兰州大学机构库 >物理科学与技术学院
过渡金属Cr中轨道霍尔效应和拓扑半金属PtSe2中自旋霍尔效应研究
Alternative TitleOrbital Hall Effect in Transition Metal Cr and Spin Hall Effect in Topological Semimetal PtSe2
谢宏斐
Subtype硕士
Thesis Advisor席力
2023-05-27
Degree Grantor兰州大学
Place of Conferral兰州
Degree Name理学硕士
Degree Discipline物理学
Keyword轨道霍尔效应 orbital Hall effect 轨道转矩 orbital torque 自旋霍尔效应 spin Hall effect 自旋轨道转矩 spin-orbital torque 磁矩翻转 magnetization switching
Abstract

在现代生活中,数字化、人工智能和大数据等诸多计算、分析和存储领域的发展给人们的生活带来了十足的便利。其中,信息存储作为一项基于数据而又服务于数据的关键领域,它的发展需要立足于存储介质的优化和创新。通过重金属的自旋霍尔效应,人们已经能够利用自旋流产生的自旋轨道转矩(SOT)操控磁矩的翻转来写入信息,并发展了存储密度较高、存储稳定性较好以及能耗较低的磁性随机存储器(MRAM)。为进一步降低器件功耗,如何提高自旋轨道转矩效率成为了科学家们深入研究的目标。在随后的研究过程中,科学家们发现由于原子的自旋轨道相互作用(SOI),轨道霍尔效应(OHE)和轨道流很有可能是过渡金属中大的自旋霍尔效应(SHE)的起源。将不同过渡金属的轨道霍尔电导(OHC)与自旋霍尔电导(SHC)的数值进行对比后发现,OHC的数值普遍高于SHC,更有甚者可以超出一个量级,这显示出轨道流以及轨道转矩(OT)在操控磁矩上的巨大潜力。为了验证、量化和分析OHE和OT对不同磁性器件中磁矩的影响,我们重点研究了过渡金属Cr的轨道霍尔效应。另外我们还研究了拓扑半金属PtSe2体系中的自旋流和SOT对面内磁化样品磁矩翻转的规律。通过原子力显微镜(AFM)、X射线衍射仪(XRD)和磁光克尔显微镜(MOKE)分别对样品的表面形貌、结构和磁性进行了表征,再通过谐波电压信号对OT/SOT效率进行了表征,最后我们利用OT/SOT实现了样品的磁矩翻转。得到的研究结果如下:

(1) 过渡金属Cr的轨道霍尔效应。利用磁控溅射技术,我们首先制备了Pt/Co/Pt对称结构,通过改变Pt层厚度,确定了能够使该结构具有稳定垂直磁各向异性的最小Pt层厚度,并以此为基础制备了Pt/Co/Pt/Cr磁性异质结,详细研究了Cr层厚度的改变对于磁性异质结垂直磁各向异性(PMA)和OT的影响。首先,我们发现Cr层厚度的变化对于异质结的PMA和样品磁矩在外磁场下的翻转过程均没有显著影响,这是因为Cr层的引入对Co/Pt界面几乎没有影响。然后,我们对Pt/Co/Pt/Cr异质结进行了反常霍尔和谐波电压信号的测试,测试结果表明,随着异质结中Cr层厚度的增加,异质结中的damping-like有效场和field-like有效场以及相应的转矩效率均有所增加,通过对比Cr层厚度为0 nm和10 nm的样品,异质结中的damping-like转矩效率增幅超过200%。随着进一步分析damping-like和field-like转矩效率随Cr层厚度的变化趋势,我们可以确定在Cr层厚度在4 - 6 nm区间内出现了相变,相变导致了damping-like转矩效率在该厚度区间内的缓慢增加,以及field-like转矩效率在该区间内的跳变。最后,通过对比Cr层厚度为4 nm和1 nm的异质结在相同测试条件下的电流诱导的磁化翻转回线,发现Cr层更厚的异质结,其临界翻转电流密度更小且翻转比例更大。这也进一步证实了Cr层产生的OT能够操控磁性异质结中的磁矩,并且可以实现更低的翻转电流密度。

