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Introduction to MFM

Scanning Probe Microscopy

Scanning probe microscopy (SPM) refers to several techniques that use the interaction of a microscopic probe (usually on a sharp tip) with the surface of a sample to measure characteristics of the sample at localized points. To measure a certain property locally, the probe is scanned relative to the sample surface in a two-dimensional pattern (like an electron beam scans a television screen) by applying voltages to the x- and y- electrodes of a piezoelectric scan unit. It is either possible to directly record the signal due to the probe-sample interaction at each position, or to use a feedback electronic. In the latter case an appropriate voltage is applied to the z-electrode in a way, that the probe-sample interaction is kept constant. Such piezoelectric scan units can achieve picometer resolution in all three dimensions.

The probe-sample interaction may be the tunnel current, all type of long- and short-range forces, temperature, etc.

Scanning Force Microscopy

The probe used in scanning force microscopy (SFM) is a cantilever with a sharp tip at its free end. Short-range interatomic forces and friction forces can be detected as well as long-range electrostatic or magnetostatic forces. The sample can be either conducting or insulating. Two principal modes of operation are used, the static and the dynamic mode.

In the static mode the cantilever is scanned relative to the sample while in contact or not in contact (e.g. several 10 nm above the surface). Attractive (repulsive) tip-sample forces bend the cantilever towards (away) from the sample. According to Hooke's law the magnitude of the tip-sample force is proportional to the cantilever deflection.

In the dynamic mode the cantilever is oscillated at or near by its resonance frequency. Depending on the way of excitation the cantilever is externally driven or self oscillating. Any tip-sample interaction influences amplitude, phase or frequency of the cantilever. During oscillation the tip may (tapping) or may not (non-contact) touch the surface at the lower turnaround point. It is not so straightforward as in the static mode to quantify the magnitude of the tip-sample interaction. In the case of long-range interactions the frequency shift is proportional to the force gradient.

Magnetic Force Microscopy

In magnetic force microscopy (MFM) a ferromagnetic tip is scanned at a certain constant height of several ten nanometers above the surface. Thereby, the magnetostatic interaction between the evanescent stray field produced by the domain structure of the sample and the ferromagnetic tip is detected. MFM can be operated in the static and dynamic mode. Typically, lateral resolution is around 50 nm and the force (force gradient) sensitivity is better 10 pN (10 µN/m).

Read more about MFM imaging.

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