Coexistence of ferromagnetism and superconductivity in ErRh₄B₄ single crystals probed by dynamic magnetic susceptibility

High-resolution measurements of the dynamic magnetic susceptibility are reported for the ferromagnetic, reentrant superconductor ErRh₄B₄. A detailed investigation of the coexisting regime reveals unusual temperature-asymmetry on warming and cooling and magnetically anisotropic behavior. The superconducting phase appears via a series of discontinuous steps upon warming from the ferromagnetic normal phase, whereas the ferromagnetic phase develops via a more gradual transition upon cooling from the superconducting phase. A model based on local field inhomogeneity is proposed to explain these observations.