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

Abstract

Ferromagnetism with high Curie temperature $T_c$, well above room\ntemperature, and very small saturation moment has been reported in various\ncarbon and boron systems. It is argued that the magnetization must be very\ninhomogeneous with only a small fraction of the sample ferromagnetically\nordered. It is shown that a possible source of high $T_c$ within the\nferromagnetic regions is itinerant electrons occupying a narrow impurity band.\nCorrelation effects do not reduce the effective interaction which enters the\nStoner criterion in the same way as in a bulk band. It is also shown how, in\nthe impurity band case, spin wave excitations may not be effective in lowering\n$T_c$ below its value given by Stoner theory. These ideas are applied to\nCaB$_6$ and a thorough review of the experimental situation in this material is\ngiven. It is suggested that the intrinsic magnetism of the B$_2$ and O$_2$\ndimers might be exploited in suitable structures containing these elements.\n