兰州大学机构库 >物理科学与技术学院
垂直各向异性磁异质结构中 电流诱导的转矩及磁化翻转研究
Alternative TitleStudy on current-induced torques and magnetization switching in magnetic heterostructures with perpendicular anisotropy
郭永海
Subtype博士
Thesis Advisor曹江伟
2023-08-28
Degree Grantor兰州大学
Place of Conferral兰州
Degree Name理学博士
Degree Discipline物理学
Keyword自旋极化 Spin polarization 无场翻转 Field-free magnetization switching 亚铁磁 Ferrimagnetic 轨道霍尔效应 Orbital Hall effect 自旋轨道耦合 Spin orbit coupling 轨道霍尔电导率 Orbital Hall conductivity
Abstract

由于自旋电子器件具备非易失性、高速读写、耐久性等特征,因此其在信 息存储、传感和存算一体器件方面吸引了人们广泛的关注。在最新的自旋轨道 矩(Spin-orbit torque, SOT)-磁 性 随 机 存 储 器(Magnetic random access memory, MRAM)中,利用电流诱导的转矩效应驱动铁磁层磁化翻转来实现信息的写入。 虽然 SOT 器件具有写入速度快、读写分离、对器件损伤较小等优势,但在该器 件实用化的过程中仍然面临着如何实现无场条件下电流驱动的磁化翻转,以及 如何进一步降低翻转所需的临界电流密度等问题,因为前者可以提高器件微型 化与集成化,而后者可以有效降低器件的功耗。本文围绕以上两个关键科学问 题,以具有垂直磁各向异性(Perpendicular magnetic anisotropy, PMA)的铁磁/非磁/铁磁三层结构为基础,研究了该体系中电流产生的自旋转矩效应以及无场条件 下的电流驱动磁化翻转;进一步地,在利用电子自旋角动量的基础上,我们利 用了电子的轨道角动量,开展了如何利用轨道流效应来降低电流驱动磁化翻转 的临界电流密度的研究,取得的主要创新性结果如下: 1、制备了 Co(PMA)/Ti/CoPt 磁异质结构,希望利用 CoPt单层中存在较大的电流 诱导 SOT 效应及界面处的自旋轨道进动(Spin orbit precession, SOP)效应来实现无 场条件下电流诱导的磁化翻转。磁阻端、霍尔端电阻的二次谐波法测量结果表 明,该体系中除了普通的横向自旋极化(??̂y)自旋流之外,还存在沿电流方向自旋 极化(??̂x )的自旋流,证明了界面处 SOP 效应的存在;但在反常霍尔效应(Anomalous Hall effect, AHE)回线偏移的测量中并未观测到垂直方向极化(??̂z )的 自旋流,也未能在该体系中实现无场条件下电流诱导的磁化翻转,这可能是由 于 Ti/CoPt 界面处 SOP 效应较弱,或者由于 AHE 曲线测量时回线偏移与外加电 流之间的非线性所导致的。 2、在优化的 Co/Ti/Tb-Co(PMA) 的三层结构中,发现电流驱动的具有垂直各向 异性 Tb-Co 层的翻转极性依赖于面内磁化的 Co 磁矩沿电流方向的分量,进而实 现了无场条件下电流诱导的确定性磁化翻转。通过二次谐波和 AHE 回线偏移的 方法测量了该体系中的类阻尼(Damping-like, DL)-SOT 有效场,结果表明该体系中存在??̂y和??̂z的自旋流,前者可能来源于 Co 层或 Tb-Co/Ti 及 Ti/Co 界面,而后 者可以用 Co/Ti 界面上的 SOP 效应来解释。 3、确认了不同铁磁层与 Cr 组成双层结构中电流诱导力矩具有不同的符号,这 是轨道流产生力矩效应(轨道矩)的一个明显特征,其归因于铁磁层中不同的轨 道-自旋转化效率。在此基础上我们研究了 Pt/Co(PMA)/Tb/Cr 结构中电流诱导转 矩随 Tb 插入层和 Cr 层厚度的变化,发现改变两者厚度都能够有效调控电流诱 导的转矩大小,其效率在 Tb 厚度为 4 nm 时达到最大值;并利用轨道流扩散模 型拟合了DL力矩效率随Cr厚度的依赖性,得到Cr的有效轨道霍尔角为−0.18 ± 0.01。电流诱导磁化翻转测量表明该体系中的利用 Cr 层产生的轨道流能够有效 降低临界翻转电流密度。 4、 发现了 Ti/Tb-Co/Cr 样品中大的 DL 力矩效率和高效的电流诱导磁化翻转。 二次谐波测量表明该体系中电流诱导力矩与 Tb-Co 层的成分相关,这与在普通 重金属/铁磁结构中的现象是不同的。通过简单的轨道流扩散模型拟合电流诱导 的 DL 力矩对 Cr 厚度的依赖性,在Ti/Tb0.85Co0.15/Cr 样品中获得了−0.57 ± 0.02的有效轨道霍尔角;而在 Ti/Co/Cr 和 Ti/Tb-Co/SiN 对照样品中仅观察到了非常 低的效率。这些结果表明,利用稀土-3d 亚铁磁合金作为磁性层能够有效提高对 于轨道流的吸收,进一步增加电流诱导转矩的效率。

