Ferromagnetism induced by Ca vacancy in CaB6

CaB6 doped with a La atom, Ca1-xLaxB6, is a ferromagnet, but the magnetic and transport properties of the nondoped mother material, CaB6, remain unclear. In order to reveal these we prepared a nondoped sample of CaB6 from CaO and B by the oxidation-reduction method. We observed that a sample synthesized at an appropriate temperature shows a diamagnetic magnetization, while a sample prepared at a higher temperature exhibits ferromagnetism which is one order of magnitude larger than that in La-doped CaB6. We have ascribed this appearance of ferromagnetism to Ca vacancies which were produced during the synthesis. When we compared experimental results of the electrical resistivity with the magnetic properties, are failed to find any correlation between them. From these observations, we wish to suggest that the Ca vacancy yields not only the carrier doping effect but also another effect such as inhomogeneity or lowering of local symmetry in the crystal, the latter being possibly more effective in terms of the appearance of the ferromagnetism.