First-principles Study On Magnetic Properties Of Two-Dimensional Heavy Metal Oxides And Iodides And Magnetocrystalline Anisotropy Of Ta|CoFeB|MgO Heterojunction

Following the successful exfoliation of graphene,atomically thin two dimensional?2D?materials have attracted a growing interest due to their intriguing physical and chemical properties.However,the intrinsic zero band gap of graphene limits its applications in electronic,spintronic and optoelectronic devices.As a consequence,great efforts have been made to search for 2D semiconducting material.Among 2D metal halogenides,lead iodide?PbI₂?is one of the most recently synthesized single layer semiconductor and is regarded as a representative of the most explored 2D metal halogenides.Monolayer PbI₂ is intrinsically nonmagnetic,which limits its application in spintronic devices.Up to now,the influence of vacancy and substitutional nonmetallic atoms on the electronic structure and magnetic properties of monolayer PbI₂ have not been reported yet.It is worth noticing that heavy elements Pb and I has strong spin orbit coupling?SOC?interactio,which has substantially effect on the band structure of PbI₂.In this work,by using first-principles calculations,we systematically investigate the influence of I vacancy and a series of substitutional nonmetallic atoms,such as Si,P,S,F,Cl,and Br,on the stability,geometric structure,electronic structure and magnetic properties of monolayer PbI₂.Furthermore,we explore the effect of SOC on the electronic band structure and magnetic properties of doped monolayer PbI₂.Because monolayer materials can be regarded as atomically thin films,it can be foreseen that applying monolayer ferromagnetic materials with a large magnetocrystalline anisotropy?MCA?to magnetic storage will help further increase the density of data storage.Recently,2D intrinsic ferromagnetic materials with large MCA,such as monolayer CrI₃,VSe₂,Fe₃GeTe₂ and few-layer Cr₂Ge₂Te₆,have been experimentally demonstrated.Meanwhile,the spin-polarized calculations indicated that the ground state of metastable monolayer 1H-RuO₂ is ferromagnetic.In view of the strong SOC interactions of 4d and 5d heavy metal elements and the similarity of the valence electrons in the d orbitals of the same-group elements Ru and Os,it is estimated that monolayer OsO₂ may be a 2D intrinsic ferromagnetic material with large MCA.In this paper,using first-principles calculations,we explore the stability,electronic structure and magnetic properties of monolayer 1T-RuO₂ and 1T-OsO₂,and investigate the effects of strain on their electronic structure and magnetic properties.The magnetic electrode materials with perpendicular magnetic anisotropy?PMA?can decrease the magnetization switching current of magnetic tunnel junction?MTJ?.The MTJ with perpendicular magnetic anisotropy based on Ta|CoFeB|MgO have attracted a growing interest due to large tunneling-magnetoresistance?TMR?and low magnetization switching current.Recently origins of the perpendicular magnetic anisotropy in MgO|CoFeB|Ta still remain controversial.Some experimental results reveal that the B diffuses out of CoFeB electrodes leaded by annealing will increase PMA in annealed Ta|CoFeB|MgO.Wang et al.conducted the EELS mapping that reveals that B diffuses out of CoFeB electrodes into Ta interstitial sites rather than MgO after annealing.In this work,by using first-principles calculations,we investigate the effect of interstitial B position on the magnetic anisotropy of heterojunction Ta|CoFeB|MgO.The obtained results are as followings:1.The calculated formation energy of I vacancy and substitutional atom and MD simulation indicate that substitutional doping of nonmetallic atom F,Cl,Br,Si,P and S at the I site in single layer PbI₂ are possible under appropriate experimental conditions and the I vacancy and nonmetallic atom doped single layer PbI₂ are thermodynamically stable at room temperature.Due to same number of valence electron of F,Cl,Br and I,the substitutional doping of F,Cl and Br at the I site do not produce magnetism in single layer PbI₂.In contrast,one I vacancy,substitutional S,P and Si atom induce the magnetic moments of 1.0,1.0,2.0 and 3.0?_B,respectively and the distribution of the magnetic moment relate to the hybridization between dopant and its neighboring Pb and I atoms or the hybridization between Pb and I atoms around vacancy.Furthermore,the SOC induce spin splitting due to the absence of inversion symmetry in I vacancy andnonmetallic atoms X doped single layer PbI₂ and the spin splitting induced by the SOC interaction is much smaller than the exchange splitting.More importantly,the spin splitting induced by the SOC does not alter the electron occupation of the bands near the Fermi level,thus the magnetic moment induced by I vacancy,S,P and Si atoms are not almost changed and the magnetic moment of F,Cl and Br doped single layer PbI₂ are still zero after the inclusion of SOC.2.The ground state structure of monolayer RuO₂ and OsO₂ is a T structure with D_3d point group and the synthesis of monolayer 1T-OsO₂ is feasible.Furthermore,it is found that both monolayer 1T-RuO₂ and 1T-OsO₂ are 2D intrinsic ferromagnetic materials with large MCA and their magnetic moment and MCA mainly come from the Ru and Os atoms.In particular,the MCA of monolayer 1T-OsO₂ is as high as 42.67 meV per unit cell due to the strong SOC of the Os atom and its Curie temperature being much higher than the liquid nitrogen temperature.It is worth noting that the MCA of monolayer 1T-OsO₂ under 2%strain increases by 19%compared with that of unstrained monolayer 1T-OsO₂.By analyzing the density of states and the d orbital-resolved MCA of Os and Ru atoms based on second-order perturbation theory,it is revealed that the large MCAs of monolayer 1T-RuO₂ and 1T-OsO₂ are mainly contributed by the matrix element differences between the opposite-spind_{x y} and d_x2-y2 orbitals of Ru and Os atoms,and the tensile strain induced enhancement in monolayer 1T-OsO₂,which originates from the change in contributions to the MCA from the matrix element differences between the d_{y z} and d_z2 orbitals of Os atoms from zero to negative.3.The PMA of heterojunction Ta|CoFe|MgO mainly come from Co atoms in MgO|CoFe interface and Co and Ta atoms in CoFe|Ta interface.Furthermore,the PMA of Ta|CoFe|MgO in which B is in Ta interstitial site is larger that that of MgO|CoFe|Ta in which B is in CoFe,and the PMA of Ta|CoFe|MgO in which B is in Ta surface of CoFe|Ta interface is larger that of Ta|CoFe|MgO in which B is Co surface of CoFe|Ta interface,.were studied systematically.Our results indicate that B diffuses out of CoFeB electrodes into Ta interstitial sites rather than MgO after annealing lead the increase of the perpendicular magnetic anisotropy in annealed Ta|CoFe|MgO.