8 Spintronics — Spin-Based Electronics CONTENTS Abstract 8.1 Spin Transport Electronics in Metallic Systems High-Density Data Storage and High-Sensitivity Read Heads • The Giant Magnetoresistance Effect • Spin Valves • Magnetic Tunnel Junctions • Device Applications for Spin Valves and Magnetic Tunnel Junctions • Magnetoresistive Random Access Memory 8.2 Issues in Spin Electronics Spin Injection • Research in New Materials for Spintronics • Optically and Electrically Controlled Ferromagnetism • Current-Induced Magnetization Switching and Spin Wave Generation • Optically Excited Spin States in Semiconductors 8.3 Potential Spintronics Devices S.A. Wolf Light-Emitting Diode • Field-Effect Transistor • Resonant Defense Advanced Research Projects Tunneling Diode • New Spintronic Device Proposals Agency and Naval Research 8.4 Quantum Computation and Spintronics Laboratory Nuclear Spin Quantum Computer • Spin-Resonance A.Y. Chtchelkanova Transistor • NMR Quantum Computer • Quantum Dots as Quantum Bits Strategic Analysis, Inc. 8.5 Conclusion D.M. Treger Acknowledgments Strategic Analysis, Inc. References Abstract Spintronics is an acronym for a spin transport electronics and was originally used as a name for the Defense Advan ced Research Projects Agency (DARPA) program. In 1994, DARPA started to develop magnetic field sensors and memory based on the giant magnetoresistance (GMR) effect and spin- dependent tunnelin g. The overall goal for the Spintronics program was to create a new generation of electronic devices where the spin of the carriers would play a crucial role in addition to or in place of the charge. Spintro nic devices can be used as magnetic memories, magnetic field sensors, spin-based switches, modulators, isolators, transistors, diodes, and perhaps some novel devices without conventional ana- logues that can perform logic functions in new ways. Spintronics brings together specialists in semicon- ductors, magnetism, and optical electronics studying spin dynamics and transport in semiconductors, metals, super conductors, and heterostructures. As always, the key question is whether any potential benefit of such technology will be worth the production costs. © 2003 by CRC Press LLC 8-2 Handbook of Nanoscience, Engineering, and Technology This paper will provide an overview of the field and the reference material to the original papers for the future in-depth reading. In Section 8.1 we briefly describe spin transport electronics in metallic systems and commercially available devices utilizing magnetization and spin transport properties in metals.1,2,3 Section 8.2 addresses the issues in semiconductor spin electronics,4 which have to be resolved to create successful devices — efficient spin injection into semiconductors and heterostructures and the search for new spin-polarized materials. It also mentions effects potentially important for spintronics devices including optical and electrical manipulation of ferromagnetism, current-induced magnetization switching and precessing, long decoherence time for optically excited spins in semiconductors, etc. Section 8.3 briefly describes proposed devices utilizing spins, and Section 8.4 covers quantum computing schemes relying on spins. Progress in spintronics would not be possible without the maturity of electron-beam and ion-beam fabrication, molecular beam epitaxy (MBE), and the ability to manufacture devices at the nanoscale with nanoimprint lithography. Advances in magnetic microscopy for direct, real-space imaging demonstrated for the past decade5 have also played a crucial role in new materials and device characterization. 8.1 Spin Transport Electronics in Metallic Systems Conventional electronic devices are based on charge transport, and their performance is limited by the speed and dissipation of the energy of the carriers (electrons). Prospective spintronic devices utilize the direction and coupling of the spin of the electron in addition to or in place of the charge. Spin orientation along the quantization axis (magnetic field) is dubbed as up-spin and in the opposite direction as down- spin. Electron spin is a major source for magnetic fields in solids. 8.1.1 High-Density Data Storage and High-Sensitivity Read Heads The spin has been an important part of magnetic high-density data storage technology for many years. Hard disk drives (HDD) store information as tiny magnetized regions along concentric tracks. Mag- netization pointing in one direction denotes a zero bit, and in the opposite direction a one bit. Areal density, expressed as billions of bits per square inch of disk surface area (Gbits/in2), is the product of linear density (bits of information per inch of track) multiplied by track density (tracks per inch) and varies with disk radius. The recent progress in the increasing storage areal density is due to high- sensitivity read heads made possible after the discovery of the GMR effect. The first commercially available disk drive using a GMR sensor head was the 1998 IBM Deskstar 16GP disk drive that had 2.69 Gbits/in2 areal density. The current commercially available density is up to 40 Gbits/in 2 and 110 Gbits/in2 is under investigation. 