Spin-Orbit Torque and Field-Free Magnetization Switching by Topological Insulators

Spin-Orbit Torque and Field-Free Magnetization Switching by Topological Insulators
Author: Xiaoyu Che
Publisher:
Total Pages: 131
Release: 2020
Genre:
ISBN:

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Spin accumulation originating from relativistic spin-orbit interaction can exert spin-orbit torque (SOT) on adjacent magnetization and switch the direction of the magnetization. Such current-driven magnetization switching holds promise for miniaturized magnetic memory devices with high speed, low power consumption, and non-volatility. One of the promising material candidates is topological insulator (TI). TI is a novel quantum state of matter with gapped bulk bands and gapless surface states protected by topology. The spin and momentum of the carriers from the surface states are locked, and the spin-momentum locked carriers can drive giant SOT on neighboring magnetization. However, there are still several issues in the study of SOT from TIs. First, there have been large discrepancies in the reported values of SOT from TIs. Second, the magnetization switching by TIs requires the assistance of an external magnetic field, which results in limited applicability. In this dissertation, I systematically investigate magnetically doped TI thin films and determine the SOT via both transport and optic approaches, collaborating with other group members. Large SOT generated by the topological surface states with consistent results is observed. The experimental results reveal a strong dependence of SOT on temperature and surface state carrier concentration. The SOT decreases drastically as temperature increases and can be manipulated by tuning the surface state carrier concentration. A competition between the top surface and bottom surface in contributing to SOT is also observed. The above phenomena could account for the large discrepancies in the reported SOT values. Utilizing the SOT from surface states, I am able to achieve current-driven magnetization switching in magnetically doped TIs. I also investigate TI/antiferromagnetic material heterostructures and demonstrate the realization of field-free magnetization switching in this material system. Accomplished by symmetry breaking with interfacial exchange-bias, the field-free switching can be driven by pulsed current with ultra-low current density. The study in this dissertation advances the understanding of SOT from TI as well as the implementation of practical and energy-efficient magnetic random-access memory.

Spin-Orbit Torques in Topological Insulator-based Magnetic Structures

Spin-Orbit Torques in Topological Insulator-based Magnetic Structures
Author: Yabin Fan
Publisher:
Total Pages: 148
Release: 2016
Genre:
ISBN:

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During recent years, enormous progress has been made in the spintronics research field which utilizes the spin degree of freedom of electrons in addition to their charge for information processing with the goal to achieve non-volatile spintronic memory and logic devices with fast speed, high density, good reliability and low power consumption. In particular, novel materials have been incorporated in the magnetic structures to realize manipulation and/or switch of magnetic moment using the least possible power. Among them, heavy metals and topological insulators (TIs), which exhibit strong spin-orbit coupling, have been employed to generate spin-orbit torques (SOTs) to enable efficient switching of magnetic moment, which may thus lead to the next generation green spintronic devices. In this work, we will explore the current-induced SOTs in TI-based magnetic structures. First, we present the magnetization switching through giant SOT induced by an in-plane current in a TI/Cr-doped TI bilayer heterostructure. The critical current density required for switching is below 8.9i 10^4 A/cm^2 at 1.9 K. Both the effective spin-orbit field to current ratio and the spin-torque efficiency are found to be three orders of magnitude larger than those reported for conventional heavy metal/ferromagnet heterostructures. Second, we show the effective electric-field control of the giant SOT in a uniformly Cr-doped TI thin film using a top-gate field-effect transistor structure. We demonstrate that the SOT strength can be modulated by a factor of 4 within the accessible gate voltage range, and it shows strong correlation with the spin-polarized surface current arising from surface spin-momentum locking in the film. Furthermore, we demonstrate the magnetization switching by scanning gate voltage with constant current and in-plane magnetic field applied in the Cr-doped TI thin film. Last but not least, we summarize the research results and discuss future potential research opportunities and challenges in this field. There are still many unresolved questions in this new research field. Nevertheless, the giant current-induced SOT we have observed in TI-based magnetic structures suggests that it might have wide implications in the next generation gate-controlled, ultralow power spintronic devices.

