Humps in <i>κ</i> ‐ <i>T</i> Curves: Evidence for the Stable Single‐Domain to Superparamagnetic Transition in Magnetite

Abstract Thermomagnetic susceptibility ( κ ‐ T ) curves are used widely for magneto‐mineralogical analysis. Humps in heating curves below the Curie temperature of magnetite are observed frequently. We present κ ‐ T curves with hump features in magnetite‐bearing flood basalt samples. Heating‐cooling experiments combined with theoretical principles suggest that these humps reflect primarily the transition from the stable single domain (SSD) to the superparamagnetic (SP) state, with possible contributions from maghemite decomposition and stress release. Scanning electron microscopy confirms magnetite intergrowths with &lt;100 nm size, which supports the presence of SSD‐SP domain states and the diversity of hump shapes. Changes in magnetic interactions, internal stress distribution, and internal subdivision due to maghemitization likely contribute to different hump shapes in heating and cooling curves. The findings indicate that humps provide a diagnostic feature associated with thermal relaxation in fine magnetite in diverse lithologies and suggest caution in interpreting κ ‐ T decreases at intermediate temperatures solely by maghemite decomposition.