Electronic Structure And Magnetic Control Of Several Two Dimensional Spintronics Materials

The discovery of graphene and graphene-like two dimensional materials brings great chance to materials science,and the two dimensional materials have attracted extensive attentions.Two dimensional materials of atomic thickness with great mechanical properties,thermodynamic properties and electronic properties can bring new possibilities.When the scales of materials and devices are down to nanoscale,the timescale get to nanosecond,pico-second or even femto-second.The energy scale approaches the level of electron-volt,namely the same scale of the local fields such as charge,molecular orbital,electron spins.Therefore,the physical fields such as strain,electric and magnetic field and substrates applied to nanomaterials is prone to a strong coupling with the local field,and thereby change the properties of whole system.For magnetic two dimensional materials,controllable magnetic properties are a very important for device design or manufacture.The two dimensional boron nitride nanotube is a widely studied material,and it can become a light-element magnetic material by fluorination.The two dimensional molybdenum nitride monolayer is a recently discovered material with intrinsic ferromagnetism,and its magnetism can also be tuned through some specific methods such as structural deformation.The two dimensional MoTeI monolayer is intrinsically a ferromagnetic semiconductor according to our results through density functional theory.The magnetic state of MoTeI monolayer is sensitive and can be significantly changed to antiferromagnetic by biaxial compressive strain.Here we studied the properties of these three materials mentioned above and explore possibility to manipulate their magnetic properties.It is well known that the most common ways to calculate light-element magnetic materials which only involved 2p orbital are local density approximation(LDA),generalized gradient approximation(GGA)or Hybrid functionals such as Heyd-Scuseria-Ernzerhof(HSE).As well known,the local density approximation and generalized gradient approximation underestimates the electron localization effects and usually tends to give misleading results.Heyd-Scuseria-Ernzerhof performs better while computationally expensive,especially for extended systems.In order to go beyond semi-local DFT without needing to calculate the expensive Fock exchange,here we explore the performance of the more efficient GGA plus the Hubbard U correction(GGA+U)approach to light-element magnetic materials by considering fluorinated boron nitride(F-BN)sheets and nanotubes as model systems.By applying the Hubbard U correction to the N-2p orbitals with the value of U determined by fitting the HSE results in a particular F-BN sheet,it is found that the GGA+U approach shows a great improvement to GGA in describing the magnetic properties and electronic properties of F-BN systems with an accuracy close to that of the HSE hybrid functional approach.The GGA+U approach also fits for other light-element magnetic materials such as BN nanotube.It indicates the possibility of using the ad hoc correction approach as an efficient alternative to study light-element magnetic materials,especially for large systems where calculations based on hybrid functionals become cost-demanding.MoN₂ monolayer is a newly discovered two-dimensional material with strong ferromagnetic ordering in its ground state.We found that the ferromagnetic spin ordering within MoN₂ varies dramatically due to moderate structural deformations.For example,absorption of a Fe atom on the top of the Moatom results in structural deformation and the magnetic moment decreases significantly,and the whole system becomes a bipolar magnetic semiconductor.Further exploration by manually moving the N atom horizontally towards the Moatom or the hollow site reveals that the ferromagnetic spin ordering vanishes only if the displacement of the N atom from its equilibrium position is larger than 0.5(?).Based on these findings,we further propose and demonstrate a nanomechanical modulation by imposing redial deformation to sensitively recover or quench the spin polarization within a MoN₂ nanotube.Our work suggests the great potential of the 2D MoN₂ material in the nanoscale electronic and spintronic applications.MoTeI is a new two dimensional material,in this work we study the details of geometry structure,thermodynamic stability magnetism and electronic properties.The structure is similar with MoS₂ which also contains three sublayer.The two-dimensional MoTeI monolayer is intrinsically a ferromagnetic semiconductor.The magnetic states of MoTeI monolayer are sensitive and can be significantly changed by biaxial strain.After the magnetic phase transition,the semiconducting characteristics are always maintained and little charge transfer is observed.We believe that the magnetic phase transition is attributed to the competition of the direct interaction and indirect superexchange interaction between magnetic centers of Moatoms.These findings suggest that the MoTeI monolayer can be a potential candidate for spintronics applications.