Observation of uniaxial strain tuned spin cycloid in a freestanding BiFeO3 film

Non-collinear spin order that breaks space inversion symmetry and allows efficient electric-field control of magnetism makes BiFeO₃ a promising candidate for applications in low-power spintronic devices. Epitaxial strain effects have been intensively studied and exhibit significant modulation of the magnetic order in BiFeO₃, but tuning its spin structure with continuously varied uniaxial strain is still lacking up to date. Here, we apply in situ uniaxial strain to a freestanding BiFeO₃ film and use scanning NV microscope to image the nanoscale magnetic order in real-space. The strain is continuously increased from 0% to 1.5% and four images under different strains are acquired during this period. The images show that the spin cycloid tilts by sim 12.6^circ when strain approaches 1.5%. A first principle calculation has been processed to show that the tilting is energetically favorable under such strain. Our in situ strain applying method in combination with scanning NV microscope real-space imaging ability paves a new way in studying the coupling between magnetic order and strain in BiFeO₃ films.