Two groups of unidirectionally solidified Co&sngbnd;Co3Nb eutectic samples containing small differences in nickel content were analyzed for their magnetic domain structures using TEM. The two groups showed no observable domain structures in the as‐solidified condition, but the group of samples containing the lower nickel content (<0.01 wt%) developed a strip‐type domain structure when the samples were thermally cycled. When favorable crystallographic and defect structures existed, the thermal agitation of the heat treatment cycles was able to assist in nucleating the magnetic domain structures that were observed.