${\mathrm{CrO}}_2$: A Self-Doped Double Exchange Ferromagnet

With the help of band structure calculations $(\mathrm{LSDA}+U)$ a clear picture of the physics behind the metallic ferromagnetic properties of ${\mathrm{CrO}}_2$ is revealed. It is concluded that ${\mathrm{CrO}}_2$ is a negative charge transfer gap material which leads to self-doping and explains why it is a metal in spite of the large Coulomb interactions. We find that there exist in ${\mathrm{CrO}}_2$ both localized and itinerant $d$ electrons, resulting in ferromagnetic ordering due to double exchange similar to colossal magnetoresistance manganates.