Other Abstract

Spintronic devices have attracted widespread attention in the field of integrated devices for information storage, sensing, and storage due to their non-volatile, highspeed read and write, and durability characteristics. In the latest spin orbit torque (SOT) magnetic random access memory (MRAM), the current induced torque effect is utilized to drive the magnetization reversal of the ferromagnetic layer to achieve information writing. Although SOT devices have advantages such as fast write speed, separation of read and write, and minimal damage to the device, they still face issues such as how to achieve current driven magnetization switching under fieldless conditions and how to further reduce the critical current density required for switching, as the former can improve device miniaturization and integration, while the latter can effectively reduce device power consumption. Based on the ferromagnetic/non-magnetic/ferromagnetic three-layer structure with perpendicular magnetic anisotropy (PMA), this paper focuses on the above two key scientific problems, and studies the spin-torque effect caused by current in the system and the current-driven magnetization switching under the condition of no field. Furthermore, on the basis of using the electron spin Angular momentum, we used the electron orbital Angular momentum to carry out research on how to use the orbital current effect to reduce the critical current density of current driven magnetization reversal. The main innovative results are as follows: 1, Co(PMA)/Ti/CoPt magnetic heterostructures were prepared, aiming to utilize the significant current induced SOT effect in CoPt monolayers and the spin orbit precession (SOP) effect on the interface to achieve current induced magnetization reversal under fieldless conditions. The measurement results of the magnetoresistive and hall terminal resistances by the second harmonic method show that in this system, except for the ordinary transverse spin polarization(??̂y ) current, there is also spin polarization along the current direction(??̂x ),The spin current proves the existence of SOP effect on the interface; However, in the measurement of Anomalous Hall effect(AHE)loop shift, the spin current of vertical polarization(??̂z )was not observed, and the current-induced magnetization switching in this system under the condition of no field was not achieved, which may be due to the weak SOP effect at the Ti/CoPt interface. Or it is caused by the nonlinearity between the shift of the loop line and the applied current during AHE curve measurement. 2, In the optimized Co/Ti/ Tb-Co (PMA) three-layer structure, it is found that the switching polarity of the current-driven Tb-Co layer depends on the component of the Co magnetic moment of the in-plane magnetization along the current direction, thus achieving the current-induced deterministic magnetization switching under the fieldfree condition. The Damping-like (DL)-SOT effective field in the system is measured by the second harmonic and AHE loop shift method. The results show that there are spin currents (??̂y) and (??̂z ) in the system, and the former may originate from the Co layer or the Tb-Co/Ti and Ti/Co interfaces. While the latter can be explained by the SOP effect on the Co/Ti interface. 3, It is confirmed that the current-induced torques have different signs in the double-layer structure composed of different ferromagnetic layers and Cr, which is an obvious feature of the orbital current generating torque effect (orbital torque), which is attributed to the different orbit-spin conversion efficiency in the ferromagnetic layer. On this basis, we study the variation of current-induced torque with the thickness of Tb insertion layer and Cr layer in Pt/Co (PMA)/Tb/Cr structure. It is found that changing the thickness of both Tb insertion layer and Cr layer can effectively regulate the current-induced torque, and the maximum efficiency is reached when the thickness of Tb is 4 nm. Furthermore, the dependence of DL torque efficiency on Cr thickness is fitted using the orbital flow diffusion model, and the effective orbital Hall Angle of Cr is -0.18±0.01. The measurement of current-induced magnetization switching shows that the critical flip current density can be effectively reduced by using the orbital current generated by the Cr layer in the system. 4, Large DL torque efficiencies and efficient current-induced magnetization switching in Ti/Tb-Co/Cr samples were found. Second harmonic measurements show that the current-induced torque is dependent on the composition of the Tb-Co layer, which is different from the phenomenon in ordinary heavy metal/ferromagnetic structures. A simple orbital current diffusion model is used to fit the dependence of DL moment on Cr thickness, and an effective orbital Hall Angle of -0.57±0.02 is obtained for Ti/Tb0.85Co0.15/Cr samples. However, only very low efficiencies were observed in Ti/Co/Cr and Ti/Tb-Co/SiN control samples. These results indicate that the use of rareearth-3d ferromagnetic alloys as a magnetic layer can effectively improve the absorption of orbital current and further increase the efficiency of current-induced torque.

MOST Discipline Catalogue理学 - 物理学 - 凝聚态物理
URL查看原文
Language中文
Other Code262010_120190906751
Document Type学位论文
Identifierhttps://ir.lzu.edu.cn/handle/262010/536647
Collection物理科学与技术学院
Affiliation
兰州大学物理科学与技术学院
Recommended Citation
GB/T 7714
郭永海. 垂直各向异性磁异质结构中 电流诱导的转矩及磁化翻转研究[D]. 兰州. 兰州大学,2023.
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