The AFM is a natural tool for exploring liquid-solid interfaces, but oscillating probe methods are limited by the viscous action of the liquid on the probe. If the probe is driven acoustically, high driving amplitudes are required to obtain a response from the probe, and this can give rise to spurious responses. The problem is solved if the probe is driven directly, using a magnetic film on the tip and a solenoid to provide a driving field. This method, Magnetic A/C mode (MacMode) has provided the highest resolution in both images of biomolecules and in force spectroscopy at the liquid-solid interface.
The interaction force is reduced because the microscope can be operated at low amplitudes of oscillation (0.5 to 5nm) with small changes (0.1 to 0.5nm) in amplitude used for sensing. This often results in greatly improved resolution, presumably because small aperities on the tip are not damaged.