Play of the facets

The atomic structure of some crystals, such as quartz, can be recognized with the naked eye by looking at their facets. In others, these are difficult to distinguish, even by x-ray diffraction.  What happens when a second material is grown, atom by atom, on only slightly different facets of a single crystal? Are the physical properties of the deposited material changed?

Researchers at the MPI-FKF pursued these questions in their study "Imprinted atomic displacements drive spin-orbital order in a vanadate", which has now been published in Nature Physics. They grew a so-called quantum material, YVO₃, which shows different magnetic ordering patterns in the bulk as a function of temperature, as a thin film on two different facets of a YAlO₃ substrate. The investigation using light scattering has shown that the magnetic patterns are actually different depending on the facet and that this is due to the subtle difference in the displacements of the Y and O atoms. This fundamental effect can be used specifically for the stabilisation of desired functional phases, for example to create novel spintronic or solar cell materials.

The flexible and comparatively simple perovskite structure of transition-metal oxides allows the combination of different compounds with ABO₃ composition in atomically sharp epitaxial heterostructures. Much of the research in this area focuses on the targeted manipulation of the diverse quantum phases through electronic and magnetic reconstructions at interfaces and thus on the realisation of new functionalities. Due to the strong coupling of the electronic degrees of freedom to the lattice, structural changes are equally influential. Small shifts in the oxygen positions can decisively modify the macroscopic properties, for example by changing magnetic exchange paths. The structural sensitivity has been demonstrated for various ABO₃ compounds by the study of their bulk phase diagrams. Slight deviations from the ideal cubic perovskite structure, caused by heteroepitaxy in thin films, are much more difficult to detect, but can effectively alter their various physical properties. While previous studies have shown that the choice of different substrate facets as a template for the growth of perovskite thin films leads to different physical properties, it was not possible to clearly separate effects such as polar discontinuity and changes in biaxial strain from the effect of subtle atomic displacements of A and O ions.

The present study shows that the direction of such displacements can be adjusted by growing on facets that are indistinguishable in the cubic structure. The difference in the direction of displacements induced in YVO₃ films grown on orthorhombic (110) and (001) facets of YAlO₃ leads to the stabilisation of different spin-orbitally ordered phases. The two facets were chosen such that other well-known control parameters, including lattice and polarity mismatch with the overlayer, remain nearly unchanged.

Using optical spectroscopy methods, the study reveals signatures of staggered orbital and magnetic order, and demonstrate distinct spin-orbital ordering patterns on different facets. The observed changes can be understood by the influence of specific octahedral rotation and cation displacement patterns, which are imprinted by the substrate facet, on the covalency of the bonds and the superexchange interactions in YVO₃. Substrate-induced templating of lattice distortion patterns constitutes a pathway for materials design beyond established strategies.

This work has been highlighted by the Nature Physics in their News & Views.