Local structural analysis of half-metallic ferromagnet CrO₂

We have performed powder neutron diffraction on the half-metallic ferromagnet CrO₂ which has a rutile-type crystal structure with a tetragonal unit cell. Although the powder diffraction pattern can be fitted by the reported crystal structure including a single Cr site, the atomic pair distribution function (PDF) can be fitted by the structural model with an orthorhombic unit cell including two kinds of inequivalent Cr sites. The difference between the valences of the two inequivalent Cr sites, δ of Cr^+4±δ, estimated from the local structural parameters is about 0.06. The shapes of the two CrO₆ octahedra are slightly different, suggesting the short-range orbital ordering of the Cr 3d orbitals. The lattice distortion and the improvement of the fitting to the PDF obtained using the locally distorted structure model are apparent in the region below about 10 Å, suggesting that the domain size or correlation length of the locally distorted structure is about 10 Å, roughly corresponding to the size of two unit cells.