Quantitative imaging of antiferromagnetic spin cycloidal textures on strain engineered BiFeO3 thin films with a scanning nitrogen-vacancy magnetometer

Antiferromagnetic thin films attract significant interest for future low-power spintronic devices [1]. Multiferroics, such as bismuth ferrite BiFeO3, in which antiferromagnetism and ferroelectricity coexist at room temperature, appears as a unique platform for spintronic [2] and magnonic devices [3]. The nanoscale structure of its ferroelectric domains has been widely investigated with piezoresponse force microscopy (PFM), revealing unique domain structures and domain wall functionalities [4]. However, the nanoscale magnetic textures present in BiFeO3 and their potential for spin-based technology remain concealed. In this report, we present two different antiferromagnetic spin textures in multiferroic BiFeO3 thin films with different epitaxial strains, using a commercial non-invasive scanning Nitrogen-Vacancy (NV) magnetometer based on a single NV defect in diamond, with a calibrated NV flying height of 60 nm and a proven DC field sensitivity of 1 T/Hz. Two BiFeO3 samples were grown on DyScO3 (110) and SmScO3 (110) substrates (later mentioned as BFO/DSO and BFO/SSO, respectively) using pulsed laser deposition. The striped ferroelectric domains in both samples are first observed by the in-plane PFM. The scanning NV magnetometry (SNVM) confirms the existence of the spin cycloid texture, with zig-zag wiggling angles of 90 and 127, and propagation wavelength of DSO=64 nm andSSO=103 nm, respectively. At the local scale, the combination of PFM and SNVM allows to identify the relative orientation of the ferroelectric polarization and cycloid propagation directions on both sides of a domain wall. For the BFO/DSO sample, the 90-degree in-plane rotation of the ferroelectric polarization imprints the 90-degree in-plane rotation of the cycloidal propagation direction along k1=[-1 1 0], corresponding to the type-I cycloid. On the contrary, in the BFO/SSO sample, the propagation vectors are found to be along k1'=[-2 1 1] and k2'= [1 -2 1] directions in the neighboring domains separated by the 71 domain wall. It is worth to mentioned that in the previous report [5], BFO/SSO, prepared in another growth chamber, showed G-type antiferromagnetic textures, compared to the observed type-II cycloid here. Our results here shed the light on future potential for reconfigurable nanoscale spin textures on multiferroic systems by strain engineering.