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Piezoresponse Force Microscopy (PFM)
fundamentals
Multiferroic materials have attracted significant attention recently due
to their possible applications as magnetoelectronic devices, such as elec-
trically tuneable spin valves or tunnel magnetoresistance sensors (TMR).
Especially thin films of multiferroics in composite devices are of interest as
electrically tuneable tunnel barriers and in potential storage applications.
Due to their natural domain sizes, it is important to understand their behav-
ior at the nanoscale.
In the family of multiferroic materials, magnetoelectric materials exhibiting
ferroelectricity and ferromagnetism simultaneously are the most interest-
ing ones. These materials can be characterized using Piezoresponse Force
Microscopy (PFM), a variant of contact mode AFM. Typically, an AC voltage is
applied to the tip and the resulting nanoscale deformation is measured. This
deformation stems from the strain generated by the application of an exter-
nal electric field (inverse piezoelectric effect). When analyzing the magnetic
field and temperature dependence of the sample, further material properties
and phase transitions can be recorded and analyzed.
For performing a PFM measurement, a conductive AFM tip is brought in con-
tact with the sample surface. Once in contact, an AC voltage is applied to the
tip resulting in a local electric field. Due to the inverse piezoelectric effect
this results in a small local deformation of the sample, which is then picked
up by the cantilever. A lock-in amplifier analyzes the signal and returns am-
plitude and phase information as a function of the sample position.
It is important to mention that due to the fiber-based interferometer for de-
flection detection in the attoAFM I, out-of-plane polarization of piezoelec-
tric domains can be directly visualized with picometer precision. In-plane
domains are only accessible indirectly via crosstalk effects. One advantage
of the interferometer of the AFM is that it yields directly an absolute length
calibration of the PFM response amplitudes, and hence enables quantitative
analysis.
Wh ile the tip has a natural resonance with a rather high Q, the coupled reso-
nance of the tip-sample system lies at much higher frequencies (~factor 5),
with considerably lower Q factor. The resonance also depends on the local
structure of the sample, hence making it difficult to track the resonance
during scanning. Off-resonance techniques are therefore widely used and
accepted in the community.
Optical fiber
U AC
Dither piezo
Sample
attoMICROSCOPY
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