Competition between magnetic and structural transitions in CrN

CrN is observed to undergo a paramagnetic to antiferromagnetic transition with N\'eel temperature TN~280 K, accompanied by a shear distortion from cubic NaCl-type to orthorhombic Pnma structure. Our first-principles plane wave calculations, based on ultrasoft pseudopotentials, confirm that the distorted antiferromagnetic phase with spin configuration arranged in double ferromagnetic sheets along the [110] direction is energetically much more stable than the paramagnetic phase, and also slightly favored over the ferromagnetic and other examined antiferromagnetic phases. The energy gain from polarization is much larger than that from distortion; nevertheless, the distortion is decisive in resolving the competition among the antiferromagnetic phases: the anisotropy in the (100) plane arising from the ferromagnetic double-sheet magnetic order allows a small but important energy gain upon the cubic-to-orthorhombic transition. Although antiferromagnetic order leads to a large depletion of states around Fermi level, it does not open a gap. The system is metallic with occupied hole and electron pockets of Fermi surface containing ~0.025 carriers of each sign per formula unit. The simultaneous occurrence of structural distortion and antiferromagnetic order, as well as the competition between antiferromagnetism and ferromagnetism, is analyzed.