<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi>CrO</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>: 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.