The nature of Fe impurity phase in ferromagnetic CaB6

Since the discovery of unexpected ferromagnetism in CaB6, the role of Fe impurity has been suspected, but its identity has never been properly determined. By an experimental study of the microscopic structure and related ferromagnetism observed in the CaB6 materials purposely doped with varying amount of iron, we found that Fe impurity forms separate borides and distributes inhomogeneously among the CaB6 matrixes. The dependence of the magnetization with the Fe content in the samples shows that almost all the magnetization can be account for the Fe impurity in the form of FeB phase in the materials. This conclusion can also be extended to nominally 'undoped' CaB6 materials and the related boron compounds, suggesting not only their weak ferromagnetic behaviors are mostly of extrinsic origin, but specifically they are caused by FeB.