[论文解读] High magnetoresistance of hexagonal boron nitride-graphene heterostructure-based MTJ through excited-electron transmission

Halimah Harfah, Yusuf Wicaksono+3 more|arXiv (Cornell University)|Apr 18, 2021

Graphene research and applications参考文献 50被引用 8

一句话总结

本研究提出了一种基于六方氮化硼-石墨烯-hBN（hBN-Gr-hBN）异质结的磁性隧道结（MTJ），在略高于费米能级的0.34 eV处实现了约1200%的记录性高隧穿磁阻（TMR）比。高TMR源于Ni/hBN界面处d^z²轨道表面态与氮原子p^z轨道的激发电子传输，该过程通过石墨烯的邻近效应得到增强，从而强化界面杂化并抑制传输损耗。

ABSTRACT

This work presents an ab-initio study of a few-layers hexagonal boron nitride (hBN) and hBN-graphene heterostructure sandwiched between Ni(111) layers. The aim of this study is to understand the electron transmission process through the interface. Spin-polarized density functional theory calculations and transmission probability calculations were conducted on Ni(111)/$n$hBN/Ni(111) with $n$ = 2, 3, 4, and 5 as well as on Ni(111)/hBN-Gr-hBN/Ni(111). Slabs with magnetic alignment in an anti-parallel configuration (APC) and parallel configuration (PC) were considered. The pd-hybridizations at both the upper and lower interfaces between the Ni slabs and hBN were found to stabilize the system. The Ni/nhBN/Ni magnetic tunnel junction (MTJ) was found to exhibit a high tunneling magnetoresistance (TMR) ratio at ~0.28 eV for $n$ = 2 and 0.34 eV for $n$ > 2, which are slightly higher than the Fermi energy. The observed shifting of this high TMR ratio originates from the transmission of electrons through the surface states of the $d_{z^2}$-orbital of Ni atoms at interfaces which are hybridized with the $p_z$-orbital of N atoms. In the case of $n$ > 2, the proximity effect causes an evanescent wave, contributing to decreasing transmission probability but increasing the TMR ratio. However, TMR ratio, as well as transmission probability, was found to be increased, by replacing the unhybridized hBN layer of the Ni/3hBN/Ni MTJ with graphene, thus becoming Ni/hBN-Gr-hBN/Ni. A TMR ratio as high as ~1200% was observed at an energy of 0.34 eV, which is higher than the Fermi energy. Furthermore, a design is proposed for a device based on a new reading mechanism using the high TMR observed just above the Fermi energy level.

研究动机与目标

• 采用从头算方法研究Ni/nhBN/Ni和Ni/hBN-Gr-hBN/Ni磁性隧道结（MTJs）中的电子传输机制。
• 理解在hBN基MTJs中，费米能级以上能量处观测到的高隧穿磁阻（TMR）比的起源。
• 探讨在hBN势垒中引入石墨烯后，通过界面杂化与邻近效应增强TMR的机制。
• 基于费米能级以上附近观测到的高TMR，提出一种新型器件机制，以期用于自旋电子学应用。

提出的方法

• 采用自旋极化密度泛函理论（SP-DFT）模拟Ni(111)/nhBN/Ni与Ni/hBN-Gr-hBN/Ni异质结的电子结构，其中hBN层数n分别为2、3、4、5层。
• 利用Landauer-Büttiker形式化方法计算透射概率，评估平行（PC）与反平行（APC）磁性构型下的TMR比。
• 分析Ni d^z²轨道与Ni/hBN界面处N p^z轨道之间的轨道杂化，以确定特定能量下透射增强的根源。
• 研究多层hBN体系中邻近效应的作用，特别是倏逝波的形成及其对透射的影响。
• 对比纯hBN势垒（Ni/nhBN/Ni）与石墨烯掺杂hBN势垒（Ni/hBN-Gr-hBN/Ni）的透射与TMR性能，以分离石墨烯的功能角色。
• 采用SIESTA代码，结合DFT+D3修正与PBE泛函，确保对范德华相互作用与电子性质的准确描述。

实验结果

研究问题

• RQ1 为何Ni/nhBN/Ni MTJ中的最高TMR比出现在~0.28 eV（n=2）和0.34 eV（n>2），而非费米能级处？
• RQ2 Ni/hBN界面处d^z²-p^z杂化在高能区实现高TMR中扮演何种角色？
• RQ3 在hBN势垒中引入石墨烯层（形成hBN-Gr-hBN）如何影响透射概率与TMR比？
• RQ4 与纯Ni/nhBN/Ni体系相比，Ni/hBN-Gr-hBN/Ni中观察到的TMR增强背后的机制是什么？
• RQ5 在费米能级以上附近观测到的高TMR是否可为自旋电子学器件提供一种新型读出机制？

主要发现

• Ni/nhBN/Ni MTJ在n=2时TMR比高达~0.28 eV，在n>2时高达~0.34 eV，两者均略高于费米能级，表明存在非费米能级的TMR极大值。
• 在0.34 eV处的高TMR源于Ni d^z²轨道与界面处N p^z轨道杂化形成的表面态所引发的电子传输。
• 对于n>2的情况，邻近效应诱导出倏逝波，虽降低透射概率，但因增强自旋滤波效应而提高TMR比。
• 将Ni/3hBN/Ni中未杂化的中心hBN层替换为石墨烯（形成Ni/hBN-Gr-hBN/Ni）后，透射概率与TMR比均显著提升。
• Ni/hBN-Gr-hBN/Ni异质结在0.34 eV处实现了高达~1200%的TMR比，显著超过费米能级，优于传统MTJ。
• 石墨烯层增强了Ni d^z²轨道与hBN中B原子p^z轨道之间的邻近效应，强化界面杂化，从而实现高TMR。