Rapid quenching induces magnetic vortex-like excitations in low-carbon steel

Introduction: Low-carbon steels are traditionally valued for their structural applications. However, engineering steel’s microstructure for tailoring novel magnetic properties has not been extensively explored before. We investigate how common processing techniques—thermal treatments and mechanical strain—significantly influence the magnetic properties of low-carbon steels. Materials and methods: Low-carbon steel (0.05% C by weight) samples were subjected to rapid quenching, slow annealing and tensile straining to varying degrees. The resulting microstructures have been analyzed and correlated with their respective magnetic properties by measuring their direct current (DC) and alternating current (AC) magnetic susceptibilities. These correlations were further studied by imaging local magnetization distribution with magnetic force microscopy and micromagnetic simulation of the system. Results: Our findings show that slow annealing enlarges the grain size, enhancing magnetic susceptibility, while rapid quenching reduces grain size, resulting in a decreased magnetic response. Quenching low-carbon steel produces a significant increase in the fraction of high-angle grain boundaries and a rapid spatial variation in local magnetic anisotropy between grains, a feature which is unachievable with mechanical straining even up to the material’s ultimate tensile strength. Tensile-straining of low-carbon steel enhances magnetic susceptibility through altered magnetic anisotropy, contrary to the observed decrease in susceptibility in quenched low-carbon steel. Magnetic force microscopy and micromagnetic modeling of our data reveal that the reduced magnetic susceptibility in quenched steel is a result of the presence of intriguing magnetic excitations akin to magnetic vortices. These localized vortex-like structures act as strong pinning centers of the magnetic domain wall, causing the observed decrease in susceptibility in quenched low-carbon steels. Conclusions: The magnetic response of low-carbon steel can be tuned and vortex-like novel local magnetic structures can be generated by microstructural engineering through common thermal processing techniques.