Observation of Uniaxial Strain Tuned Spin Cycloid in a Freestanding BiFeO3 Film

Abstract

Bismuth ferrite (BiFeO3) possesses a non-collinear spin order while the ferroelectric order breaks space inversion symmetry. This allows efficient electric-field control of magnetism and makes it 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 bismuthBiFeO3, but tuning its spin structure with continuously varied uniaxial strain is still lacking at this moment. Herein, in situ uniaxial tensile strain is applied to a freestanding BiFeO3 film by mechanically stretching an organic substrate. A scanning nitrogen-vacancy (NV) microscopy is applied 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 ≈12.6° when strain approaches 1.5%. A first principle calculation is processed to show that the tilting is energetically favorable under such strain. The 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.