(2) 拓扑半金属PtSe2中的自旋霍尔效应。我们首先通过AFM和XRD对不同厚度的PtSe2衬底进行了表面形貌和晶体结构的表征,结果表明在蓝宝石上通过化学气相沉积方式生长的PtSe2是具有(0 0 1)晶向择优取向的多晶,其中2 nm厚的PtSe2表面平均粗糙度比10 nm厚的PtSe2表面平均粗糙度大,其主要原因可能和Pt初期生长的岛状生长结构有关。然后,通过磁控溅射的方法将不同厚度的PtSe2制成结构均为PtSe2/FeNi/Ta的磁性异质结。MOKE测量结果表明两者在面内易磁化方向上的磁矩翻转过程存在差异,其根本原因是由于PtSe2/FeNi界面平整度的差异而导致的。随后,为了量化体系内部产生的自旋流,我们测量了谐波霍尔电压与外磁场之间的角度依赖关系,基于理论推导得到了不同自旋极化方向自旋流对谐波霍尔电压的依赖关系,我们确定了体系内部的自旋流由PtSe2的体SHE效应和PtSe2与FeNi的界面效应共同产生,其中PtSe2(10 nm)/FeNi/Ta 的有效自旋霍尔角θSH 为0.15,PtSe2(2 nm)/FeNi/Ta的θSH 为0.07。最后,我们对PtSe2(10 nm)/FeNi/Ta进行了电流翻转磁矩的测试,测试得到的临界翻转电流密度为7.7×106 A/cm2。得到的翻转回线表现出明显的不对称性,这可能与FeNi不是严格的单畴有关。实验得到较大的自旋霍尔角和较低的临界翻转电流密度说明PtSe2作为一种新型二维材料在自旋电子学领域拥有巨大的潜力和研究价值。

Other Abstract

In modern life, the development of data computing, analysis, and storage fields has brought full convenience to people's lives. Among them, information storage is a key field based on and serving data, and its development needs to be based on optimization and innovation of storage media. Through the spin Hall effect of heavy metals, people have been able to use spin-orbit torque (SOT) generated by spin current to manipulate the magnetization switching to write information, and have developed magnetic random-access memory (MRAM) with high storage density, good storage stability, and low energy consumption. In order to further reduce device power consumption, how to improve spin-orbit torque efficiency has become the goal of in-depth research by scientists. In the subsequent research process, scientists discovered that due to the atomic spin-orbit interaction (SOI), the orbital Hall effect (OHE) and orbital current are likely the origin of the large spin Hall effect (SHE) in transition metals. After comparing the orbital Hall conductivity (OHC) and spin Hall conductivity (SHC) of different transition metals, it was found that OHC are generally higher than SHC, and even can exceed an order of magnitude, indicating the great potential of orbital current and orbital torque (OT) in controlling magnetic moments. In order to verify, quantify, and analyze the effects of OHE and OT on magnetic moments in different magnetic devices, we focused on the orbital Hall effect of transition metal Cr. In addition, we have also studied the spin Hall effect and SOT from the topological semimetal PtSe2 system, which could induce magnetization switching of in-plane magnetized moments. The surface morphology, structure, and magnetism of the sample were characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), and magneto-optical Kerr microscope (MOKE), respectively. Then, the efficiency of OT/SOT was characterized by harmonic voltage signals. Finally, we realized the magnetization switching of samples by using OT/SOT. The obtained research results are as follows:

