Strain Tuned The Magnetism And Structure Of Two-dimensional Transition Metal Nitrides

It is inspiring that 2D intrinsic magnetic materials,such as antiferromagnetic monolayer FePS₃ as well as ferromagnetic monolayer CrI₃,few-layer Cr₂Ge₂Te₆ and monolayer VSe₂,have been experimentally demonstrated in recent year.monolayer 1H-MoN₂ was theoretically proposed to be a 2D intrinsically ferromagnetic material with a Curie temperature of nearly 420 K and it can be exfoliated using similar methods as for other 2D materials.Remarkably,using first-principles calculations,Wang et al.found that as the lattice constant is increased under strain,monolayer 1H-MoN₂ undergoes an isostructural phase transition at which the N-N distance suddenly drops,corresponding to the transition from a N-N nonbonding state to a N-N bonding state.More interestingly,the N-N nonbonding-bonding switching in monolayer 1H-MoN₂ also induces a magnetic phase transition,during which the magnetic moments transfer from the N atom to the Mo atom and the magnetic coupling is changed from ferromagnetic?FM?to antiferromagnetic?AFM?.The obtained results as fellow:Both of the unstrained monolayer 1H-NbN₂ and 1H-TcN₂ are 2D intrinsically ferromagnetic metal,in which the magnetic moment of monolayer 1H-NbN₂ and 1H-TcN₂ mainly come from the d orbitals of Nb atom and the p orbitals of N atom,respectively.Interestingly,two neighboring N atoms in the unstrained monolayer 1H-NbN₂ form N-N bond,while those in 1H-TcN₂ do not form N-N bond.When lattice constant is larger than 3.16 ?,monolayer 1H-TcN₂ undergoes N-N nonbonding-bonding switching at which dN-N suddenly drops by almost 25 %.More interestingly,due to the bonding between two N atoms in 1H-TcN₂,the magnetic moment of N atoms are quenched and the ground state of monolayer 1H-TcN₂ is transferred to non-magnetic.In contrast,when lattice constant is smaller than 3.19 ?,N-N bonding-nonbonding switching occurs at monolayer 1H-NbN₂,at which d N-N suddenly increases from 1.78 ? to 1.97 ?.It is worth noticing that the N-N bonding-nonbonding switching in monolayer 1H-NbN₂ induces the magnetic moments to transfer from the d orbitals of Nb atom to the p orbitals of N atom,while ground state of monolayer 1H-NbN₂ remains FM metal.