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

TL;DR

This paper proposes that high-temperature ferromagnetism in certain materials arises from itinerant electrons in narrow impurity bands, challenging traditional views and applying these ideas to CaB$_6$ with implications for magnetic material design.

Contribution

It introduces a model where impurity band electrons cause high $T_c$ ferromagnetism, emphasizing the role of narrow bands and inhomogeneous magnetization, specifically applied to CaB$_6$.

Findings

High $T_c$ ferromagnetism can originate from impurity band electrons.

Spin wave excitations may not suppress $T_c$ as in bulk systems.

Intrinsic magnetism of B$_2$ and O$_2$ dimers could be harnessed.

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.