Spintronics for Next Generation Innovative Devices

Spintronics (short for spin electronics, or spin transport electronics) exploits both the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-state devices. Controlling the spin of electrons within a device can produce surprising and substantial changes in its properties.

Drawing from many cutting edge fields, including physics, materials science, and electronics device technology, spintronics has provided the key concepts for many next generation information processing and transmitting technologies. This book discusses all aspects of spintronics from basic science to applications and covers:

• magnetic semiconductors
• topological insulators
• spin current science
• spin caloritronics
• ultrafast magnetization reversal
• magneto-resistance effects and devices
• spin transistors
• quantum information devices

This book provides a comprehensive introduction to Spintronics for researchers and students in academia and industry.

• Language: English
• Publisher: Wiley
• Release date: Jul 22, 2015
• ISBN: 9781118751794

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This edition first published 2015

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List of Contributors

Matthias Althammer, Walther-Meißner-Institut für Tieftemperaturforschung, Bayerische Akademie der Wissenschaften, Germany

Tomoteru Fukumura, Department of Chemistry, Graduate School of Science, University of Tokyo, Japan

Sebastian Goennenwein, Walther-Meißner-Institut für Tieftemperaturforschung, Bayerische Akademie der Wissenschaften, Germany

Kohei Hamaya, Department of Electronics, Faculty of Information Science and Electrical Engineering, Graduate School of Information Science, Kyushu University, Japan

Hans Huebl, Walther-Meißner-Institut für Tieftemperaturforschung, Bayerische Akademie der Wissenschaften, Germany

Norikazu Mizuochi, Graduate School of Engineering Science, Osaka University, Japan

Shuichi Murakami, Department of Physics, Tokyo Institute of Technology, Japan

Theo Rasing, Institute for Molecules and Materials, Radboud University Nijmegen, the Netherlands

Eiji Saitoh, Institute for Materials Research and WPI Advanced Institute for Materials Research, Tohoku University, Japan

Masashi Shiraishi, Solid State Electronics, Department of Systems Innovation, Graduate School of Engineering Science, Osaka University, Japan

Yukiko Takahashi, Magnetic Materials Group, Magnetic Materials Center, National Institute for Materials Science, Japan

Koki Takanashi, Magnetic Materials Lab, Institute for Materials Research, Tohoku University, Japan

Tomoyasu Taniyama, Materials and Structures Laboratory, Division of Novel Functional Ceramics, Tokyo Institute of Technology, Nagatsuta, Japan

Arata Tsukamoto, Department of Electronic Engineering, College of Science and Technology, Nihon University, Japan

Ken-ichi Uchida, Institute for Materials Research, Tohoku University, Japan

Mathias Weiler, Walther-Meißner-Institut für Tieftemperaturforschung, Bayerische Akademie der Wissenschaften, Germany; and National Institute of Standards and Technology, CO, USA

Jiang Xiao, Department of Physics, Fudan University, China

Akinobu Yamaguchi, Laboratory of Advanced Science and Technology for Industry, University of Hyogo, Japan

Series Preface

Wiley Series in Materials for Electronic and Optoelectronic Applications

This book series is devoted to the rapidly developing class of materials used for electronic and optoelectronic applications. It is designed to provide much-needed information on the fundamental scientific principles of these materials, together with how these are employed in technological applications. The books are aimed at (postgraduate) students, researchers and technologists, engaged in research, development, and the study of materials in electronics and photonics, and industrial scientists developing new materials, devices and circuits for the electronic, optoelectronic, and communications industries.

The development of new electronic and optoelectronic materials depends not only on materials engineering at a practical level, but also on a clear understanding of the properties of materials, and the fundamental science behind these properties. It is the properties of a material that eventually determine its usefulness in an application. The series therefore also includes such titles as electrical conduction in solids, optical properties, thermal properties, and so on, all with applications and examples of materials in electronics and optoelectronics. The characterization of materials is also covered within the series in as much as it is impossible to develop new materials without the proper characterization of their structure and properties. Structure–property relationships have always been fundamentally and intrinsically important to materials science and engineering.

