The basic idea of Piezoresponse Force Microscopy
(PFM) is to effect locally the piezoelectric sample surface by the
electric field and to analyse resulting displacements of the sample
surface[1].
The PFM technique is based on the converse
piezoelectric effect, which is a linear coupling between the electrical
and mechanical properties of a material. Since all ferroelectrics
exhibit piezoelectricity, an electric field applied to a ferroelectric
sample results in changes of its dimensions.
To detect the polarization orientation the AFM tip is used as a top electrode, which is moved over the sample surface. In the Intro1
animation one can see the reaction of out-of-plane and in-plane domains
in the ferroelectric film on the voltage applied to the scanning tip in
Contact Constant Force Mode.
The electric field generated in the sample causes the domains with the
polarization parallel to the field to extend and the domains with
opposite polarization to contract.
If the polarization vector is
perpendicular to the electric field, there is no piezoelectric
deformation along the field direction, but a shear strain appears in
the ferroelectric, leading to displacements of the sample surface
parallel to itself, along the polarization direction.
The AFM
probe tip moving according to the surface displacement causes
cantilever normal or torsion (because of friction) deflections.
Direction of the deflection depends on the mutual orientations of the
electric field and domain polarization. Correspondingly in the case of
the AC electric field (see Intro2 animation) phase
lag between the electric field and cantilever deflections will depend
on the their mutual orientations. In general case by analyzing the
amplitudes and phases of the normal and torsion cantilever vibrations
one can reconstruct the sample domain structure.
References
M. Alexe, A. Gruverman (Eds.). Nanoscale Characterisation of
Ferroelectric Materials. Scanning Probe Microscopy Approach. Springer,
2004.
