Control of growth orientation in as-deposited epitaxial iron-rich nickel ferrite spinel

The authors here report epitaxial growth of the ferrimagnet Ni_xFe_3–xO₄ (NFO) by atomic layer deposition at low temperatures. Films grow epitaxially at a reactor temperature of 250 °C and require no further postannealing treatment. (100)-, (110)-, and (111)-oriented thin films of NFO are attainable by using MgO (100)/LaAlO₃ (100), LaAlO₃ (110), and Al₂O₃ (001) substrates, respectively. This means that the direction of the easy axis of magnetization relative to the film surface can be selected by the choice of substrate. The authors carefully study nucleation onset and growth dynamics by varying the number of cycles used to deposit the films. This shows that the initial growth is mostly inhibited by the Fe-species, creating a small composition gradient for the first few nanometers of the film. The growth rate varies as a result of the choice of substrate, explained by the direction of the close packed layers in the film. The authors believe that the ability to deposit epitaxial ferrimagnetic films with angstrom thickness control at low temperatures will make a major impact on the possibility to grow multiferroic stacks and on low temperature monolithic device integration where a spontaneously magnetized sample is required.