Materials science is well known for being one of the most interdisciplinary sciences. It is the interdisciplinary aspect of materials science that has led to many exciting discoveries, new materials, and new applications. It is not unusual to find scientists with a chemical engineering background working on materials projects with applications in electronics. In selecting titles for the series, we have tried to maintain the interdisciplinary aspect of the field, and hence its excitement to researchers in this field.

Arthur Willoughby

Peter Capper

Safa Kasap

Preface

Semiconductor manufacturing technologies are indivisibly related to nanotechnology, since they become more and more sophisticated as exemplified by the fact that the manufacturing accuracy of the CMOS microprocessing plunges into the 10 nm range. Consequently the technology is opposing a limitation due to (1) consumption of energy and (2) transmission delay in wiring. International Technology Roadmap for Semiconductors (ITRS) published a roadmap to overcome the limit, which suggested three directions of research and development to overcome the limit: (1) More Moore: extension of the limit by invention of novel technologies, (2) More than Moore: addition of higher functionalities by integration of different technologies, and (3) Beyond CMOS: development of devices based on new concept. Among these three directions, the beyond CMOS direction should be most eagerly pursued because of its innovative nature.

Diverse materials and processes have been proposed for the beyond CMOS technology, such as low-dimensional materials (nano-mechanical memory, nanotube, nanowire, grapheme, etc.), macromolecules (molecular memory, molecular devices, resists, imprint polymers, etc.), self-assembled materials (sublithographic patterns, selective etch, etc.), spintronics materials (MRAM by spin injection, semiconductor spin transport, ferromagnetic semiconductors, etc.), complex metal oxides (multiferroics), and interfaces and hetero-interfaces (electrical and spin contacts).

Majority of the contents of this book are based on the achievements of researchers and advisors affiliated with the PRESTO (Precursory Research for Embryonic Science and Technology) project titled Materials and Processes for Next Generation Innovative Devices, which was promoted by Japan Science and Technology Agency (JST) between FY 2007 and 2013. The PRESTO project for which Prof. Katsuaki Sato, one of co-editors of this book, dedicated himself as a Research Supervisor covered wide area of materials including spintronics materials, materials of strongly correlated system such as high-temperature superconductors, high-mobility wide-gap semiconductors quantum dots, nano-carbons, and organics. All these fields have provided successful achievements; some of which are highly evaluated and already in the second stage of R&D involving industries. Among them, the most prominent innovative achievements were obtained in spintronics, since it has opened up the most advanced areas of physics, leading to completely new concepts for electronic devices. For this field Prof. EijiSatoh, another co-editor, dedicated himself as a strong promoter of this field.

This book is planned to give comprehensive insight into spintronics with a particular reference to materials and processes for the next-generation innovative devices. We hope this book is helpful to not only scientists who started researches in spintronics, but also engineers who are pioneering the next-generation devices beyond CMOS.

Sincere gratitude is due to Prof. Koki Takanashi, an advisor of the PRESTO project, and all the PRESTO members who joined the hot discussions on all the topics of the project, as well as other authors who cooperated to publish this book. Also, thanks to the editorial office of Wiley Publishing Company.

Katsuaki Sato and Eiji Saitoh

May 2015

Tokyo

Introduction

Spintronics or electronics using spin-related phenomena has been attracting attention because of its potential applicability to new functional devices combining the charge transport and the spin properties. Magnetic semiconductors and ferromagnetic/nonmagnetic hybrid structures are now the most important topics of investigation in the field of new functional semiconductor devices. There is a long history of research on this category of materials. The first-generation materials were europium chalcogenides and chalcogenides of chromium with spinel-type crystal structures, which were studied intensively in the late 1960s and early 1970s. Interesting physical properties of magnetic semiconductors, such as magnetic red shift of the absorption edge and huge negative magnetoresistance (MR) around the Curie temperature, were discovered at that time. However, researchers lost interest in these materials because their Curie temperatures were far below the room temperature and because growth of good-quality single crystals was very difficult. The second-generation materials are II–VI-based diluted magnetic semiconductors (DMSs), among which c0x-math-001 was the focus of attention due to its capability to accommodate a high percentage of Mn atoms (as high as 77%) and its appropriate energy gap for optical application. The magnetic properties of most of these materials are either paramagnetic or spinglass. Although the controllability of transport properties is relatively poor, the material shows a good optical property that led to its application to optical isolators.

