Bimodal magnetic force microscopy with capacitive tip-sample distance control

Abstract

A single-passage, bimodal magnetic force microscopy technique optimized for scanning samples with arbitrary topography is discussed. A double phase-locked loop (PLL) system is used to mechanically excite a high quality factor cantilever under vacuum conditions on its first mode and via an oscillatory tip-sample potential on its second mode. The obtained second mode oscillation amplitude is then used as a proxy for the tip-sample distance, and for the control thereof. With appropriate z-feedback parameters two data sets reflecting the magnetic tip-sample interaction and the sample topography are simultaneously obtained.

Magnetic force microscopy with frequency-modulated capacitive tip–sample distance control

“…where the approximation is valid for small β. Similar to our previous work [9], we use the amplitudes of the second side bands at f 2 ac for the distance control, because their amplitudes are independent of U dc , and therefore of the contact potential difference. Provided that the tip-sample capacitance and its dependence on tip-sample distance are known, β can be calculated as…”

“…However, as already discussed in [9], a tip-sample distance control based on the A 2 -signal would fail if the second mode quality factor Q 2 of the cantilever changed. There are various mechanisms that can affect Q 2 .…”

“…The second MFM operation method recently described by our group [9] overcomes these limitations. Again, the cantilever is excited mechanically on its first resonance to map the magnetic tip-sample interaction via the shift of the first mode resonance frequency.…”

“…Data acquired in this mode facilitates a posteriori quantitative data analysis [10][11][12]. The variation of A 2 is a measure of the sample topography and can for example be used to align images measured in different external magnetic fields [9].…”

Amplitude dependence of image quality in atomically-resolved bimodal atomic force microscopy

“…11 Schwenk and coworkers used bimodal AFM to increase the contrast stemming from magnetic interaction with a ferromagnetic tip. 18,19 Kawai and coworkers explicitly demonstrated the advantage of a higher flexural mode oscillating at smaller amplitudes (amplitudes less than 100 pm) with a standard Si cantilever on a KBr(100) surface in UHV. 20 Moreno and coworkers achieved intramolecular resolution in UHV conditions at low temperature.…”