First-principles Study Of The Magnetic Properties In Van Der Waals Intrinsic Magnets And Magnetic Tunnel Junctions

As the fundamental units of spintronic devices,the enhancement of performance of magnetic tunnel junctions（MTJs）plays a crucial role in high integration and miniaturization of the device.Two-dimensional van der Waals（vd W）materials have atomic-level thickness,which significantly reduce device size and increase integration density.Additionally,its atomic-layer surfaces are smooth and flat,without dangling bonds,which effectively avoid interface defects.Due to the weak vd W forces between layers in vd W materials,the limitation of lattice mismatch between materials is overcome,resulting in a richer variety of high-performance tunnel junctions.Therefore,two-dimensional vd W materials offer new opportunities for the development of high-performance spintronic devices.Some works reported that the interlayer magnetic coupling of two-dimensional magnetic materials is one of the key factors in achieving high tunneling magnetoresistance（TMR）values in vd W MTJs,thus it is one of the focal issues in the research of two-dimensional magnetic materials.Additionally,antiferromagnetic materials can exhibit spin polarization dependent on momentum space.Due to the absence of stray fields and terahertz frequency of spin dynamics responses,antiferromagnetic materials hold huge potential applications in high-speed and high-density storage fields.Using first-principles calculations,this thesis investigates the interlayer magnetic coupling mechanism of two-dimensional magnetic materials Sc O₂ and NbN₂,spin transport properties of two designed MTJs,and spin polarization properties in antiferromagnetic materials M₂X₂O（M=Cr,Mn;X=S,Se）and its application in all-antiferromagnetic tunnel junction（AFMTJ）.The main research findings are as follows:1.We investigate the electronic structures,magnetic properties,and the interlayer magnetic coupling mechanism of bilayer Sc O₂ and NbN₂.It is found that AB stacking is the most preferred stacking order of bilayer Sc O₂ and NbN₂.Furthermore,the ground state of AB-stacked bilayer Sc O₂ is a semiconductor with intralayer ferromagnetic（FM）and interlayer antiferromagnetic（AFM）couplings,and the magnetic moments primarily originate from theorbits of the O atoms.Conversely,the ground state of AB-stacked bilayer NbN₂ is a metal with both intra-and inter-layer FM couplings,and the magnetic moments are mainly provided by the(92 orbits of the Nb atoms.In particular,the interlayer AFM coupling of AB-stacked bilayer Sc O₂ is much larger than intralayer FM coupling,which arises from the overlap of spin-polarizedorbitals of O atoms in adjacent layers.In contrast,the interlayer magnetic coupling in AB-stacked bilayer NbN₂ is negligible,which attributed to the lack of overlap between the spin-polarized(92 orbits of Nb atoms in adjacent layers.The results will benefit a deep understanding of the interlayer coupling of two-dimensional vd W intrinsic magnets and the exploration of high performance spintronic devices based on two-dimensional vd W intrinsic magnets.2.We investigate the spin-dependent transport and the TMR effect of the spin-filter van der Waals magnetic tunnel junctions（sf-vd W MTJs）formed by sandwiching bilayer and trilayer NiBr₂ barriers between two graphite electrodes（Gr/NiBr₂/Gr sf-vd W MTJs）.Similar to experimental findings in sf-vd W MTJs with few-layer Cr I₃barriers,the TMR ratios of the Gr/NiBr₂/Gr sf-vd W MTJs increase first with the increase of bias voltage and decrease with the further increase of bias voltage after reaching the highest points because the conduction bands of the interlayer FM NiBr₂barrier at the K points enter the bias window earlier than those of the interlayer AFM NiBr₂ barrier with the increase of bias voltage.Compared to the TMR ratios of approximately 170%and 206%at zero bias voltage,the TMR ratios of Gr/NiBr₂/Gr sf-vd W MTJs with bilayer and trilayer NiBr₂ barriers are significantly enhanced by approximately 34 and 67 times by the optimized bias voltage,respectively.Consequently,a remarkable TMR ratio of about 6000%and 14,000%can be achieved in Gr/NiBr₂/Gr sf-vd W MTJs with bilayer and trilayer NiBr₂ barriers at bias voltages of 0.14 and 0.125 V,respectively.Our findings elucidate the mechanism of bias voltage induced giant TMR ratios in Gr/NiBr₂/Gr sf-vd W MTJs and offer promising avenues for the development of MTJs with high TMR ratios.3.We explore the spin-dependent transport in vd W MTJ composed of two vd W FM CrSe₂ electrodes and an interlayer AFM bilayer NiCl₂ barrier（CrSe₂/NiCl₂/CrSe₂vd W MTJ）.In contrast to the large resistance-area（RA）products higher than several kilohms square micrometer in crystalline Mg O based MTJs with high tunneling magnetoresistance（TMR）ratios,the large TMR ratio of about 5200%and the low RA products ranging from 0.11 to 6Ω·μm² are simultaneously achieved in the CrSe₂/NiCl-₂/CrSe₂ vd W MTJ at zero bias,which is attributed to the momentum-space-dependent spin polarization in the CrSe₂ electrodes and the strong spin-filtering effect in the half-metallic NiCl₂ barrier.Moreover,the TMR ratio of the CrSe₂/NiCl₂/CrSe₂ vd W MTJ increases first with increasing bias voltage and then decreases with the further increase of bias voltage after reaching the highest value.The optimized bias voltage can significantly promote the TMR ratio up to 12000%.Our findings provide a promising pathway for developing high-performance vd W MTJs with both large TMR ratios and low RA products for future spintronic applications.4.We investigate the magnetic properties,electronic structure,and spin polarization in momentum space of monolayer M₂X₂O（M=Cr,Mn;X=S,Se）.It is found that the magnetic ground state of monolayer M₂X₂O（M=Cr,Mn;X=S,Se）is AFM state with spin splitting in the energy band.Taking monolayer Mn₂Se₂O as an example,the overall spin polarization of monolayer Mn₂Se₂O in momentum space is zero,thus is unable to generate spin current.However,there is spin polarization dependent on the wave vector k_||,which makes monolayer Mn₂Se₂O promising for constructing all-antiferromagnetic tunnel junctions,thereby generating TMR effect.These research findings have guiding significance for exploring two-dimensional vd W AFM materials with spin polarization and their applications in spintronic devices.