MAGNETIC FORCE MICRSCOPY - MFM
fundamentals

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Historically, the Magnetic Force Microscope (MFM) has been derived from the Atomic Force Microscope (AFM) one year after its invention in 1986. Unlike AFM, MFM uses a magnetic tip to measure magnetic tip-sample interaction forces. Such MFM probes are readily prepared by evaporating a thin ferromagnetic layer on top of a formerly unmagnetic AFM tip. Nowadays, also anti-ferromagnetic probes are being used in order to reduce the magnetic stray field of the probe.

Since magnetic interaction forces are long-range in nature, they can be separated from topographical effects by scanning at a certain tip-sample separation. Typically, distances ranging from 10-100 nm are used meaning that MFM is always operated in non-contact.
There are two distinct modes of operation for an MFM: In constant height mode, the tip is scanned across the sample at a certain elevation with the z-feedback switched off. In this mode, the scanning plane and the surface plane need to be aligned parallel using an electronic tilt correction beforehand. During the scan, the MFM is typically (but not necessarily) operated in amplitude mode, i.e. the cantilever is excited with constant frequency f0 and amplitude a0. The phase-shift measured between excitation source and cantilever then reflects the magnetic field gradient. Constant height mode is restricted to cases where the roughness of the sample surface is small compared to the tip-sample separation. For nonflat surfaces, or for cases where the tip needs to be scanned relatively close to the surface, the dual-pass mode is superior to the constant height mode.
In dual-pass mode, the tip is first scanned over the surface in close proximity and then retracted by a predefined amount. In a second scan, the tip follows the recorded surface topography at constant separation and the phase/frequency shift due to magnetic interaction forces is recorded. To avoid problems associated with drift, dual-pass mode is executed in a line-by-line fashion.

Apart from AM mode, both constant height and dual-pass modes can be operated in combination with a phase-locked loop (PLL). In this case, the cantilever is always excited at resonance, i.e. 90° phase shifted with respect to its detected phase at anyone time. Due to this fixed-phase condition, a frequency shift is then observed (and recorded) whenever the tip is scanned over the surface. This technique is most frequently used at high vacuum conditions where the Q-factor of the MFM cantilever is high, or whenever tuning fork sensors are used.

With the MFM, a lateral resolution below 50 nm is routinely observed. Under right conditions, however, a resolution of down to 11 nm has already been demonstrated (attocube application labs 2009. MFM on NiFe pads in dual-pass mode with 20 nm tip-sample separation).

attocube systems MFMs

All attocube microscope systems are compatible with cryogenic and vacuum environments as well as high magnetic fields. The MFM is also suited to be used in combination with a He3 insert allowing measurement temperatures down to 300 mK.

attoMFM I:
The attoMFM I is designed particularly for the use at extreme environmental conditions such as ultra low temperature, high magnetic fields, and high vacuum. To perform low temperature microscopy, the attoMFM I is cooled by a controlled exchange gas atmosphere in a liquid Helium bath cryostat. Alternatively, the MFM can be operated under vacuum conditions. The attoMFM I is compatible with commercially available cantilevers.