Linear dichroic soft X-ray microscopy of ferroelectric stripe domains in epitaxial K$_\mathbf{0.6}$Na$_\mathbf{0.4}$NbO$_\mathbf{3}$
Functional properties of ferroelectric thin films are governed by domains that can be engineered by epitaxial strain. Soft X-ray microscopy can image domain structures with elemental and electronic sensitivity, but hitherto its application to strain-stabilized domains has been hindered by the absorption of soft X-rays in epitaxial substrates. Here, it is demonstrated how this limitation can be overcome by locally back-thinning the (110) TbScO$_3$ substrate of epitaxial K$_{0.6}$Na$_{0.4}$NbO$_3$ ferroelectric thin films to achieve soft X-ray transparency at the O K-edge around 530 eV. Strain-induced ferroelectric stripe domains with periods down to 44 nm were resolved by scanning transmission X-ray microscopy and coherent diffractive imaging by exploiting the X-ray linear dichroism of hybridized O 2p-Nb 4d states, providing sensitivity to in-plane polarization components under normal incidence. The results establish soft X-ray microscopy for nanoscale imaging of epitaxial ferroelectric domains structures and open perspectives for time-resolved studies thereof.