Possible co-existence of superconductivity and weak ferromagnetism in ErNi2B2C

Detailed measurements of the field H and temperature T dependent, anisotropic magnetization M(T, H) on single crystal ErNi2B2C indicate that there is a phase transition to a magnetically ordered ground state that has a weak ferromagnetic component of 0.33 mu(B)/Er for T < 2.3 K. This ordering is similar to the transition to weak ferromagnetism seen in TbNi2B2C, but, unlike TbNi2B2C, the ferromagnetic component exists in ErNi2B2C for temperatures and fields that are well below the T-c and H-c2. Therefore, in ErNi2B2C there appears to be the coexistence of superconductivity and weak ferromagnetism for T < 2.3 K and H < 12 kG.