First-principles study of the metal-insulator transition in the Ti-substituted rutile CrO2

Abstract

The effect of Ti-substitution for Cr in CrO2 is extensively studied by employing first-principles electronic structure calculations. The host material is a ferromagnetic half-metal. This material encounters first step transition from half-metal to a metallic phase at 50% Ti-substitution for Cr. The application of Coulomb interaction U significantly changes the electronic and magnetic properties of Cr0.5Ti0.5O2. This material remains in its metallic phase up to U = 2 eV and encounters a second step transition from the metallic to a half-metallic phase at U = 3 eV. Eventually, this system exhibits a metal-insulator transition (MIT) at U = 4 eV with a band gap of 0.15 eV. Nevertheless, Cr0.5Ti0.5O2 preserves its ferromagnetism (FM) in all the metallic, half-metallic and insulating phases. The metal to half-metal transition in Cr0.5Ti0.5O2 is observed due to full spin polarizations accompanied by strong dynamical correlations of Cr-dyz/xz electrons. Besides this, weak static but the strong dynamical correlation of electrons in the bonding and anti-bonding components of Cr-dyz/xz orbitals is accounted for the key element of MIT in Cr0.5Ti0.5O2. The FM in the half-metallic Cr0.5Ti0.5O2 arises from the double exchange interaction of electrons in the partially occupied bonding and anti-bonding components of Cr-dyz/xz orbitals. In addition, the double exchange interaction of electrons in the bonding components of Cr-dyz/xz orbitals triggers FM in the insulating phase of Cr0.5Ti0.5O2. The ferromagnetic Curie temperature increases due to Ti-substitution in CrO2. Finally, a trivial structural distortion is observed due to Ti-substitutions in CrO2.