Frequency modulation detection using high-<i>Q</i> cantilevers for enhanced force microscope sensitivity

TLDR

In the FM technique, the cantilever acts as the frequency‑determining element of an oscillator, and force gradients modulate its frequency, which is demodulated by an FM detector. The authors aim to extend the range of problems accessible by force microscopy through the increased sensitivity provided by this technique. The method achieves higher sensitivity by operating the cantilever in a moderate vacuum (<10⁻³ Torr), thereby increasing its quality factor. The new FM technique enhances attractive mode force microscopy sensitivity by more than an order of magnitude, and experimental comparisons in vacuum (Q∼50 000) versus slope detection in air (Q∼100) show over tenfold improvement for a fixed bandwidth, confirmed by images of magnetic transitions on a thin‑film CoPtCr disk.

Abstract

A new frequency modulation (FM) technique has been demonstrated which enhances the sensitivity of attractive mode force microscopy by an order of magnitude or more. Increased sensitivity is made possible by operating in a moderate vacuum (&amp;lt;10−3 Torr), which increases the Q of the vibrating cantilever. In the FM technique, the cantilever serves as the frequency determining element of an oscillator. Force gradients acting on the cantilever cause instantaneous frequency modulation of the oscillator output, which is demodulated with a FM detector. Unlike conventional ‘‘slope detection,’’ the FM technique offers increased sensitivity through increased Q without restricting system bandwidth. Experimental comparisons of FM detection in vacuum (Q∼50 000) versus slope detection in air (Q∼100) demonstrated an improvement of more than 10 times in sensitivity for a fixed bandwidth. This improvement is evident in images of magnetic transitions on a thin-film CoPtCr magnetic disk. In the future, the increased sensitivity offered by this technique should extend the range of problems accessible by force microscopy.