Magnetization anomaly in metamagnetic superconductors

The magnetization in the tetragonal c-plane was measured below T_N using a single crystal of bct ErRh₄B₄. The magnetization decreases with increasing magnetic field in a field range just below the metamagnetic transition. This magnetization anomaly is explained by the fact that the entrance of the vortices is suppressed by an increase of the vortex core energy due to the precursor metamagnetic transition in the core around metamagnetic transition field.