Piezoresponse force microscopy of domains and walls in multiferroic HoMnO3

We report ambient piezoresponse force microscopy (PFM) studies of the multiferroic hexagonal manganite HoMnO3 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.