A new frequency modulation (FM) technique has been demonstrated that enhances the sensitivity of attractive mode force microscopy by an order of magnitude or more. This technique operates in a moderate vacuum (<10⁻³ Torr), which increases the quality factor (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 an 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. The FM technique allows the measurement of much weaker force gradients, opening new applications of ac force microscopy. The conventional slope detection method involves driving the cantilever at a fixed frequency slightly off resonance. Force gradients are detected as variations in the amplitude or phase of the cantilever vibration. However, increasing the Q of the cantilever restricts the bandwidth of the system. In vacuum, the Q can be >50,000, offering excellent sensitivity, but the bandwidth may be <1 Hz, which is too slow for most applications. The FM detection system, on the other hand, allows increased sensitivity through higher Q without restricting bandwidth or dynamic range. The FM system uses a high-Q cantilever vibrating on resonance as the frequency-determining component of an oscillator. Changes in the force gradient cause instantaneous changes in the oscillator frequency, which are detected by an FM demodulator. The FM detection system is simpler to calibrate and is disturbed less by drifts than the slope detection system. The FM modulator has a linear output over a fairly large range, and the calibration of the demodulator remains constant. With slope detection, drifts in the system can cause the operating point to move along the slope of the response curve, changing the sensitivity of the system. Thus, quantitative force gradient measurements are easier with the FM system. The FM technique allows the measurement of much weaker force gradients, opening new applications of ac force microscopy.A new frequency modulation (FM) technique has been demonstrated that enhances the sensitivity of attractive mode force microscopy by an order of magnitude or more. This technique operates in a moderate vacuum (<10⁻³ Torr), which increases the quality factor (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 an 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. The FM technique allows the measurement of much weaker force gradients, opening new applications of ac force microscopy. The conventional slope detection method involves driving the cantilever at a fixed frequency slightly off resonance. Force gradients are detected as variations in the amplitude or phase of the cantilever vibration. However, increasing the Q of the cantilever restricts the bandwidth of the system. In vacuum, the Q can be >50,000, offering excellent sensitivity, but the bandwidth may be <1 Hz, which is too slow for most applications. The FM detection system, on the other hand, allows increased sensitivity through higher Q without restricting bandwidth or dynamic range. The FM system uses a high-Q cantilever vibrating on resonance as the frequency-determining component of an oscillator. Changes in the force gradient cause instantaneous changes in the oscillator frequency, which are detected by an FM demodulator. The FM detection system is simpler to calibrate and is disturbed less by drifts than the slope detection system. The FM modulator has a linear output over a fairly large range, and the calibration of the demodulator remains constant. With slope detection, drifts in the system can cause the operating point to move along the slope of the response curve, changing the sensitivity of the system. Thus, quantitative force gradient measurements are easier with the FM system. The FM technique allows the measurement of much weaker force gradients, opening new applications of ac force microscopy.