A First Principle Investigation For MnN And CrN

The first-principles calculations have investigated the physical properties of Manganese Nitride?MnN?and Chromium Nitride?CrN?.With the development of new magnetic materials,such as its wide application in magnetic regulation,magnetic storage and other fields,new magnetic materials have attracted more attention.In this paper,the first-principle calculations have studied the magnetic properties of MnN and CrN,including the magnetic ordering,the coupling of magnetism and structure,the magnetic and electronic structures,and the changes of the magnetic properties under high pressure for MnN and CrN.These research will be helpful for the further applications of nitride magnetic materials.MnN shows a distorted face-centered tetragonal structure at ambient pressure.The calculation indicates that MnN has a structural phase transition induced by magnetic phase transition under high pressure.As the pressure increases,the reduction ratio of the lattice constants is different,which could lead to the structural phase transition from the distorted face-centered tetragonal structure to the cubic structure when MnN is compressed to 16?³or the calculated transformation pressure at about 30 GPa.Moreover,the calculations indicate that the structural transition is accompanied by the magnetic transition of MnN from antiferromagnetic?AFM:Antiferromagnetic?to ferromagnetic?FM:Ferromagnetic?under high pressure.The analysis of the magnetic interaction in MnN by the Heisenberg model shows that the ferromagnetic coupling between the Mn-e_g orbital and the N-p orbital in the xy plane is enhanced with the increase of pressure,which resulting in the FM state of MnN above 30 GPa.The structure of MnN therefore transform from the distorted face-centered tetragonal structure to the face-centered cubic structure driving by the changed magnetic stress under high pressure.The structural and magnetic properties of CrN has also been investigated.It is found that the Cr and N atoms in the orthorhombic phase both shift from their ideal position along the[100]direction of the orthogonal structure,and then form a zigzag atomic chain in the orthorhombic phase.However,these atomic displacements had not been considered by most of previous calculations of CrN.Our calculations show that the atomic displacements can reduce the total energy of the orthorhombic phase by 0.125 eV,which is more stable.After considering the atomic displacements,the calculated structural parameters such as lattice length and bulk elastic modulus are in better agreement with the experimental values.The magnetic ground state of the orthorhombic phase is an asymmetric antiferromagnetic structure between[100]layers,so the atomic displacements would be driven by the asymmetric magnetic stress between the layers,and the atomic displacements could compensate the magnetic interaction force between the layers as well.The atomic displacements would not change the Mott-insulator properties of CrN,but slightly reduce the band gap in the band structure.In addition,the calculations shown that the atomic displacements also exist in the orthorhombic phase under high pressure,and phonon calculations show that the structure with atomic displacements is dynamic stable under 50 GPa.Its atomic displacements value increase with pressure,as well as the increased a/b ratio under high pressure.Furthermore,the atomic displacements does not change the Mott-insulator properties of CrN under pressure as well,but the band gap decreases with the pressure.At pressures above 89 Gpa,the structural distortion induced by high pressure and atomic displacements would contributes to the overlap of the Cr-t_2g orbital and Cr-e_g orbital and the electron redistribution in CrN,then resulting in the structural phase transition of CrN from orthorhombic structure to a hexagonal structure under high pressure.