Bulk pyrochlore vanadates are ferromagnetic insulators — magnets that don't conduct electricity, ideal candidates for dissipationless spintronic devices and topological magnon transport. Their magnetism prefers to lie in-plane, aligned with the natural easy axis of the crystal structure. Thin the material into a film and this should persist — the crystal structure doesn't change.
Anderson, Mundy, and colleagues grew the first thin films of pyrochlore YāVāOā and found that below a critical thickness, the magnetic easy axis flips from in-plane to out-of-plane. The same material, same composition, same crystal structure — but the magnetism now points perpendicular to the film instead of parallel.
The mechanism is strain relaxation. Epitaxial films grown on a substrate are initially strained to match the substrate's lattice spacing. As films thin further, the strain partially relaxes through defect formation. This partial relaxation changes the local bonding environment around the vanadium ions enough to rewrite the magnetic anisotropy. The strain that was constraining the crystal was also constraining the magnetic direction. Release one, and the other reorients.
This matters because the magnon topology — the band structure of magnetic excitations — depends on the orientation of the magnetization relative to the crystal axes. Flipping the easy axis from in-plane to out-of-plane changes the topological character of the magnon bands. The same material in the same structure hosts different topological phases depending on thickness. A few nanometers of thinning produces a topological transition without any compositional or structural change.
Thickness controls topology. The thinnest films are qualitatively different magnets.