Abstract Self‐reversed magnetization, acquired antiparallel to the external field, can complicate the interpretation of paleomagnetic directional records, particularly in altered greigite‐bearing sediments with anomalous magnetizations. The underlying physical process for this self‐reversal remains poorly constrained. Here, we construct core‐shell micromagnetic models to investigate the remanence acquisition in greigite particles with neoformed magnetite shells at room temperature. We find that while small single‐domain particles align with the external field, larger single‐vortex particles can acquire a self‐reversed magnetization. In vortex particles, the core generates strong local magnetostatic fields that can exceed the weak external field, causing the newly formed shell to magnetize in the opposite direction. We propose a new domain‐state‐dependent mechanism for self‐reversed chemical remanent magnetization in chemically altered particle assemblages. Our micromagnetic analysis links nanoscale magnetic structures to anomalous paleomagnetic signals and provides a physical basis for diagnosing unreliable recorders.