【作者】 陈婷；

【导师】 陈洪；

【作者基本信息】 西南大学 ， 理论物理， 2018， 硕士

【摘要】 新型自旋随机磁存储器件作为嵌入式存储器具有记录密度高、读写速度快、能量消耗低等优势,有望全面取代传统的磁存储设备。MnGa合金被认为是一种用于制作新型自旋随机磁存储器件的理想电极材料。四方结构的MnGa合金拥有足够大的垂直磁各向异性能、低饱和磁化强度、超高的电子自旋极化率、低的磁阻尼系数、较高的居里温度,是制作高隧道磁电阻率的磁隧道结的候选。然而,实验研究发现MnGa/MgO/MnGa垂直磁隧道结的隧道磁电阻率严重偏低,根本问题在于MnGa薄膜电极与MgO势垒层之间存在较大的晶格失配度以及Mn原子在界面处发生扩散现象。为解决MnGa薄膜电极与MgO势垒层的晶格不匹配以及Mn原子在界面扩散的问题,许多立方结构的Co基Heusler合金被制成MnGa与MgO之间的嵌入层。大量的研究表明Co₂MnSi与MnGa之间拥有强烈的界面反铁磁交换耦合作用是理想的嵌入层材料。Co₂MnSi的优势是作为典型的Heusler合金与MnGa的晶体构型同源。Co₂MnSi还是完全的半金属材料在室温下能够保持较高的磁电阻率。Co₂MnSi具有远高于室温的居里温度和低磁阻尼系数保证了良好的热稳定性和较低的能耗。此外,Co₂MnSi薄膜作为软磁材料能够很好地适应MnGa电极磁矩的变化。最后,MnGa/Co₂MnSi双层膜复合电极表现出良好的结构稳定性不仅缓解了界面的晶格失配问题,还成功地克服了Mn原子的界面扩散效应。MnGa薄膜与Co₂MnSi嵌入层之间的界面交换耦合机制是研究的关键。本文基于密度泛函理论对D0₂₂-Mn₃Ga/Co₂MnSi界面和L1₀-MnGa/Co₂MnSi界面的热力学稳定性、界面电子结构和界面磁矩行为进行了详细计算。它们的热力学稳定性计算结果表明实验上最有可能合成的界面构型是MnGa合金的MnMn原子端面与Co₂MnSi合金的MnSi原子端面以桥位方式拼接而成MM-MS_B型界面。MM-MS_B型界面倾向于形成反铁磁交换耦合并且拥有较高的居里温度。详细的界面原子磁矩计算显示,MnGa和Co2MnSi在界面附近区域的原子磁矩受到强烈反铁磁交换耦合作用的影响均有所增强。界面电子结构的分析结果则表明,Co₂MnSi嵌入层有增强MnGa薄膜费米面附近电子自旋极化率的作用,从而使得整个隧道结的隧道磁电阻率升高。最后的界面电荷分布情况的计算分析表明,MnGa薄膜与Co₂MnSi嵌入层之间存在接触电势差,界面区域形成的内建电场促使MnGa薄膜内的电荷朝向Co₂MnSi区域迁移。电荷的聚散行为主要集中紧靠界面的前三层原子。综上所述,Co₂MnSi薄膜作为嵌入层扮演了自旋偏振片的角色,起到了增强MnGa薄膜的自旋极化率的作用。本文的研究工作阐明了Co₂MnSi嵌入层与MnGa薄膜之间的界面电子结构和界面原子的磁行为,对于Co₂MnSi嵌入层与MnGa薄膜之间的界面交换耦合机制给出了合理的解释。最后,我们建议利用电子自旋极化率100%的半金属Heusler合金制作成自旋偏振片嵌入到自旋器件当中提高电流的自旋极化程度。我们相信该方案有助于新型自旋器件的理论设计,对于相关的实验研究也具有十分重要的指导意义。更多还原

【Abstract】 The novel spin transfer torque magnetic random access memory for embedded memory combining the advantages of high recording density,high speed performance and low energy consumption.It is expected to completely replace the traditional magnetic storage device.MnGa alloys are an ideal electrode material for making novel spin random magnetic memory devices.The tetragonal MnGa alloy meets the requirement of stability for magnetic tunnel junctions due to the large perpendicular magnetic anisotropy.With low saturation magnetization,MnGa alloys are suitable for the free layers in magnetic tunnel junctions.The high spin polarizability of MnGa alloys provides a prerequisite for making magnetic tunnel junctions with a high tunnel magnetoresistance.The ultralow damping constant of MnGa alloy supports low power current-induce switching,and it is a key factor to achieve low power consumption.In addition,MnGa alloys are appropriate for high thermal stability since their high Curie temperature.However,the investigations on MnGa/MgO/MnGa perpendicular magnetic tunnel junction show its tunnel magnetoresistance is serious low.One reason is a large lattice mismatch between MnGa electrode and MgO barrier layer;another is that the interfacial diffusion of Mn atoms may bring magnetic impurities into MgO barrier.In order to solve the lattice mismatch and interfacial diffusion,large cubic Co-based Heusler alloys were formed as interlayer because the structure of MnGa alloys derive from Heusler.Co₂MnSi is a promising candidate interlayer due to the strong interfacial antiferromagnetic exchange coupling between MnGa and Co₂MnSi.The half-metallicity of Co₂MnSi can lead to a high magnetoresistance at room temperature.Co₂MnSi has a high Curie temperature and low damping constant,which guarantees high thermal stability and low power consumption.Moreover,the Co₂MnSi as a soft magnetic film is sensitive to the magnetic moment of the MnGa electrode.Finally,the MnGa/Co₂MnSi bilayer composite electrode not only alleviates the lattice mismatch but also overcomes the interfacial diffusion of Mn atoms.The key of research is the interfacial exchange coupling between the MnGa alloy and Co₂MnSi interlayer.The structure stability,electronic and magnetic properties of D0₂₂-and L1₀-MnGa/Co₂MnSi bilayers were studied by performing first-principles calculations based on density functional theory.The calculation results of the thermodynamic stability of MnGa/Co₂MnSi bilayers show that the interface configuration is likely the MM-MS_B model with antiferromagnetic exchange coupling in interface.The atom magnetic moments near the MnGa and Co₂MnSi interface are enhanced by strong antiferromagnetic exchange coupling.The electronic structure of interface reveal the Co₂MnSi embedded layer enhances the spin polarizability of MnGa electrode.Finally,there is contact potential difference between the MnGa electrode and the Co₂MnSi layer.The built-in electric field in the interface region leads to the charge migrate from the MnGa to the Co₂MnSi.In summary,the Co₂MnSi film plays a role of a spin-polarizer in enhancing the spin polarizability of the MnGa electrode.This research elucidates the electronic structure and magnetic behavior in MnGa/Co₂MnSi interface.A reasonable explanation is given for the interfacial exchange coupling mechanism between the MnGa thin-film and the Co₂MnSi interlayer.We propose a new scheme that a spin-polarizer is embedded in the spin device to increase spin polarizability of the electric current.We believe that this scheme has very important guiding significance for the design and research of spin devices.更多还原