The third-generation materials are III–V-based diluted semiconductors, in which magnetic properties have been found to be strongly dependent on the carrier concentration in the material. This series of materials can only be produced by using an MBE technique with very low substrate temperatures. Since III–V compound semiconductors are widely used in electronic devices, the III–V-based DMSs are inherently capable of device integration. Despite these efforts, magnetic semiconductors have not been put into practical use due to their low Curie temperature. Recently, a room-temperature ferromagnetic semiconductor FET has been realized by Fukumura using a transition metal-doped oxide semiconductor, which enabled the control of magnetic properties by the gate voltage.

Another direction of spintronics research is a spin FET proposed by Datta and Das, in which the spin polarization of an electron injected from a ferromagnetic source electrode into a semiconductor is modulated by application of the gate voltage using the Rashba effect, and analyzed by a ferromagnetic drain electrode. However, the Datta–Das device has not been realized yet in the original setups.

The innovative development of spintronics had its gates opened in 1988, with the giant magnetoresistance (GMR) effect of the magnetic metal/nonmagnetic metal hybrid structure by Fert and Grünberg, et al. Over a few years, the GMR contributed to the increase in the density of the magnetic storages as a spin valve, and humanity obtained a means to efficiently convert magnetic information into electric signals without using a coil.

After that, Miyazaki et al. and Moodera et al. discovered the tunnel magnetoresistance (TMR) effect at room temperature, providing an opportunity for the emergence of nonvolatile solid-state memory devices, magnetoresistive random-access memory (MRAM). Furthermore, the TMR was significantly improved and it greatly advanced due to the study by Yuasa et al. and Parkin et al., in which MgO was employed as the tunnel barrier.

The next innovative development was brought on by a theoretical prediction and an empirical validation of the magnetization reversal phenomena that uses spin-transfer torque (STT). The STT-MRAM that utilizes this phenomenon does not require any electric current wires for generation of a magnetic field, allowing for a high-density integration that is more than DRAM, and finally, a sample was able to be released to the market. At last, humanity had obtained a way to convert electric signals into magnetic information without a coil.

Up to the point, the spin current had been something that accompanied the electrical charge current, but then a theoretical prediction of the existence of a pure spin current that was not accompanied by the electric charge current was made, and it has been empirically validated in the last 10 years. Using the pure flow of spin allows information transmission without having Joule heat, so the expectation is that this can solve the issues of energy dissipation from the metal wire of high-density minute integrated circuits.

Theory and its empirical validation of the spin Hall effect (SHE) greatly contributed to the generation and detection of the spin current. These effects are based on the concept of Berry's phase and it is assumed that a cosmology can be established in solid matter. Furthermore, theories that are the basis of spintronics, such as the discovery of the spin Seebeck effect with the thermal spin current, thespin current carried by a spin valve, the interaction between the spin current and phonons, and topological insulators, are experiencing a great leap forward. In addition, recently, a path to control the magnetic properties by using an electric voltage instead of an electric current using a principle that is different from the STT, is about to open up.

In the following, topics included in this book are briefly reviewed as a guide to those readers who are not familiar with the field of spintronics. First part is devoted to materials for spintronics and the rest to various functions associated with spintronics.

MATERIALS FOR SPINTRONICS

For practical applications of spintronics, research and development of materials for both spin injection and spin guideline are important.

Spin Injector Materials with High Polarization

As spin-injector electrodes, full-Heusler compounds have been intensively studied. The compounds are predicted by electronic structure calculations as half-metals, in which at Fermi level, a half of the spin-polarized band has a finite value of density of state while the other half has zero density of state, leading to a 100% degree of spin polarization in ideal cases. Dr. Yukiko Takahashi gives a comprehensive review of the Heusler compounds and describes methods how to measure the degree of spin polarization, as well as how to increase the degree of spin polarization by appropriate combination of elements.

Carbon Spintronics

As a guideline for spin current, the spin diffusion lengths are rather short in metals such as Cu and Al which have predominantly been used. On the other hand, longer spin diffusion length is expected in lighter atoms which are subjected to a reduced spin–orbit coupling due to smaller relativistic effect. In this context, nano-carbon materials such as carbon nanotubes (CNT), graphene sheets, and organic compounds are promising, for which Prof. Seiji Shiraishi provides comprehensive descriptions.

