Noncoplanar magnetic structures in Mn4N epitaxial films evaluated by alternating magnetic force microscopy

Noncoplanar magnetic structures in the Mn4N epitaxial thin films grown on the 001-oriented MgO and SrTiO3 (STO) substrates were studied, based on the measurements of topological Hall effect (THE) and the observation of magnetic domain nucleation. The typical nucleation diameter of domain was determined using an alternating magnetic force microscope, which proved advantageous for the visualization of the domain with an out-of-plane magnetic component. The nucleation diameter of the domains on the MgO substrate were similar to 150 nm for the thickness of 30 nm and similar to 110 nm for 10 nm, while similar to 130 nm for 30 nm on the STO substrate. The value of THE was one or two orders of magnitude larger than that estimated based on the nucleation diameter, indicating that the existence of a noncoplanar magnetic structure is the primary factor contributing to the THE in the Mn4N films, comparing to the effect from domain nucleation. The noncoplanar magnetic structure was more pronounced with decreasing thickness and substrate-induced strain.