MFM image of a constant frequency test pattern on a hard disk. The Magnetic Force Microscope senses the magnetic field just above the disk surface. 15 micron scan. Bit position and amplitude measurements from 1 MFM image. In a constant frequency test pattern, the bits ideally have equal amplitude and spacing.
Variation is a measure of media noise and other micromagnetic characteristics.
These results are tools used by disk engineers to set the maximum bit density.