8.1.2 The Giant Magnetoresistance Effect The GMR effect was discovered in the late 1980s when two research groups performed magnetoresistivity studies6,7 of heterostructures comprised of alternating thin (10–100 Å) metallic layers of magnetic and nonmagnetic metals in the presence of high magnetic fields (2T) at low temperatures (4.2 K). They saw resistance changes up to 50% between the resistivities at zero field and in the saturated state. At room temperature the GMR effect was smaller but still significant. The resistance of the GMR structure is lower if the directions of the magnetization of the ferromagnetic (FM) layers are aligned than when they are anti- aligned. At zero field with the thickness of nonmagnetic metal in the range of 8–18 Å, indirect electron exchange provides a mechanism to induce an anti-parallel alignment of the magnetic layers.8 Application of an external magnetic field (100–1000 Oe) overcomes the anti-parallel coupling between the magnetic layers and, as a result, the resistance changes in the range of 10–60%. The GMR effect is measured as the change in resistivity divided by the resistivity at large fields, typically termed ∆R/R. GMR is observed when the current is in the plane of the layers (CIP) and perpendicular to the plane of the layers (CPP). GMR is attributed to spin-dependent scattering at the interfaces and spin-dependent conductivity. © 2003 by CRC Press LLC Spintronics — Spin-Based Electronics 8-3 It has long been known that electrons in metals have two spin states and that, when electric field is applied to a metal, two approximately independent currents flow. In nonmagnetic metals these two spin channels are equivalent because they have the same Fermi energy density of states (DOS) and the same electron velocities, but in ferromagnetic transition metals they are quite different. Spin polarization is defined as the ratio of the difference between the up- and down-spin channel population to the total number of carriers: P = (n↑ – n↓)/(n↑ + n↓) For most electronics applications, only the total conductivity resulting from these two parallel currents or spin channels is important. Recent technologically important effects such as GMR take advantage of the differences in electron transport between the two spin-channels. The physics of spin-dependent transport in magnetic multilayers is explained in Reference 9. 8.1.3 Spin Valves A GMR structure widely used in HDD read heads is a spin valve (SV), originally proposed by IBM in 1994.10 An SV has two fe rromagnetic layers (alloys of nickel, iron, and cobalt) sandwiching a thin nonmagnetic metal (usually copper) with one of the two magnetic layers being “pinned,” i.e., the magnetization in that lay er is relatively insensitive to moderate magnetic fields.11 The other magnetic layer is called the free lay er, and its magnetization can be changed by application of a relatively small magnetic field. As the alignment of the magnetizations in the two layers changes from parallel to anti- parallel, the resistance of the spin valve typically rises from 5% to 10%. Pinning is usually accomplished by using an antiferromag netic layer that is in close contact with the pinned magnetic layer. In SV read heads for high-density re cording, the magnetic moment of the pinned layer is fixed along the transverse direction by exchange coupling with an antiferromagnetic layer (FeMn), while the magnetic moment of the free layer rotates in response to signal fields. The resultant spin-valve response is given by ∆R ~ cos(θ1 – θ2), where θ1 and θ2 are the angles between the magnetization direction of the free layer and pinned layer and the direction parallel to the plane of the media magnetization, as seen in Figure 8.1. When a weak magnetic field, such as from a bit on a hard disk, passes beneath the read head, the magnetic orientation of the free layer changes its direction relative to the pinned layer, generating a change in electrical resistance due to the GMR effect. Because an SV is so important for industrial applications, there have been many improvements in recent years. The simple pinned layer is replaced with a synthetic antiferromagnet — two magnetic layers separated by a very thin (~10Å) nonmagnetic conductor, usually ruthenium.12 The magnetizations in the two magnetic layers are strongly anti-parallel coupled and are thus effectively immune to outside magnetic fields. The second innovation is the nano-oxide layer or NOL, which is formed at the outside surface of the soft magnetic film. This layer reduces resistance due to surface scattering,13 thus reducing background resistance and thereby increasing the percentage change in magnetoresistance of the structure. The magnetoresistance of spin valves has increased dramatically from about 5% in early heads to about 15 to 20% today, using synthetic antiferromagnets and NOLs. 8.1.4 Magnetic Tunnel Junctions A magnetic tunnel junction (MTJ) is a device in which a pinned layer and a free layer are separated by a very thin insulating layer, commonly aluminum oxide.