Spin-Orbit Torques in Topological Insulators

Spin-Orbit Torques in Topological Insulators
Author: Qiming Shao
Publisher:
Total Pages: 75
Release: 2015
Genre:
ISBN:

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Spin torque can toggle magnetization state of nanomagnets in magnetic tunnel junctions. In heavy metals, giant Spin Hall Effect gives rise to a large spin-orbit torque (SOT) enabling magnetization switching of adjacent ferromagnets. Spin torque ratio is the key parameter in determining energy efficiency. Recently, a giant SOT was demonstrated in topological insulators (TIs), which are a class of materials owing insulating bulk and metallic surface states. Although several theoretical studies show that spin-momentum locking of surface states generates this giant SOT, experimental clarification is still lacking. In this thesis, we use the second-harmonic anomalous Hall resistance to access the current-induced spin-orbit fields (and thus SOTs). Heavy metal/ferromagnet hetoerstructure, Ta/CoFeB/MgO, is used to verify the validation of this method. Then, we quantitatively estimate the SOT in different TI/chromium-doped TI heterostructures with different magnetic dopant positions, which shows the evidence of surface states originated SOTs. At last, we show that the magnitude of SOTs can be controlled by the gate voltage due to its semiconducting nature. All these studies may contribute to the future low power spintronics.

A Study of Spin-orbit Torques and Deterministic Field-free Switching of Perpendicular Magnetization

A Study of Spin-orbit Torques and Deterministic Field-free Switching of Perpendicular Magnetization
Author: Seyed Armin Razavi
Publisher:
Total Pages: 160
Release: 2020
Genre:
ISBN:

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Spintronic devices provide an energy-efficient platform for implementing non-volatile memory and logic. In spintronic memory devices, the information is stored in the magnetization state of the free magnetic layer, and current, voltage or strain-induced mechanisms are used for switching the magnetization. Among the operation methods, current-induced spin-orbit torque (SOT) is a promising mechanism for magnetization switching with faster dynamics, higher endurance, and potentially higher energy efficiency compared to the conventional spin-transfer torque. For magnetic memory applications, perpendicular magnetic anisotropy is desirable since it enables high memory densities. However, for deterministic SOT switching of a perpendicular magnet, an external magnetic field collinear with the current is required, which hampers the applicability of SOT switching. Although there have been several prior effects aimed at solving this problem, but most involve structural asymmetries or additional layers that are not practical for large wafer scale applications. Consequently, a practical realization of deterministic field-free SOT switching of perpendicular magnetization remains a challenge.In this dissertation, we present two main methods for realizing deterministic field-free SOT switching for practical uses. First, we show that the external in-plane field can be replaced by a built-in exchange bias field using antiferromagnetic materials. We also show that certain antiferromagnets can create the SOTs themselves, serving as the layer providing both the exchange bias and SOTs. As the second approach, we use the concept of structural asymmetry reported previously, and modify the conventional SOT heterostructure by inserting a slightly asymmetric light-metal at the heavy-metal/ferromagnet interface. The broken structural symmetry enables the creation of current-induced out-of-plane effective magnetic fields, which break the symmetry between the up and down states for each current polarity and allow for deterministic SOT switching at zero external magnetic field. We also apply the asymmetry concept to a second material system with a minimal structural asymmetry, resulting in an enhanced magnetic uniformity across large wafer areas. We show that the latter approach for field-free SOT switching has almost all the characteristics of a practical solution that could be used in applications. We also present a better understanding of the deterministic switching process enabled by the structural asymmetry by examining its microscopic origins. In the last part of this dissertation, we discuss several other interesting aspects of the antiferromagnet-based material system (IrMn/CoFeB/MgO) that we primarily developed for the field-free SOT switching. We show that this material system has several unique properties, including the simultaneous presence of Dzyaloshinskii-Moriya interaction, voltage-control of magnetic anisotropy (VCMA), exchange bias field, and spin-orbit torques. We use this material system as a platform for studying magnetic skyrmions, where we can create/annihilate skyrmions using single voltage pulses without any external magnetic fields via the VCMA effect. Furthermore, we use spin-orbit torques to move the skyrmions in the same structure, providing a promising platform for skyrmion-based device applications.

