Scanning Hall probe imaging of ErNi₂B₂C

We report scanning Hall probe imaging of ErNi₂B₂C in the superconducting, antiferromagnetic, and weakly ferromagnetic regimes in magnetic fields up to 20 Oe, well below H_c1, with two results. First, imaging isolated vortices shows that they spontaneously rearrange on cooling through the antiferromagnetic transition temperature T_N=6 K to pin on twin boundaries, forming a striped pattern. Second, a weak, random magnetic signal appears in the ferromagnetic phase below T_WFM=2.3 K, and no spontaneous vortex lattice is present down to 1.9 K. We conclude that ferromagnetism coexists with superconductivity either by forming small ferromagnetic domains or with oscillatory variation of the magnetization on sub-penetration-depth length scales.