Silicon Spintronics

Silicon is also a very important candidate for a spin current guideline, since silicon is a dominantly used material in the semiconductor device technology so that spintronics can easily be combined to conventional electronics circuits. Until recently, a tunnel contact with heavily doped degenerate silicon materials has been used for spin-injection experiments. However, for the purpose of electrical control of spin currents by application of the gate-voltage (spin-FET operation), nondegenerate materials should be pursued. For this purpose, improvement of spin injection efficiency is necessary. Prof. Kohei Hamaya describes how he succeeded in realization of a spin-MOSFET using high-quality Schottky tunnel contact.

SPINTRONICS FUNCTIONS

Spin-Dependent Transport

The first epoch-making issue in the field of spintronics function was introduced by the Nobel-Prize winners Albert Fert and Peter Grünberg, who independently observed prominent spin-dependent transport phenomena in superstructures consisting of magnetic and nonmagnetic layers, which was named giant magnetoresistance (GMR). Next important issue was an observation of spin-dependent tunneling conductance, tunnel magnetoresistance (TMR), at room temperature by Terunobu Miyazaki. Astonishingly it took no longer than 5 years before these discoveries are applied to practical devices in both GMR and TMR; the former being used as magnetic sensors for hard disk drives, while the latter being used as MRAM. Anomalous magnetoresistance (AMR) effects, even though small, have been investigated for long time, which have been providing an important background for present-day spintronics. Physical and technical aspects for magnetoresistance phenomena including AMR, GMR, and TMR are described in detail by Prof. Koki Takanashi.

Spin Current, Spin Hall Effect, and Spin Pumping

Dramatic reduction of energy consumption in the LSI wiring is expected by using the spin current instead of the charge current. The spin current is defined as a difference between a flow of up-spin electrons and that of down-spin electrons. If the charge current direction of up-spins is opposite to that of down-spins, net charge current vanishes and only the spin current exists; that is a pure spin current. Not only the conventional spin current, but also the spin-wave spin current and the topological spin current are recently proposed and experimentally confirmed. In addition, new paradigm of spin-related physics has been opened up such asthe concept of spin Hall effect (SHE) and inverse spin Hall effect (ISHE), which can be used for mutual conversion between the spin current and the charge current. The latter is used to detect the spin injection, and other spin current-related phenomena. Detailed description of diverse types of spin current is provided by Prof. Eiji Saitoh, one of the editors of this book. Theoretical backgrounds for concept of the SHE and ISHE and topological insulators are given by Prof. Shuichi Murakami. Concept of spin pumping is also a significant issue for injection of spin-wave spin current to insulators and semiconductors, for which Prof. Sevastian Goennenwein gives physical and technical backgrounds.

Spin Torque

Since the spin is a quantum of the angular momentum, a spin-polarized electron transfers a torque when injected into magnetized materials as a current flow. The torque is called spin-transfer torque (STT). The torque can be used to invert the direction of magnetization in MRAMs instead of the current-induced magnetic field used in conventional MRAMs, which has been an obstacle to miniaturization of the device due to increase in the current density. MRAM making use of spin-transfer torque, STT-MRAM, is considered to be a promising candidate for a nonvolatile memory component, required in realization of normally off computing technology. Not only the STT-MRAM, but also the spin torque technology is expected to be applicable to domain-wall drive memories, spin-torque oscillators, etc. Theoretical and experimental descriptions of spin torque phenomena are given by Prof. Akinobu Yamaguchi.

Spin Seebeck Effect

It is recently observed that temperature gradient in a slab of magnetic materials induces a pure spin current without a flow of the charge current. By the use of ISHE, the spin current can be converted to the electric current, which in turn can be converted to voltage. The effect is named spin Seebeck effect and is promising for energy harvesting, which converts an environmental heat to an electric energy. Dr. J. Xiao gives theoretical and experimental details for the spin Seebeck effect.

Electric Control of Spin Phenomena

Tunable Spin Sources

In conventional spintronics devices, spin polarization of an injector electrode is solely dependent on magnetization of the material and cannot be controlled externally. A few ideas have been proposed to obtain tunable spin sources using some sort of magnetic phase change. One idea is to use an antiferromagnetic (AF)/ferromagnetic (FM) phase transition induced by spin injection and another is to use temperature-induced phase transition in magnetite electrode for LED. Prof. Tomoyasu Taniyama describes experimental observation of AF/FM transition in FeRh alloy by spin injection from Co electrodes and of spin injection to GaAs/GaAlAs quantum well from the f05-math-002 electrode which undergoes the famous Verway transition around 120 K.