       (1) Orbital Hall effect of transition metal Cr. Using magnetron sputtering technology, we first prepared a Pt/Co/Pt symmetric structure. By changing the thickness of the Pt layer, we determined the minimum thickness of Pt layer that can make the structure have stable perpendicular magnetic anisotropy. Based on this, we prepared a Pt/Co/Pt/Cr magnetic heterostructures, and carefully studied the effect of changing the thickness of the Cr layer on the perpendicular magnetic anisotropy (PMA) and OT of the magnetic heterostructures. First, we found that the change of the Cr layer thickness has no significant effect on the PMA of the heterostructures, because the insertion of the Cr layer does not destroy the flat Co/Pt interface, so there is no significant difference in the switching process of the sample magnetization under the external magnetic field. Then, we tested the anomalous Hall and harmonic voltage signals on the Pt/Co/Pt/Cr heterostructures, the results showed that the damping like effective field and field like effective field in the heterostructures increased, as well as the corresponding torque efficiency with the increase of Cr layer thickness in the heterostructures. With further analysis of the trend of damping-like and field-like torque efficiency with Cr layer thickness, we can determine that there is a phase transition in the range of Cr layer thickness from 4 to 6 nm, which leads to a slow increase in damping-like torque efficiency within this thickness range, as well as a jump in field-like torque efficiency within this range. Finally, by comparing the current-induced magnetization switching loops of heterostructures with Cr layer thickness of 4 nm and 1 nm under the same test conditions, it is found that heterostructures with thicker Cr layers have smaller critical switching current density and larger switching ratio. This result further confirms that the OT generated by the Cr layer can manipulate the magnetization in this heterostructures, and achive a lower switching current density.

      (2) Spin Hall effect in topological semimetal PtSe2. We first characterized the surface morphology and crystal structure of PtSe2 substrates with different thicknesses by AFM and XRD. The results showed that PtSe2 grown on sapphire by chemical vapor deposition has polycrystalline structures with (001) preferred orientation, and the average surface roughness of PtSe2 with a thickness of 2 nm is greater than that of PtSe2 with a thickness of 10 nm. The main reason may be related to the island like growth structure of Pt at the initial growth stage. Then, PtSe2 with different thickness were made into magnetic heterostructures with the structure of PtSe2/FeNi/Ta by magnetron sputtering. The MOKE measurement results show that two samples have a difference in the magnetization switching process at the in-plane easy magnetization direction, which is due to the difference in the flatness of the PtSe2/FeNi interface. Subsequently, in order to quantify the spin current generated inside the system, we measured the angular dependence between the harmonic Hall voltage and the external magnetic field. According to the theoretical approach based on the dependence of the spin current which has different polarization directions on the harmonic Hall voltage, we determined that the spin current inside the system is generated jointly by the bulk SHE of PtSe2 and the interface effect between PtSe2 and FeNi, where the effective spin Hall angle θSH  of PtSe2(10 nm)/FeNi/Ta is 0.15, and the θSH  of PtSe2(2 nm)/FeNi/Ta is 0.07. Finally, we conduct the current-induced magnetization switching test on PtSe2(10 nm)/FeNi/Ta, and the critical switching current density obtained from the test is 7.7×106 A/cm2. The obtained switching loop exhibits significant asymmetry. It maybe explained by the non-single domain states of FeNi at the Hall cross. However, the large spin Hall angle and low critical switching current density still indicate that PtSe2, as a new two-dimensional material, has great potential in the field of spintronics.

Subject Area自旋电子学
MOST Discipline Catalogue理学 - 物理学 - 凝聚态物理
URL查看原文
Language中文
Other Code262010_220200938001
Document Type学位论文
Identifierhttps://ir.lzu.edu.cn/handle/262010/535504
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
谢宏斐. 过渡金属Cr中轨道霍尔效应和拓扑半金属PtSe2中自旋霍尔效应研究[D]. 兰州. 兰州大学,2023.
Files in This Item:
There are no files associated with this item.
Related Services
Recommend this item
Bookmark
Usage statistics
Export to Endnote
Altmetrics Score
Google Scholar
Similar articles in Google Scholar
[谢宏斐]'s Articles
Baidu academic
Similar articles in Baidu academic
[谢宏斐]'s Articles
Bing Scholar
Similar articles in Bing Scholar
[谢宏斐]'s Articles
Terms of Use
No data!
Social Bookmark/Share
No comment.
Items in the repository are protected by copyright, with all rights reserved, unless otherwise indicated.