  Ferroelectric materials have remained one of the foci of condensed matter
physics and materials science for over 50 years. In the last 20 years, the
development of voltage-modulated scanning probe microscopy techniques,
exemplified by Piezoresponse force microscopy (PFM) and associated time and
voltage spectroscopies, opened a pathway to explore these materials on a
single-digit nanometer level. Consequently, domain structures, walls and
polarization dynamics can now be imaged in real space. More generally, PFM has
allowed studying electromechanical coupling in a broad variety of materials
ranging from ionics to biological systems. It can also be anticipated that the
recent Nobel prize in molecular electromechanical machines will result in rapid
growth in interest in PFM as a method to probe their behavior on single device
and device assembly levels. However, the broad introduction of PFM also
resulted in a growing number of reports on nearly ubiquitous presence of
ferroelectric-like phenomena including remnant polar states and
electromechanical hysteresis loops in materials which are non-ferroelectric in
the bulk, or in cases where size effects are expected to suppress
ferroelectricity. While in certain cases plausible physical mechanisms can be
suggested, there is remarkable similarity in observed behaviors, irrespective
of the materials system. In this review, we summarize the basic principles of
PFM, briefly discuss the features of ferroelectric surfaces salient to PFM
imaging and spectroscopy, and summarize existing reports on ferroelectric like
responses in non-classical ferroelectric materials. We further discuss possible
mechanisms behind observed behaviors, and possible experimental strategies for
their identification.
