High-temperature ferromagnetism of sp electrons in narrow impurity bands: application to CaB6

Ferromagnetism with high Curie temperature Tc, well above room temperature, and very small saturation moment has been reported in various carbon and boron systems. It is argued that the magnetization must be very inhomogeneous with only a small fraction of the sample ferromagnetically ordered. It is shown that a possible source of high Tc within the ferromagnetic regions is itinerant electrons occupying a narrow impurity band. Correlation effects do not reduce the effective interaction which enters the Stoner criterion in the same way as in a bulk band. It is also shown how, in the impurity band case, spin wave excitations may not be effective in lowering Tc below its value given by Stoner theory. These ideas are applied to CaB6 and a thorough review of the experimental situation in this material is given. It is suggested that the intrinsic magnetism of the B2 and O2 dimers might be exploited in suitable structures containing these elements.