Magnetic force microscopy: High quality-factor two-pass mode

Magnetic force microscopy (MFM) is a well-established technique in scanning probe microscopy that allows for the imaging of magnetic samples with a spatial resolution of tens of nm and stray fields down to the mT range. The spatial resolution and field sensitivity can be significantly improved by measuring in vacuum conditions. This improvement originates from the higher quality-factor (Q-factor) of the cantilever's oscillation in vacuum compared to ambient conditions. However, while high Q-factors are desirable as they directly enhance the magnetic measurement signal, they pose a challenge when performing standard MFM two-pass (lift) mode measurements. At high Q-factors, amplitude-based topography measurements become impossible, and the MFM phase response behaves non-linearly. Here, we present a modified two-pass mode implementation in a vacuum atomic force microscope that addresses these issues. By controlling the Q-factor in the first pass and using a phase-locked loop technique in the second pass, high Q-factor measurements in vacuum are enabled. Measuring the cantilever's frequency shift instead of the phase shift eliminates the issue of emerging nonlinearities. The improvements in MFM signal-to-noise ratio are demonstrated using a nano-patterned magnetic sample. The elimination of non-linear responses is highlighted through measurements performed on a well-characterized multilayer reference sample. Finally, we discuss a technique that avoids topography-induced artifacts by following the average sample slope. The newly developed, sensitive, and distortion-free high quality-factor two-pass mode has the potential to be widely implemented in commercial setups, facilitating high-resolution MFM measurements and advancing studies of modern magnetic materials.