First-principles Study Of Magnetoresistance Effect Of Tunnel Junctions Based On Two-dimensional Magnetic Materials And Perpendicular Magnetic Anisotropy Of Co₂FeAl/NiFe₂O₄ Superlattices

Magnetic tunnel junctions?MTJs?based on spin transfer torque are promising candidates for applications in non-volatile magnetic random access memory?MRAMs?.Compared to MTJs with in-plane magnetic anisotropy?IMA?,the MTJs with perpendicular magnetic anisotropy?PMA?are beneficial for obtaining sufficiently high thermal stability and low critical switching current in MRAMs.Some researches indicate that both a high TMR and interface PMA may be achieved in spinel oxide-based MTJs with Heusler alloy electrodes.Magnetic spinel oxide NiFe₂O₄ is a very attractive candidate for spintronic applications due to its high magnetic ordering temperatures,large saturation magnetizations and small lattice mismatch with typical ferromagnetic electrodes.So far,no study has addressed the magnetic anisotropy in magnetic spinel/Heusler alloy heterostructures.In this work,the magnetic anisotropy of Co₂FeAl/NiFe₂O₄ superlattices is studied by first-principles calculations based on density functional theory.The discovery of two-dimensional?2D?intrinsic magnetic materials,including ferromagnetic?FM?semiconductors Cr₂Ge₂Te₆ and CrI₃ as well as FM metals Fe₃GeTe₂ and 1T-VSe₂,has provided a new way to explore spintronic devices.Recently,two-dimensional intrinsic magnets have been used to construct van der Waals?vdW?heterostructures and lateral heterojunctions to develop high-performance spintronic devices,such as spin valves,spin-filtered MTJs lateral MTJs,and spin field effect transistors.We design vdW MTJ,formed by a ferromagnetic?FM?monolayer CrI₃ barrier sandwiched between two 2D FM 1T-VSe₂ electrodes,as well as lateral MTJ and vdW MTJ based on FM monolayer CrI₃ barrier and FM Li_0.5CrI₃ electrodes,respectively.Furthermore,tunneling magnetoresistance?TMR?effect of these MTJs are investigated.The main works are as follows:1.The Co₂FeAl?001?/NiFe₂O₄?001?superlattice is constructed in a tetragonal supercells.The structure and magnetic anisotropy of Co₂FeAl/NiFe₂O₄ superlattice are studied by first-principles calculation.The results show that the most energetically favorable configurations for Co₂FeAl?001?/NiFe₂O₄?001?interfaces are when the interface O atoms in NiFe₂O₄ are on top of the interface metal atoms in Co₂FeAl due to the bonding between interface O atoms in NiFe₂O₄ and interface metal atoms in Co₂FeAl.It is worth noting that a large PMA of up to 1.07 mJ/m² can be obtained at the interface between Co-terminated Co₂FeAl and NiO-terminated NiFe₂O₄ and the interface Co atoms play an important role in establishing the large interfacial PMA at the Co₂FeAl?001?/NiFe₂O₄?001?interface.Meanwhile,the interface Ni atoms also provide a non-negligible contribution to the large PMA of the superlattice.The d-orbital-resolved magnetic anisotropy energy?MAE?of interface and surface Co atoms show that compared to surface Co,the matrix element differences between the dz² and dyz as well as dx²–y² and dxy orbitals of the interface Co provide large contributions to the PMA of interface Co.Furthermore,due to the bonding between interface Co atoms in Co₂FeAl and interface O atoms in NiFe₂O₄,the electron occupation of dz²,dyz,dx²–y² and dxy orbitals for the interface Co is obviously different to that for the surface Co,thus the contribution to the interfacial PMA from matrix element differences between dyz and dz² orbitals changes from a negative value for the surface Co to a large positive value for the interface Co,and the contribution from the matrix element difference between dx²–y² and dxy orbitals of the interface Co is larger than that of the surface Co.Compared to the in-plane magnetic anisotropy of the surface Ni atoms,the change of matrix element difference between dx²–y² and dxy orbitals of Ni from a negative value for the surface Ni to a positive value for the interface Ni and the decrease of the negative contribution to PMA from the matrix element difference between dz² and dyz orbitals lead to a nonnegligible PMA of the interface Ni.2.We design a vdW MTJ 1T-VSe₂/CrI₃/1T-VSe₂ formed by a FM monolayer CrI₃ barrier sandwiched between two 2D FM 1T-VSe₂ electrodes and investigate the magnetic anisotropy and the tunneling magnetoresistance?TMR?effect of this vdW MTJ by using first principles calculations.It is found that if the magnetic moment of the top electrode is pinned to be[001]axis,four magnetic configurations can be realized by applying a large magnetic field to the MTJ due to the same order of magnitude of the estimated magnetocrystalline anisotropy field of CrI₃ barrier and VSe₂ electrode.Moreover,the conductance of vdW MTJ 1T-VSe₂/CrI₃/1T-VSe₂ is the highest and lowest when the magnetic moments of barrier and bottom electrode are all along[001]andé_?00 1ù_?axes,and a highest TMR ratio of 178%can be obtained in this vdW MTJ.The large changes of tunneling conductance with different magnetic configurations originate mainly from the large variation of the effective majority-and minority-spin transmission channels of FM 1T-VSe₂ for different magnetic configurations.Our results suggest that vdW MTJ based on 1T-VSe₂/CrI₃/1T-VSe₂heterostructure holds great potential in multi-states magnetic storage for spintronics.3.We design two lateral MTJs and a vdW MTJ based on FM monolayer CrI₃barrier and FM Li_0.5CrI₃ electrodes,respectively,and investigate the TMR effect of these MTJs.The calculation results show that for the vdW MTJ,the Li_0.5CrI₃electrode is FM semi-metal,while the monolayer CrI₃ barrier becomes FM semi-metal.As a result,the conductance of the vdW MTJ is about 0.36 e²/h when the magnetic moments of two FM electrodes are in the parallel configuration,while the conductance in the antiparallel configuration is only 2.44×10^-13 e²/h,thus the vdW MTJ can produce an ultrahigh TMR ratio of up to 10¹⁴%.For two lateral MTJs,the Li_0.5CrI₃ electrode is FM semi-metal and the CrI₃ barrier is FM semiconductor.The conductance of two lateral MTJs is 1.50×10^-03 and 2.00×10^-03 e²/h when the magnetic moments of two FM electrodes are in the parallel configuration,respectively,which is two order of magnitude smaller than that of the vdW MTJ.At the same time,the conductance of two lateral MTJs in the antiparallel configuration is only 9.92×10^-12and 7.00×10^-14 e²/h,respectively.Therefore,the TMR of two lateral MTJs is as high as 10¹⁰%and 10¹²%,respectively.Our calculations show that lateral and vdW MTJs based on FM monolayer CrI₃ barrier and FM Li_0.5CrI₃ electrodes are potential candidates for applications in spintronic devices.