Optimization of thin-film tips for magnetic force microscopy

We report results of a quantitative investigation of MFM sensitivity vs. tip coating thickness. Etched Si tips on cantilevers 225 μm in length were sputter-coated with Co-Cr films of various thicknesses 150 Å<t<1500 Å. 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 in sensitivity (effective moment) up to a critical film thickness near 500 Å beyond which saturation occurs. For 500 Å 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