Structural distortion and collinear-to-helical magnetism transition in rutile-type FeO2

Abstract

The recent discovery of $\mathrm{Fe}{\mathrm{O}}_{2}$ under high pressures has aroused great interest. With the fully anisotropic density-functional theory+U method, we present a predictive study of structural and magnetic transitions of rutile-type $\mathrm{Fe}{\mathrm{O}}_{2}$ after reproducing the experimental spiral wave vector in isostructural $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Mn}{\mathrm{O}}_{2}$. A second-order structural distortion (tetragonal to orthorhombic) involving octahedral rotation occurs at a critical pressure of $3\ensuremath{\sim}4\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. From a global search in the Brillouin zone, the ground-state spin order of rutile-type $\mathrm{Fe}{\mathrm{O}}_{2}$ is found to be collinear below 22 GPa and transforms to a helix at higher pressures. The phases remain insulating throughout the whole pressure range, with a change from an indirect gap in the high-spin state to a direct gap in the low-spin state. Our work extends the fundamental understanding of iron oxides and provides schemes that treat strongly correlated magnetic systems in a proper and effective way.