Enhancement of Rashba Effect

In spin-transistors proposed by Datta and Das, the spin polarization direction is modulated by using the Rashba effect, in which a spin–orbit coupling is controlled by an application of the gate voltage. Since the effect is so small that considerably long channel length is necessary to invert electron spins flowing between source and drain electrodes. Therefore, the Datta–Das device requires a long spin diffusion length, which is not the case in conventional semiconductors. To overcome the problem, Dr. Hiroyuki Nakamura proposed a method to enhance the spin–orbit interaction by means of dielectric materials. He succeeded to realize an enhancement of spin–orbit interaction by using the third-order Rashba effect in f05-math-003 dielectric.

Ferromagnetic Semiconductor

Electrical control of magnetism has been a long-lasting wish of researchers from the age of the first-generation magnetic semiconductors. Although dramatic voltage-controlled change in the magnetization curves was reported in InMnAs semiconductors, the operation temperature was as low as order of 10 K due to low ferromagnetic transition temperature. Quite recently, the electric field-induced ferromagnetism was observed in ferromagnetic oxide semiconductors at room temperature. Prof. Tomoteru Fukumura describes how he succeeded in control of magnetism by an electric field in f05-math-004 :Co room temperature ferromagnetic semiconductor with an FET structure. He also refers to some of recent developments in voltage-controlled spin states in metals and other materials.

Spin Photonics

Spin Quantum Devices using Diamond

Room-temperature operation of quantum information processing has been a dream of researchers of this field. The diamond f05-math-005 center consisting of a carbon vacancy (V) and a nitrogen atom (N) substituting a carbon site has been attracting attention as a promising quantum information device operating at room temperature, since the center forms a deep center due to wide-gap nature of the material. By using optically controlled magnetic resonance technique, the coupling of an electron spin and a nuclear spin becomes available, which in turn enables optical initialization of the quantum system. Professor Norikazu Mizuochi describes how the f05-math-006 centers can be coupled to enable 5 qubit operations, as well as how he managed to realize a current-operated single photon source by using a specially designed LED composed of a diamond p–i–n junction.

Ultrafast Light-Induced Spin Reversal

Magnetic devices are facing a limitation of data-transfer rate determined by the magnetization-reversal time associated with the ferromagnetic resonance, which is of the order of GHz. Recently, optically induced ultrafast spin-reversal phenomenon of subpicosecond range has been observed in a few materials. The most prominent achievement was the realization of the ultrafast light-induced magnetization reversal in amorphous rare earth-transition metal alloy films around the angular moment compensation temperature. Prof. Arata Tsukamoto describes the details of the phenomenon and gives a physical background of dynamical behavior of spins in nonequilibrium conditions.

Finally, we sincerely hope that this book may provide a perspective guide to spintronics for young researchers who are pioneering the science and technology for next-generation innovative devices.

Chapter 1

Fundamentals of Magnetoresistance Effects

Koki Takanashi

Institute for Materials Research, Tohoku University, Aoba-ku, Sendai, Japan

1.1 Giant Magnetoresistance (GMR) Effect

Giant magnetoresistance (GMR) effect is the most fundamental phenomenon in the field of spintronics. In general, the electric resistance of a material is changed when a magnetic field is applied. This phenomenon is known as a magnetoresistance (MR) effect. Although there are many types of MR effects, the MR effect showing a particularly large resistance change among them is called the GMR effect. However, GMR is different from the conventional MR effects that had been known before, not only quantitatively, but also qualitatively. In 1988, GMR was first reported in the experiment of Fe/Cr superlattices by Fert and his collaborators [1]. That was followed by an enormous amount of studies concerning GMR in nanometer-scaled layered structures containing various kinds of ferromagnetic metals, and GMR was practically used for a read head of a hard disk drive (HDD) in only 10 years after the discovery of GMR. The GMR-based read head had dramatically improved the storage density of HDD until GMR was replaced by tunnel magnetoresistance (TMR)