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

## TL;DR

This study demonstrates that biaxial stress induces a structural and electronic phase transition in rutile CrO₂, transforming it from a half-metallic ferromagnet to a semiconductor while maintaining a high Curie temperature, useful for spintronics.

## Contribution

First-principles analysis reveals stress-driven symmetry breaking and phase transitions in CrO₂, enabling control over its electronic and magnetic properties for spintronic applications.

## Key findings

- Biaxial compressive stress causes a structural phase transition at -5.6% strain.
- Electronic transition to a semiconductor occurs at -6.1% strain.
- Curie temperature remains near 400 K under strain.

## Abstract

It is highly desirable to combine the full spin polarization of carriers with modern semiconductor technology for spintronic applications. For this purpose, one needs good crystalline ferromagnetic (or ferrimagnetic) semiconductors with high Curie temperatures. Rutile CrO$_2$ is a half-metallic spintronic material with Curie temperature 394 K and can have nearly-full spin polarization at room temperature. Here, we find through first-principles investigation that when a biaxial compressive stress is applied on rutile CrO$_2$, 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 CrO$_2$ epitaxially on appropriate substrates. These results should be useful for realizing full (100\%) spin polarization of controllable carriers as one uses in modern semiconductor technology.