Atomically flat epitaxial Cr₂O₃ films on atomically flat Pd electrodes for magnetoelectric and topological applications.

Cr₂O₃ is one of the best magnetoelectric antiferromagnetic materials and is also an excellent substrate for many hexagonal topological materials. To fully utilize these functionalities, it is essential to have epitaxial pin-hole-free Cr₂O₃ films on conducting electrodes. So far, V₂O₃ has been identified as the best conducting bottom electrode for epitaxial Cr₂O₃ due to their matching crystal structures. However, V₂O₃ is a poor conductor at room temperature and becomes insulating at low temperatures, so its utility is very limited.

Here, we demonstrate atomically flat epitaxial Cr₂O₃ films with excellent electronic properties on highly conducting Pd films grown on Al₂O₃ (0001) substrates. It turns out that achieving epitaxial and atomically flat Pd film is critical to grow the high quality Cr₂O₃ films. Along the way, we have found that growth of epitaxial Pd film on sapphire substrate is highly challenging despite their excellent lattice match because Pd film has a strong tendency to ball up on sapphire. However, with a new growth recipe, we were able to convert the morphology of epitaxial Pd film from discontinuous, microscopic islands to contiguous, atomically flat regime. The availability of epitaxial Cr₂O₃ films on metallic electrodes will open many new opportunities for both magnetoelectric and topological applications.