Room Temperature Spin-orbit Torque Switching Induced by a Topological Insulator

Room Temperature Spin-orbit Torque Switching Induced by a Topological Insulator
Author: Jiahao Han
Publisher:
Total Pages: 63
Release: 2018
Genre:
ISBN:

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Recent studies on the topological insulators have attracted great attention due to the rich spin-orbit physics and promising applications in spintronic devices. The strongly spin-moment coupled electronic states have been extensively pursued to realize efficient spin-orbit torque switching. However, so far current-induced magnetic switching with topological insulators has been observed only at cryogenic temperatures. Whether the topologically protected electronic states in topological insulators can benefit to spintronic applications at room temperature remains a controversial issue. In this thesis, spin-orbit-torque-induced magnetic switching is realized in topological insulator/ferrimagnet heterostructure at room temperature. Ferrimagnetic CoTb alloy with robust bulk perpendicular magnetic anisotropy is directly grown on a classical topological insulator Bi2 Se3. The low switching current density provides definitive proof of the high spin-orbit torque efficiency from topological insulators. The comparison between Bi2Se3 and (Bi,Sb)2Te3 with less bulk conductivity suggests the surface states plays a significant role in generated the efficient spin-orbit torques. Furthermore, the effective spin Hall angle of topological insulators is determined to be several times larger than commonly used heavy metals. These results demonstrate the robustness of topological insulators as a spin-orbit torque switching material and provide an avenue towards applicable topological insulator-based spintronic devices.

Spin Current

Spin Current
Author: Sadamichi Maekawa
Publisher: Oxford University Press
Total Pages: 541
Release: 2017
Genre: Science
ISBN: 0198787073

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In a new branch of physics and technology, called spin-electronics or spintronics, the flow of electrical charge (usual current) as well as the flow of electron spin, the so-called "spin current", are manipulated and controlled together. This book is intended to provide an introduction and guide to the new physics and applications of spin current.

Spin Orbit Torque Driven Magnetic Switching for Low Power Computing and Memory

Spin Orbit Torque Driven Magnetic Switching for Low Power Computing and Memory
Author: Debanjan Bhowmik
Publisher:
Total Pages: 123
Release: 2015
Genre:
ISBN:

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Spintronics has rapidly emerged as a highly pursued research area in solid-state physics and devices owing to its potential application in low power memory and logic as well as the rich physics associated with it. Traditionally in spintronics, spin transfer torque in magnetic tunnel junctions and spin valves has been used to manipulate ferromagnets. Spin orbit torque has recently emerged as an alternative mechanism for manipulating such ferromagnets, which offers advantages of lower energy consumption, simpler device structure, etc. For a ferromagnet- heavy metal bilayer, electrons flowing through the heavy metal separate based on the direction of their spin. This results in the accumulation of spin polarized electrons at the interface, which in turn applies a torque, known as spin orbit torque, on the ferromagnet. A typical such heavy metal is tantalum (Ta) and typical such ferromagnet is CoFeB. The research presented in this dissertation shows how in a perpendicularly polarized Ta/CoFeB/MgO heterostructure, spin orbit torque at the interface of the Ta and CoFeB layers can be used to manipulate the magnetic moments of the CoFeB layer for low power memory and logic applications. The main results presented in this dissertation are fourfold. First, we report experiments showing spin orbit torque driven magnetic switching in a perpendicularly polarized Ta/CoFeB/MgO heterostructure and explain the microscopic mechanism of the switching. Using that microscopic mechanism, we show a new kind of ferromagnetic domain wall motion. Traditionally a ferromagnetic domain wall is known to flow parallel or antiparallel to the direction of the current, but here we show that spin orbit torque, owing to its unique symmetry, can be used to move the domain wall orthogonal to the current direction. Second, we experimentally demonstrate the application of this spin orbit torque driven switching in nanomagnetic logic, which is a low power alternative to CMOS based computing. Previous demonstrations of nanomagnetic logic needed an external magnetic field, the generation of which needed a large amount of current rendering such logic scheme uncompetitive compared to its CMOS counterpart. Here we show that spin orbit torque eliminates the need of an external magnetic field for nanomagnetic logic and hence spin orbit torque driven nanomagnetic logic consumes 100 times lower current than magnetic field driven nanomagnetic logic at room temperature. Though we can demonstrate magnetic logic with spin orbit torque in the absence of the magnetic field, spin orbit torque driven deterministic switching of a perpendicular magnet from up to down and down to up still needs the application of an external magnetic field unless the symmetry of the system is broken. This renders such switching scheme not very useful for real memory devices. In the third part of the thesis, we show through micromagnetic simulations that if the magnet has a wedge shape, the symmetry of the system is broken and the magnet can be deterministically switched from up to down and down to up even in the absence of an external magnetic field. Our simulations are supported by recent experiments, performed in our group. In the last part, we show how a bilayer of two heavy metals (Ta and Pt) can be used to increase the spin orbit torque efficiency. Interfaces of ferromagnet with Ta and that of ferromagnet with Pt exhibit spin orbit torques in opposite directions, so it is expected that their effects will cancel. Instead, in our experiments we find that the spin orbit torque efficiency at the Ta/CoFeB interface increases if a Pt layer exists under the Ta layer. Modeling of the system based on conventional spin transport physics cannot explain this result.

