Room temperature direct imaging of topological spin textures in gradient-DMI engineered CoPt single layer*

Magnetic skyrmions are topological spin textures that occur in magnetic systems where strong spin orbit coupling (SOC) and broken inversion symmetry lead to Dzyaloshinskii-Moriya interaction (DMI) [1]. Recently, it was demonstrated that creating a gradient of heavy metal/ferromagnet composition in the alloy film along the growth direction can induce DMI via bulk magnetic asymmetry and SOC [2]. Furthermore, the resulting DMI has a sign and proportionality dependence with the magnetization gradient [3]. In this work we employ magnetic force microscopy (MFM) and nitrogen vacancy (NV) scanning probe microscopy [4] to characterize the effect of the compositional cobalt gradient and thickness on the resulting size and density of stabilized skyrmions in CoPt single layers (with thickness of 10, 20 and 30 nm) grown on SrTiO₃ (111) substrates. Using both techniques, we directly image room temperature isolated skyrmions which remain stable at a wide range of applied magnetic fields [5]. Skyrmions were confirmed to be Bloch type from micromagnetic simulation and analytical magnetization reconstruction profiles extracted from NV images. We finally discuss the observation and stabilization of pairs, which may be described by skyrmion anti-skyrmion interactions. [1] A. Fert, et al., Nat. Nano. 8, 152–156 (2013). [2] L. Liu, et al., Nat. Comm. 13, 3539 (2022). [3] Q. Zhang et. al., Phys. Rev. Lett. 128, 167202 (2022). [4] A. Erickson et al., RSC advances 13 (1), 178-185 (2023). [5] A. Erickson et al., under preparation.