We report results of a quantitative investigation of MFM sensitivity vs. tip coating thickness. Etched Si tips on cantilevers 225 mu m in length were sputter-coated with Co-Cr films of various thicknesses 150 Angstrom < t <1500 Angstrom. Tip response was measured by scanning a specially-written hard disk with tracks of selected bit reversal densities. Tracking the shift in cantilever resonant frequency caused by the disk stray fields gave a measure of relative sensitivity, and hence tip moment. The main finding is a roughly linear increase insensitivity (effective moment) up to a critical film thickness near 500 Angstrom beyond which saturation occurs. For 500 Angstrom coatings, sensitivity varies about 15% from tip-to-tip, indicating the degree of uniformity that can be expected from batch fabrication, We estimate the effective tip moment to be about 10(-12) emu, implying that only a small portion of the tip's magnetic volume plays a role.
