Piezoresponse force microscopy of domains and walls in multiferroic HoMnO ₃

We report ambient piezoresponse force microscopy (PFM) studies of the multiferroic hexagonal manganite HoMnO ₃ performed on the cleaved (110) surface of a single-crystal specimen. By changing the sample orientation with respect to the cantilever, we observed an unexpected out-of-plane PFM signal at domain walls, which depends on domain wall orientation, in addition to the expected in-plane PFM signal in domains. Further studies confirmed that the domain wall PFM signal results from an out-of-plane displacement, which can be explained by a simple model of local elastic response with the conservation of unit cell volume at head-on domain walls.