Atomic Force & Confocal Microscopy (AFM/CFM)
fundamentals
After decades of evolution in magnetic
imaging, combining the sensitivity needed
to detect single electron or nuclear spins
with a spatial resolution of a few nanometers may soon get within reach of current
state-of-the-art instrumentation: optically detected magnetic resonance (ODMR)
is commonly considered to be the most
promising candidate to achieve this goal
by using the unique properties of single
nitrogen-vacancy (NV) centers in diamond.
While there is huge scientific activity to reliably prepare appropriate nano-diamonds
with tailored NV center characteristics and
to attach them to atomic force microscope
tips, attocube as a commercial supplier
of scanning probe microscopes is complementing these efforts by providing an
ideal platform for ODMR: the attoAFM/CFM
combines a cantilever based atomic force
microscope (AFM) with a high-numerical
aperture confocal microscope (CFM) (see
figure).
compact that it fits in between the sample
and an objective with high numerical aperture and low working distance. The tip
is typically operated in non-contact mode
using a phase-locked loop (PLL) to excite
the probe at resonance and track any frequency shifts due to tip-sample interactions. The PLL keeps this shift at a constant
value while scanning over the surface and
hence keeps the force on the tip constant.
Simultaneously to the information provided by the AFM probe, the CFM reveals
complementary optical information of the
sample surface. To maintain a constant
distance between the low temperature
compatible lens and the sample, upon
force changes, only the AFM tip is adjusted
in height by a dedicated scanner.
Local magnetic fields are subsequently
measured via the Zeeman shifts of the NV
defect spin sublevels which are directly
proportional to the local magnetic fields
encountered by the tip. This condition can
be detected by a decrease in photoluminescence intensity of the NV center under
resonant microwave excitation, referred to
as ODMR. Moreover, using a lock-in technique allows tracking of the resonance shift
while rastering the sample, thus enabling to
record a local iso-magnetic field map with
nanometer resolution.
The attoAFM/CFM features an Akiyama
probe to investigate any tip-sample interaction forces on the nanometer scale.
These probes combine the convenience of
conventional AFM cantilevers with electrical detection of tip-sample interactions
via a tuning fork, and thus eliminate the
need for a laser-based AFM detection
mechanism. As a result, the AFM tip is so
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