Abstract
In a step towards routinely achieving 10 nm spatial resolution with magnetic force microscopy, we have developed a robust method for active tip–sample distance control based on frequency modulation of the cantilever oscillation. It allows us to keep a well-defined tip–sample distance of the order of 10 nm within better than nm precision throughout the measurement even in the presence of energy dissipative processes, and is adequate for single-passage non-contact operation in vacuum. The cantilever is excited mechanically in a phase-locked loop to oscillate at constant amplitude on its first flexural resonance mode. This frequency is modulated by an electrostatic force gradient generated by tip–sample bias oscillating from a few hundred Hz up to a few kHz. The sum of the side bands’ amplitudes is a proxy for the tip–sample distance and can be used for tip–sample distance control. This method can also be extended to other scanning probe microscopy techniques.
Highlights
A magnetic force microscope (MFM) is a scanning probe tool that is well suited for imaging stray magnetic fields emanating from a sample surface with high spatial resolution [1] and in applied fields
In order to test the performance of frequency-modulated capacitive distance control method discussed in section 2, we used a low temperature magnetic force microscope operated in UHV [19]
Provided that the A-feedback is set to keep the fundamental oscillation mode amplitude constant, we show that the method is effective when the quality factor of the cantilever unexpectedly changes, so that the distance control is unaffected by dissipative processes
Summary
A magnetic force microscope (MFM) is a scanning probe tool that is well suited for imaging stray magnetic fields emanating from a sample surface with high spatial resolution [1] and in applied fields. Its performance is conditioned on various physical constraints that can be met in most cases with existing techniques. Due to the fact that the high spatial frequency components of the stray field decay rapidly with increasing distance from their source, the MFM tip preferably scans at small tip–sample distances [3,4,5,6]. Controlling this small distance, is challenging because magnetic and non-magnetic forces act on the tip simultaneously [7]. A serious drawback of this technique is being incompatible with operation under vacuum conditions
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