Topological protected magnetic skyrmions are promising for the application of energy efficient memory and logic devices [1]. They occur in magnetic materials where strong spin orbit coupling (SOC) and broken inversion symmetry lead to Dzyaloshinskii-Moriya interaction (DMI) [1]. Recently, considerable bulk DMI was achieved at room temperature (RT) by a compositional gradient engineering in FM/HM single layer films, referred to as gradient DMI (g-DMI) [2]. The resulting g-DMI is believed to originate from bulk magnetic asymmetry (BMA) and SOC and has sign and strength dependence with the magnetization gradient [2]. The low crystal symmetry property at the FM platelet/HM interfaces and the composition gradient along the thickness direction is used to realize magnetic field-free spin-orbit torque switching in CoTb [3] and CoPt [4] films, extending the application of FM/HM single layer systems to spintronics. In this work, we use topological Hall Effect (THE), magnetic force microscopy (MFM), and nitrogen-vacancy (NV) scanning magnetometry [5, 6] to characterize the effect of the compositional cobalt gradient and thickness on the resulting size and density of stabilized skyrmions in CoPt single layer films (with thickness of 10, 20 and 30 nm) grown on SrTiO3 (111) substrates. We directly image RT isolated skyrmions which remain stable at a wide range of applied magnetic fields [7]. Skyrmions were confirmed to be Bloch type from micromagnetic simulation and analytical magnetization reconstruction profiles extracted from NV maps. Furthermore, we observe skyrmion pairs which may be explained by skyrmion-antiskyrmion interactions [7]. Our findings expand the family of materials hosting magnetic skyrmions and offer an effective detection techniques of topological spin textures.

[1] A. Fert, et al., Nat. Rev. Mater. 2, 1–15 (2017).

[2] Q. Zhang et. al., Phys. Rev. Lett. 128, 167202 (2022).

[3] Z. Zheng, et al., Nat. Commun. 12, 4555 (2021).

[4] L. Liu, et al., Nat. Commun. 13, 3539 (2022).

[5] A. Laraoui, al., Appl. Phys. Lett. 121, 060502 (2022).

[6] A. Erickson, et al., RSC Advances 13, 178-185 (2023).

[7] A. Erickson, al., Direct imaging of room  temperature magnetic skyrmions in gradient-DMI engineered CoPt single layers, under review.