Scaling behavior of the spin cycloid in BiFeO₃ films

Understanding and manipulating complex spin textures in multiferroics can provide new opportunities in electric field-controlled spintronics. In BiFeO3, a well-known room-temperature multiferroic, competition between various exchange interactions gives rise to non-collinear spin order, manifesting as an incommensurate spin cycloid with period of 64 nm. We report on the stability and scaling behaviour of the cycloid in (110)-oriented epitaxial Co-doped BiFeO3 thin films. Neutron diffraction shows i) this cycloid, despite its out of plane propagation vector, can be stable in films as thin as 50 nm, smaller than the cycloid period itself; ii) the cycloid period increases significantly for the thinnest film of 50 nm, iii) for all film thicknesses investigated, the cycloid has a unique [11-2] cycloid propagation direction (different from the bulk); and iv) the cycloid period increases upon approach to T_N. These observations are supported by Monte Carlo theory based on a first-principles effective Hamiltonian method. Our results offer new perspectives for nanoscale magnonic devices.