Quantitative Imaging of Antiferromagnetic Spin Cycloidal Textures on Strain Engineered BiFeO3 Thin Films with a Scanning Nitrogen-Vacancy Magnetometer

Abstract

Antiferromagnetic thin films attract significant interest for future low-power spintronic devices. They are largely insensitive to external magnetic fields, exhibit negligible cross-talk between neighboring memory cells and they can be switched at THz frequencies [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, 5], but nanoscale magnetic textures present in BiFeO3 and their potential for spin-based technology remain concealed. Depending on the strain, growth conditions and crystal orientation, the magnetic state of BiFeO3 thin films can either show different types of non-collinear cycloids, canted G-type antiferromagnetic orders, or even a mixture of these [6].In this report, we present two different antiferromagnetic spin textures in multiferroic BiFeO3 thin films with different epitaxial strains, using a non-invasive scanning Nitrogen-Vacancy magnetometer (Qnami, ProteusQTM) based on a single nitrogen-vacancy (NV) defect in diamond (Qnami, QuantileverTM MX), with a calibrated NV flying height of ~35 nm and proven dc field sensitivity of ~1 μT/√Hz. The two BiFeO3 samples were grown on DyScO3 (110) and SmScO3 (110) substrates using pulsed laser deposition. The striped ferroelectric domains in both samples are first observed by the in-plane PFM as shown in Figs. (a) and (b), respectively. The corresponding scanning NV images in Figs. (c) and (d) confirm the existence of the spin cycloid texture, with zig-zag wiggling angles of 90° and 127°, and the propagation wavelength of λDSO≈64 nm and λSSO≈94 nm, respectively. At the local scale, the combination of PFM and scanning NV magnetometry allows to identify the relative orientation of the ferroelectric polarization and cycloid propagation directions on both sides of a domain wall. For the BiFeO3 grown on DyScO3 (110) substrate, 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] direction, corresponding to the type-I cycloid as shown in Fig. (c). On the contrary, in the BiFeO3 film grown on SmScO3 (110) substrate, 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, cf. Fig. (d). It is worth to mentioned that in the previous report [6], BiFeO3 grown on SmScO3 (110), prepared in another growth chamber, showed G-type antiferromagnetic textures, compared to the observed type-II cycloid here in Fig (d). Our results here shed the light on future potential for reconfigurable nanoscale spin textures on multiferroic systems by strain engineering. **