First Principles Study Of Magnetic Electrode Materials In Perpendicular Magnetic Tunnel Junctions

Magnetic tunnel junction?MTJ?is an important element in spintronics,in order to meet the demand of low critical current,high thermal stability and high-density nonvolatile memory in next-generation current induced magnetization switching magnetic memory,the new type perpendicular magnetic tunnel junction?p-MTJ?provides the probability.Recent years,many researches have focused on p-MTJ,however,the origin of perpendicular magnetic anisotropy?PMA?also need to be clarified.Moreover,the tunnel magnetioresistance?TMR?is small in p-MTJs,whether we can enhance the TMR in these MTJs is attractive.Above all,in this thesis both TMR and underlying mechanism of PMA in some heterostructures are studied from first principles calculations,some main results are as follows:1.Compared to the magnetic anisotropy energy?MAE?of bare Co film,the MAEs of Co/Graphene and Co/BN hetherstructures were enhanced by 47.9%and decreased by 19.9%,respectively.In the analysis of layer-resolved MAE of the two structures,we found that the surface Co atoms contribute most to the MAE.Thereafter,we do research on the influence of strain on magnetic anisotropy,results show that when the in-plane compressive strain is larger than-2%,there is a jump in MAE curve.This is due to that compressive strain enlarges the distance between Co and Graphene?BN?to nearly 4?,at this time there is no bonding between the two layers,at this time the MAE of the two structures are similar to that of Co film.Furthermore,we found that 5%tensile strain can increase the MAE of Co/Graphene and Co/BN by 48.5%and 80.8%,respectively.By analysis the orbital-resolved MAE and DOS combined second-order perturbative theory,we find that the enhanced MAE mainly comes from the matrix element of(d_xy,d_x2-y2)in the surface Co.These results show that large PMA could be obtained in light element composed materials,besides,strain is an effective way in manipulating the PMA.2.Results show that the PMA of Mn₃X?X=Ga,Ge?mainly comes from the Mn atoms in Mn-Mn layer.In the analysis of orbital-resolved MAE,we find that nearly all of the Mn-d orbital matrix elements display a positive contribution to the MAE.This also explains why the PMA of Mn₃X could get a large value in experiment.As we know,the in-plane lattice constant of Mn₃X will be changed when epitaxial grown on some substrates.The calculated results reveal that when the in-plane lattice constant is larger than 3.95?,the PMA decreases rapidly and when it is larger than 4.10?,the easy axis of Mn₃X rotated to in-plane direction.Correspondingly,the easy axis of Mn₃X/SrTiO₃?a=3.905??and Mn₃X/MgO?a=4.211??is lying in out-of-plane and in-plane orientation.This indicates that the mismatch between Mn₃X and the substrate has a large influence on the MAE of Mn₃X based heterostructures.3.We do calculations on density of states,band structure and magnetic property of M₄N?M=Fe,Co,Ni?.In the analysis of perpendicular magnetic anisotropy of Fe₄N/MgO and Co₄N/MgO as well as in-plane anisotropy of Ni₄N/MgO,we find that the magnetic anisotropy mainly comes from the interface magnetic atoms.In the spin dependent transport calculations,the TMR of Fe₄N-,Co₄N-and Ni₄N-composed M₄N/MgO/M₄N MTJ were 24711%,4438%and 1514%,respectively.The symmetry electron transport and interface resonance states contribute most to the spin-dependent conductivity.Furthermore,the TMR of Fe₄N/MgO/Fe₄N MTJ maintain a high level regardless of the disorder in the interface,the thickness of MgO barrier and in-plane lattice constant of the MTJ.The high conductivity of these MTJs could be explained by the branches of band which cross the Fermi level in the electrode.Besides,the output voltage increase monotonously with the applied bias.These results are helpful to the understanding on spin-dependent transport properties and the larger TMR ratio is useful for designing of MTJ-based spintronic devices.4.Using first principles calculations,we investigate a series of transition metal?TM??Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Y,Nb and Mo?,alkali metals?Li and Na?and alkaline-earth?Ca?dopants on the electronic structure and magnetic properties of a ZrS₂monolayer.It is shown that Ca,Sc,Ti,Ni and Y dopants cannot induce the magnetism in the doped ZrS₂ monolayer and the stability of the magnetic state of the Na doped system is very small,while the ground state of Li,V,Cr,Mn,Fe,Co,Nb and Mo-doped systems are magnetic and the magnetic moment induced by 3d TM,4d TM and Li dopants are mainly contributed by the 3d states of the 3d TM atom,the 4d states of the4d TM atom and its nearing Mo atoms,and the 3p states of the S atoms and the interstitial region around Li,respectively.The evolutions of the magnetic moments with the different TM dopants,such as Ti,V,Cr,Mn,Fe,Co,Ni,Nb and Mo,can be interpreted in terms of a simple analysis based on the molecular orbitals and electron filling.Furthermore,the magnetic coupling between the moments induced by two Li,V,Cr,Fe,Co,Nb and Mo is long-range ferromagnetic and the coupling can be attributed to the hybridization interaction involving polarized electrons or holes,whereas the coupling between the moments induced by two Mn is anti-ferromagnetic.The ferromagnetic dopants show half metal property and the high spin polorazied magnetic electrode materials are beneficial for the achievement of MTJ with high TMR.