High-temperature ferromagnetism of $sp$ electrons in narrow impurity bands: Application to CaB${}_6$

Abstract

Ferromagnetism with high Curie temperature $T_c$, 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 $T_c$ 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 $T_c$ below its value given by Stoner theory. These ideas are applied to CaB${}_6$ and a thorough review of the experimental situation in this material is given. It is suggested that the intrinsic magnetism of the B${}_2$ and O${}_2$ dimers might be exploited in suitable structures containing these elements.

arXiv comment26 pages, 2 figures

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