  Due to the high magnetic coupling strength between the Cr elements, the bulk
phase CrO2 is one of several ferromagnetic oxides known to have the highest
Curie temperature. When the dimensionality of the material is reduced from 3D
to 2D, the 2D CrO2 system material is expected to maintain a high Curie
temperature. In this work, we predict two new phases of CrO2 monolayer (D3d and
D2h) by using first-principles calculations. We have found that the Curie
temperature of 2D CrO2 is much lower than that of its bulk phase, but still
remains as high as 191K, which is comparable to that of Fe2Cr2Ge6. In addition,
1L D3d-CrO2 is in the ferromagnetic state, while 1L D2h-CrO2 is in the
antiferromagnetic state. Also, the different geometric structure affects its
electrical properties: the 1L D3d-CrO2 is a half-metal while 1L D2h-CrO2 is a
semiconductor. Our studies have shown that there is a wealth of electrical and
magnetic properties in CrO2.
