Ferromagnetism in lanthanum doped CaB6: is it intrinsic?

Magnetism of flux grown single crystals of undoped and lanthanum doped Ca1−xLaxB6 was systematically investigated, taking especial care in the preparation and quality control of crystals. Ca1−xLaxB6 (x=0.0029, 0.0051, 0.015) crystals showed ferromagnetism at 300 K, which could be removed by treatment with hydrochloric HCl acid, indicating that the phenomena is due to iron impurities rather than being intrinsic. Chemical analysis which has been lacking in previous reports, was performed on HCl treated and untreated Ca1−xLaxB6 crystals and showed that the magnitude of observed ferromagnetism can be explained by the iron content. Crystals grown using HCl treated aluminum flux revealed that undoped CaB6, which has a higher resistivity, accumulates less iron on the surface compared to Ca1−xLaxB6 and is diamagnetic, agreeing with our previous idea that iron is electrochemically plated onto the crystal surface during the flux removal procedure. The temperature dependence of the spurious magnetism of Ca0.985La0.015B6 crystals was measured at high temperatures and exhibited anomalies around 780 K but also at 640 K. Previously, differences of the transition temperature with bulk iron metal were given as one of the reasons to support the ferromagnetism of Ca1−xLaxB6 to be an intrinsic effect, but this result indicates that this shift can be attributed to the environment of iron impurities on the crystal surface. These results strongly indicate that the ferromagnetism in lanthanum doped CaB6 is not an intrinsic phenomenon.