Magnetic Force Microscopy (MFM)
fundamentals
MFM is a technique derived from AFM, in
which an etched silicon cantilever/tip
combined with optical deflection detection is used to precisely measure local
forces such as those caused by van der
Waals or Coulomb interaction. MFM uses
cantilevers with very low spring constant K
and with magnetic coatings (typically NiCr
or cobalt), sensitive to the magnetic interaction between tip and sample.
The figure below shows a schematic of
attocube’s cantilever-based attoMFM,
designed particularly for low temperature
and high magnetic field applications. The
attoMFM uses a single-mode, fiber-based
interferometer to detect tip deflections
with noise densities as low as 0.5 pm/Hz1/2.
As with most MFMs, the attoMFM applies an
AC modulation technique to achieve highest detection sensitivity. In AC mode, the
cantilever is mechanically excited at its
natural resonance frequency f0 using an
oscillator swinging perpendicular to the
sample surface. The magnetic interaction
offsets the equilibrium position of the tip.
This effect is hard to detect, and therefore
ignored in most cases. In addition to the
pure DC offset, the natural resonance frequency (as well as amplitude and phase) of
the cantilever is also affected by the magnetic interaction. This frequency shift
Δf = fres - f0 can be detected by classical
lock-in techniques and is the most relevant
physical quantity to measure due to its
direct proportionality to the derivative of
the local force F: ∂Fz /∂z ~ 2 K Δf / f0. The
measurement therefore yields information
about the actual local magnetic stray field:
∂Fz /∂z ~ mtip,z ∂2Hz /∂z2 (where mtip,z is the
magnetization of the tip perpendicular to
the sample surface) with very high spatial
resolution. Using a phase-locked loop
(PLL) technique, resonance frequency
shifts as small as 1 μHz can be detected.
To separate magnetic information from
other influences, two techniques are
commonly used: constant height mode
and constant distance mode. In constant
height mode, the MFM tip is scanned at a
fixed height above the mean sample plane.
In constant distance mode on the other
hand, the distance between tip and sample
is kept precisely constant to compensate
any surface corrugation. Constant height
mode is typically applied on sufficiently
flat samples, after the sample and scan
planes have been aligned parallel to
each other. The MFM tip is positioned at
typically 10 – 100 nm above the sample
surface, and is then scanned across the
surface with scan speeds of up to several
10 μm/s. Any shifts in resonance frequency
or phase are recorded simultaneously. Due
to its ease of use and high scan speeds,
this technique best fits a variety of samples
such as hard discs and superconductors. In
contrast, constant distance mode tracks the
surface topography at a defined distance.
This technique best suits samples with
rough surfaces. However, this mode leads to
considerably longer acquisition times than
constant height mode.
attoMICROSCOPY
Sophisticated Tools for Science
attoMICROSCOPY
PAGE 99