Emergent Transport Properties of Magnetic Topological Insulator Heterostructures

Emergent Transport Properties of Magnetic Topological Insulator Heterostructures
Author: Kenji Yasuda
Publisher: Springer Nature
Total Pages: 109
Release: 2020-09-07
Genre: Computers
ISBN: 981157183X

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This book reveals unique transport phenomena and functionalities in topological insulators coupled with magnetism and superconductivity. Topological insulators are a recently discovered class of materials that possess a spin-momentum-locked surface state. Their exotic spin texture makes them an exciting platform for investigating emergent phenomena, especially when coupled with magnetism or superconductivity. Focusing on the strong correlation between electricity and magnetism in magnetic topological insulators, the author presents original findings on current-direction-dependent nonreciprocal resistance, current-induced magnetization reversal and chiral edge conduction at the domain wall. In addition, he demonstrates how the coupling between superconductivity and topological surface state leads to substantial nonreciprocal resistance. The author also elucidates the origins of these phenomena and deepens readers’ understanding of the topologically nontrivial electronic state. The book includes several works which are published in top journals and were selected for the President’s Award by the University of Tokyo and for the Ikushi Prize, awarded to distinguished Ph.D. students in Japan.

Handbook of Magnetic Materials

Handbook of Magnetic Materials
Author: Ekkes Bruck
Publisher: Academic Press
Total Pages: 82
Release: 2022-11-17
Genre: Science
ISBN: 0323986021

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Handbook of Magnetic Materials, Volume 31 highlights new advances in the field, with this new volume presenting interesting chapters on a variety of timely and field specific topics, each contributed to by an international board of authors. Provides the authority and expertise of leading contributors from an international board of authors Presents the latest release in the Handbook of Magnetic Materials series

Spin-orbit Torque Switching of Compensated Ferrimagnetic Cobalt-terbium Alloys

Spin-orbit Torque Switching of Compensated Ferrimagnetic Cobalt-terbium Alloys
Author: Joseph Tyler Finley
Publisher:
Total Pages: 54
Release: 2017
Genre:
ISBN:

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Spintronic devices promise to be an energy efficient alternative to complementary metal oxide semiconductor devices for logic and memory. However, in order to be more competitive, further reductions in switching energy and switching speed are needed. Recently, there has been interest in using antiferromagnetically coupled materials instead of ferromagnetic materials to store information. Compared with ferromagnetic materials, antiferromagnetically coupled systems exhibit faster dynamics and are more stable against external magnetic field perturbations, which could potentially enable spintronic devices with higher speed and density. Despite the potential advantages of information storage in antiferromagnetically coupled materials, it remains uncertain whether one can efficiently control the magnetic state because of the cancelled net magnetic moment. This thesis reports spin-orbit torque induced magnetization switching of ferrimagnetic Co1-xTbx thin films with perpendicular magnetic anisotropy. By varying the relative concentrations of the two atomic species, one can reach compensation points where the net magnetic moment or angular momentum goes to zero. We demonstrate current induced switching in all of the studied film compositions, including those near the magnetization compensation point. We then quantify the spin-orbit torque induced effective field, where we find that close to the compensation point, there is a divergent behavior that scales with the inverse of the magnetization, consistent with angular momentum conservation. The large effective spin-orbit torque, previously demonstrated fast dynamics, and small net magnetization in these ferrimagnetic systems promise spintronic devices that are faster and more scalable than traditional ferromagnetic systems.