Visualization of ferromagnetic domains in ErNi₂B₂C single crystals: Weak ferromagnetism and its coexistence with superconductivity

The magnetic flux structure in the basal plane, (001), of single crystals of superconducting ErNi₂B₂C was studied by high resolution Bitter decoration at temperatures below T_c (superconducting transition) and/or T_N (antiferromagnetic transition). Two sets of domain boundaries, in {110} and {100} planes, were observed. The temperature range in which the {100} domain boundaries were observed coincides with the weak ferromagnetic (WFM) ordering in this material. On the other hand, the {110} twin boundaries—the antiferromagnetic domain boundaries—were observed below T_N. For comparison, TbNi₂B₂C, the related compound with higher magnetic ordering temperatures but no superconductivity, was also studied. The possibility of interpretation of {100} boundaries as Bloch domain walls in the weakly ferromagnetic phase, for T<T_WFM<T_N (TbNi₂B₂C) or T<T_WFM<T_N<T_c (ErNi₂B₂C), is discussed.