Bi-Stability and Orientation Change of a Thin α-FeO Layer on a ε-FeO (004) Surface.

This study reports the key ingredients that influence the orientation and stability of a α-FeO layer that grows on a metastable ε-FeO during pulsed laser deposition. Depending on the substrate temperature, two different α-FeO orientations arise on the ε-FeO (004) surface. At 800 °C, (2-10)-oriented α-FeO is stabilized, whereas at 700 °C, (006) orientation occurs. The (2-10)-oriented α-FeO layer possesses an interface with densely packed Fe ions with presumably considerable number of oxygen vacancies. On the other hand, the (006)-oriented α-FeO layer is stabilized, as in the case of the YSZ (100) substrate, due to the domain pattern with an in-plane rhombic shape, which is known to become an effective nucleation site. Growth with the unexpected (2-10) orientation can be understood based on a model that takes into account the surface energy as the dominant factor, which mainly stems from the presence of dangling bonds on the surface and the atomic vibration of the surface atoms. As the surface is one of the critical elements related to the specific functionality of a material, the present study will offer valuable insights into the designs of functional devices with novel surface properties.