the scan — 300 G then 0 G out of plane, and 250, 125 then 0 G in plane — so each region of one MFM image corresponds to one field value. How are electrostatic artefacts removed from MFM images? With Electric Field Cancelling (EFC), which compensates the electrostatic tip–sample force so that the MFM signal shows magnetic contrast only. Magnetic structures followed from 50 to 470 G show the rotation of domains and domain walls; on a thin magnetic film, the field was changed within a single scan (300 → 0 G out of plane; 250 → 125 → 0 G in plane). MFM resolves the magnetic contrast of patterned permalloy devices from the 20 µm overview down to a 4 µm close view (Figures 5 and 6).
