Biaxial-stress driven tetragonal symmetry breaking in and high-temperature ferromagnetic semiconductor from half-metallic CrO₂

Here, we find through first-principles investigation that when a biaxial compressive stress is applied on rutile CrO₂, the density of states at the Fermi level decreases with the in-plane compressive strain, there is a structural phase transition to an orthorhombic phase at the strain of -5.6%, and then appears an electronic phase transition to a semiconductor phase at -6.1%. Further analysis shows that this structural transition, accompanying the tetragonal symmetry breaking, is induced by the stress-driven distortion and rotation of the oxygen octahedron of Cr, and the half-metal-semiconductor transition originates from the enhancement of the crystal field splitting due to the structural change. Importantly, our systematic total-energy comparison indicates the ferromagnetic Curie temperature remains almost independent of the strain, near 400 K. This biaxial stress can be realized by applying biaxial pressure or growing the CrO₂ epitaxially on appropriate substrates.