Study the Effect of Film Thickness and Substrate on Antiferromagnetic Domains in Epitaxial Cr₂O₃ Films Using Nitrogen-Vacancy Scanning Probe Microscopy

Chromia (Cr₂O₃) is a magnetoelectric oxide, has an antiferromagnetic (AFM) order, and hosts a surface magnetization of uncompensated spins rigidly coupled to the Néel vector [1-3]. The magnetic properties associated with Cr₂O₃ film thickness, doping, and growth conditions lead to the tuning of its Néel temperature (T_N) [3,4]. Here, we use a superconducting quantum interferometer device (SQUID) and nitrogen-vacancy scanning probe microscopy (NV-SPM) to probe the surface magnetization and study the effect of film thickness and substrate on the evolution of AFM domains in Cr₂O₃ films. Cr₂O₃ films (thicknesses ~7 – 100 nm) were grown epitaxially on sapphire and Pd/sapphire substrates using molecular beam epitaxy. SQUID thermoremanent magnetization (TRM) measurements show a reduction of T_N with a decrease of the film thickness (T_N ~ 309 K for 100 nm films to < 280 K for 40 nm films). Room temperature (RT) NV magnetic imaging reveals 180-degree AFM domains in 70 and 100 nm Cr₂O₃/Al₂O₃ films with an average magnetic domain size of ~200-300 nm. RT NV imaging on Cr₂O₃ (40 nm)/Al₂O₃ film shows no magnetic contrast, corroborating the TRM results. We then discuss the effect of substrate (Pd/sapphire) on the AFM domains. [1] A. Mahmood, et al., Nat. Commun. 12, 1674A (2021). [2] A. Erickson et al., RSC Adv. 13(1), 178-185 (2023). [3] A. Erickson et al., Adv. Funct. Mater. 34(48), 2408542 (2024). [4] P. Appel et al., Nano Lett. 19(3), 1682-1687 (2019).