【作者】 刘远；

【导师】 王立英；

【作者基本信息】 天津大学 ， 物理学， 2022， 硕士

【摘要】 铁磁/氧化物界面的垂直磁各向异性（PMA）是实现高热稳定、高存储密度、低临界电流以及非易失性磁存储器件的关键因素。具有100%自旋极化的电极材料可以提高垂直磁性隧道结的隧穿磁电阻（TMR）效应。哈斯勒合金Mn_2.25Co_0.75Ga_0.5Sn_0.5（MCGS）是一种新型自旋零能隙半导体材料,具有高自旋极化率和725 K的高居里温度等优良特性,成为磁性隧道结铁磁性电极的良好候选材料。本论文利用密度泛函理论和非平衡格林函数的方法,详细地研究了MCGS/MgO异质结构的电子结构、磁性及输运特性,为MCGS在自旋电子学器件上的应用提供理论依据。主要工作如下:（1）研究了界面终端、界面氧化和薄膜厚度对MCGS/MgO异质结构的磁各向异性的影响。发现四种界面终端MCGS/MgO异质结构（AC1、AC2、BD1和BD2模型）均表现PMA。在高达±50%的界面氧化浓度范围内,PMA均得以保持。MgO厚度对异质结的磁各向异性几乎没有影响。随着MCGS厚度从五层增加到十六层,AC型和BD型表面终端的MCGS/MgO异质结构的PMA呈现不同程度的减小,易磁化轴始终指向面外。结果表明MCGS/MgO异质结构具有稳定且可调的PMA,为其在非易失性磁存储器件的应用提供了理论依据。（2）通过施加面内双轴应力来调控（MCGS）₂/（MgO）₁超晶格的电子结构和磁各向异性。发现在3%到5%的压缩应力和1%到2%的拉伸应力下,AC2终端超晶格表现100%自旋极化的半金属性质。随压缩应力的增大,超晶格的PMA呈线性增大。在2%拉伸应力下PMA转变为面内磁各向异性（IMA）,且IMA随拉伸应力的增大呈现先增大后减小趋势。这些结果为设计应力调控的新型自旋电子学器件提供了理论依据。（3）在平衡态下,四种不同终端的MCGS/MgO/MCGS隧道结均具有较高的TMR,其中BD1终端的TMR最大为6.51×10⁶%。非平衡态下,AC2和BD1终端隧道结的TMR随偏压增大而减小。在0～0.1 V偏压下,两电极相对磁化取向平行时,AC2和BD1终端隧道结均表现-100%的自旋注入效率。通过调节偏振光角度和光子能量,可以获得几乎完全自旋极化的光电流,且线偏振光比圆偏振光照射更有利于产生高自旋极化的光电流。这些结果为设计自旋和光协同调控的MCGS基的自旋电子器件提供了理论依据。更多还原

【Abstract】 Perpendicular magnetic anisotropy（PMA）of ferromagnetic/oxide interface is a key factor in magnetic random access memory devices with good thermal stability,high density,low critical current and nonvolatile.The tunneling magnetoresistance（TMR）effect of perpendicular magnetic tunneling junctions（MTJs）can be enhanced by electrode materials with 100%spin polarization.The novel spin gapless semiconductor Heusler alloy Mn_2.25Co_0.75Ga_0.5Sn_0.5（MCGS）is an ideal electrode material of MTJ because of its high spin polarization and high Curie temperature.In this dissertation,the electronic structure,magnetic and transport properties of MCGS/MgO heterostructures are detailedly investigated by density functional theory and non-equilibrium Green’s function.The results provide theoretical guidance for the application of MCGS in spintronic devices.The main work is as follows:（1）The effects of interfacial termination,oxidation and film thickness on magnetic anisotropy of MCGS/MgO heterostructures are investigated.It is found that all the AC1-,AC2-,BD1-and BD2-terminated MCGS/MgO heterostructures present PMA.The PMA of the MCGS/MgO heterostructures can be preserved in a large range of interfacial oxidation（up to±50%）.MgO thickness has no effect on the magnetic anisotropy of MCGS/MgO heterostructures.With MCGS thickness increasing from 5to 16 monolayers,the PMA of MCGS/MgO heterostructures with AC-and BD-type surface shows different decreasing trend,and the easy magnetization axis always points to out-of-plane direction.The robust and tunable PMA in MCGS/MgO heterostructures provides theoretical guidance for their applications in nonvolatile data memory devices.（2）The electronic structure and magnetic anisotropy of the（MCGS）₂/（MgO）₁superlattices are modulated by applying in-plane biaxial strain.It is found that 3%to5%compressive strain and 1 to 2%tensile strain induce the AC2-terminted（MCGS）₂/（MgO）₁ superlattice to present half-metallic character with 100%spin-polarization.The PMA of AC2-terminted（MCGS）₂/（MgO）₁superlattice increases almost linearly with the increasing compressive strain,while the PMA transforms into in-plane magnetic anisotropy（IMA）under 2%tensile strain,and IMA shows a trend of initially increasing and then decreasing.These results provide a theoretical guidance for designing novelty strain switchable spintronic devices.（3）At equilibrium,the MCGS/MgO/MCGS MTJs whit four different terminations all present high TMR,and the TMR of BD1-terminated MTJ is up to6.51×10⁶%.At non-equilibrium,the TMR of AC2-and BD1-terminated MTJs decreases with the increase of bias voltage.Under 0 to 0.1 V bias voltage,the AC2-and BD1-terminated MTJs present nearly 100%spin injection efficiency in the parallel magnetization configuration.The nearly 100%spin-polarized photocurrent can be obtained by tailoring the polarization angle and photon energy of polarized light.The linearly polarized light is more favorable than circularly polarized light to generate high spin-polarized photocurrent.These results provide a theoretical guidance for designing the spin-and optical-tunable MCGS-based spintronic devices.更多还原