Abstract
Band-structure calculations predict a gap of 1.5 eV in the minority-spin density of states of the half-metallic ferromagnet ${\mathrm{CrO}}_{2}.$ The magnitude of ${T}_{C},$ the magnons, and the activation gap in the resistivity cannot be explained with this band picture. We propose a double-exchange mechanism for two electrons per Cr site invoking strong Hund's rule correlations and the distortion in the ${\mathrm{CrO}}_{6}$ octahedra, which localize one electron into the $\mathrm{xy}$ orbital at each Cr site, while the electrons in the $\mathrm{xz}$ and $\mathrm{yz}$ orbitals are itinerant. The effective exchange interaction naturally leads to a ferromagnetic ground state with low-energy magnon excitations and several branches of excitations separated by energy gaps from the ground state, reducing ${T}_{C}.$ Quantum fluctuations suppress the orbital long-range order, giving rise to an orbital resonant valence bond-type ground state.
Published Version
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