Extrinsic Origin of High-Temperature Ferromagnetism in CaB₆

Abstract

The discovery of ferromagnetism in CaB₆ doped with La was surprising, because there is no magnetic element in the compound nevertheless the system involved a ferromagnetic Curie temperature as high as 600 K. In order to resolve the origin of the ferromagnetism, we have made magnetization measurements and chemical analysis of CaB₆ and LaB₆ samples synthesized under various conditions. We have found that the ferromagnetism disappears almost completely by the acid etching, and that there is strong correlation between the magnitude of reduced magnetization and the iron concentration involved in the acid. Therefore we conclude that the ferromagnetism is of extrinsic origin and can be ascribed to iron impurity phases, probably FeB and Fe₂B that have nearly the same Curie temperature as CaB₆. We suggest that these iron impurities originate from the crucible used in the synthesis. Furthermore we have measured the thermoelectric power at high temperatures including the Curie temperature. There seems to be no relation between the appearance of the ferromagnetism and the transport properties, and no evidence for the bulk ferromagnetism has been observed.