Porous CrO$_2$: a ferromagnetic half-metallic member in sparse hollandite oxide family

TL;DR

This paper predicts a new stable porous CrO$_2$ polymorph that is a ferromagnetic half-metal, distinct from similar manganese oxides, with potential topological features and robust magnetic properties under pressure.

Contribution

It introduces a novel ferromagnetic half-metallic CrO$_2$ polymorph in the hollandite family, highlighting its electronic, magnetic, and topological properties through first-principles calculations.

Findings

Predicted stable porous CrO$_2$ polymorph with ferromagnetic half-metallicity.

Identified topological Weyl nodes and their pressure evolution.

Demonstrated robustness of magnetic and electronic properties under strain.

Abstract

A stable polymorph of CrO$_2$ is predicted using PBE+U method. The porous material is isostructural with $\alpha-$MnO$_2$ making it the second transition metal oxide in sparse hollandite group of materials. However, unlike the anti-ferromagnetic semiconducting character of the $\alpha-$MnO$_2$, it is found to be a ferromagnetic half-metal. At Fermi level, the hole pocket has ample contribution from O$-2p$ orbital, though, the electron pocket is mostly contributed by Cr$-3d_{xy}$ and Cr$-3d_{x^2-y^2}$. A combination of negative charge transfer through orbital mixing and extended anti-bonding state near Fermi level is responsible for the half-metallic ferromagnetic character of the structure. A comparative study of rutile and hollandite CrO$_2$ and hollandite MnO$_2$ structures delineate the interplay between structural, electronic and magnetic properties. The material shows a robust magnetic character under hydrothermal pressure, as well as, the band topology is conserved under uniaxial strain. Moderate magneto-crystalline anisotropy is observed and it shows a correspondence with the anisotropy of elastic constants. Occurrence of type$-$II Weyl nodes and their evolution under pressure is explored.