Eighty years ago theoretical physicists had a problem: they lacked a mathematical description of elementary particles that was consistent with the principles of both Einstein's special theory of relativity and the newly formed theory of quantum mechanics. In 1927 Erwin Schrödinger had written down the quantum mechanical equation of motion for the electron, but this did not take into account the fact that electrons are relativistic particles. Troubled by this situation, Paul Dirac set about finding a solution.
Spin injection: Doping gallium arsenide with manganese atoms gives the semiconductor ferromagnetic properties, thereby enabling its use as a spin injector. Source: A Yazdani and D Kitchen, Princeton University |
The equation Dirac arrived at the following year was a mathematical tour de force, which predicted two totally unexpected physical phenomena. The first was the existence of antiparticles, which was proved in 1932 with the discovery of the positron (an anti-electron). The second was that the electron must have an intrinsic angular momentum or "spin" that has only two possible orientations in an applied magnetic field: aligned with the field, or "up"; and anti-aligned, or "down".
The electron lies at the heart of the microelectronics revolution, where it is shuttled around in semiconductors (usually silicon) to allow transistors and other such devices to operate. Yet these devices — which underpin everything from microwave ovens to cosmological probes — only exploit the charge of the electron, while for 70 years following Dirac's groundbreaking discovery the electron's spin has largely been ignored by the device and semiconductor industry.
One reason for this is the phenomenal success in miniaturizing devices. For the last 40 years the number of transistors per unit area that can be etched onto a silicon chip — which, for example, governs the processing power of a computer — has doubled every 18 months, a trend known as Moore's law. But we are now rapidly approaching the limit of how small and closely packed these transistors can become before the heat that they generate cannot be dissipated fast enough, or unwanted quantum-mechanical effects prevent them from functioning properly.
If Moore's law is to continue, we need to find an alternative to conventional microelectronics — at long last it is time to exploit the electron's spin in semiconductor devices. Whereas conventional electronic devices rely on only controlling the flow of charge, a "spintronic" device would also control the flow of electron spins (the so-called spin current) within the device, thereby adding an extra degree of freedom.
Spin based devices: The goal of spintronic devices is to exploit the spin as well as the charge of the electrons that pass through them. The spin light-emitting diode (spin-LED, top left), for example, in which spin-polarized electrons injected from a ferromagnetic layer (blue) into a semiconductor structure (orange) recombine with holes in the active region (yellow) to produce circularly polarized light (pink, where the arrow indicates the direction of polarization), has already been demonstrated in the lab and could be useful for encrypted communication. However, it could be several years before the most immediately useful device — a spin-based transistor — is built. In a lateral spin transistor (top right) spin-polarized electrons are injected from a ferromagnetic source into a narrow semiconductor channel (yellow) in which the electron spins can only move in 2D. Here, the spin can be switched between up and down by an applied magnetic field or the gate voltage, which thus determines the output spin current in the ferromagnetic "drain" material. An alternative approach is the single-electron spin transistor (bottom left), in which a ferromagnetic source injects a polarized electron into a semiconductor nanostructure called a quantum dot, where its spin state — and thus the output current in the ferromagnetic drain — is controlled by an applied gate voltage. A third design for a spintronic transistor is the magnetic tunnel transistor (bottom right), in which the injected electrons are filtered depending on their spin as they tunnel through a thin insulating layer (red), as happens in a magnetic tunnel junction, before passing through a Schottky barrier. The output current in the "collector" semiconductor can therefore be controlled by changing the spin alignment of the "emitter" and "base" ferromagnetic layers. |
Because the spin of an electron can be switched from one state to another much faster than charge can be moved around a circuit, spintronic devices are expected to operate faster and produce less heat than conventional microelectronic components. One of the ultimate goals is to build a spin-based transistor that would replace conventional transistors in integrated logic circuits and memory devices, thus allowing the miniaturization trend to continue. However, spintronics also opens the door to entirely new types of device, such as a light-emitting diode (LED) that generates left or right circularly polarized light for use in encrypted communication (see "Spin-based devices"). Looking further into the future, spintronic devices could even be used as quantum bits, the units of information processed by quantum computers.
For the spintronics revolution to happen, however, researchers need to find a way to inject, manipulate and detect the spin of electrons in semiconductors, since these materials are likely to remain central to device physics for the foreseeable future. Spin manipulation should in theory be relatively straightforward, but injecting and detecting spin under practical conditions are huge challenges.
Giant achievement
The ability to transport electron spins between two metals also underpins magneto-resistive random access memory (MRAM) — a novel type of computer memory that can retain information without requiring any power. MRAM is based on a similar effect to GMR known as tunnel magneto-resistance (TMR), which arises when two layers of ferromagnetic metal are separated by a thin layer of insulating material, such as aluminum oxide or magnesium oxide. Instead of the spin-polarized electrons diffusing slowly from one ferromagnetic layer to the other as happens in GMR, in TMR they tunnel quantum mechanically (a classically forbidden process in which a particle passes through a potential barrier higher than its kinetic energy) through the barrier layer — as such these devices are called magnetic tunnel junctions (MTJs) (see "Magnetic tunnel junction"). The Pauli’s exclusion principle then comes into play. Tunneling — and therefore spin transport across the barrier — can only occur if empty (i.e. unoccupied) wave states with the same spin are available on the other side of the barrier: the result is spin-dependent tunneling.
Magnetic tunnel junction: The magnetic tunnel junction, which consists of two ferromagnetic layers (blue) separated by an insulator (red), exploits tunnel magneto-resistance (TMR) to switch the output spin current between high and low. As such, the device can be used as memory to store information even when the power is turned off. When the magnetization of the two magnetic layers is parallel, spin-up electrons can tunnel through the barrier because many unoccupied states are available in the second ferromagnetic layer (top). When the two layers are anti-parallel, however, fewer up-spin states are available, so tunneling is suppressed (bottom). The difference in the tunnel current as the spin alignment of the ferromagnetic layers is switched between parallel and anti-parallel is known as the TMR ratio, and to be useful for building practical devices a TMR of about 500% is necessary. Since 1995, when room temperature TMR was first demonstrated, researchers have obtained much higher TMR values by changing the insulating material and its interface. |
Such spin-dependent tunneling was demonstrated at low temperatures in 1975 by Michel Jullière at the Institut National des Sciences Appliquées de Lyon in France. More recently, the ability to fabricate atomically flat interfaces between the metal and the oxide layers has enabled Stuart Parkin's group at IBM's Almaden Research Center in California, and Shinji Yuasa and colleagues at the AIST in Japan to independently achieve TMR values of about 400% via coherent tunneling. TMR-based commercial MRAM arrays are already starting to become available, and these could one day be used to build PCs that switch on instantly.
TMR relies on a large number of electrons with the desired spin state being transmitted across interfaces between ferromagnetic metals and insulating metal oxides. To make semiconductor spintronic devices possible, however, we need to achieve such behavior across interfaces formed between a semiconductor and a material that can serve as a spin injector or detector.
Magnetic appeal
Silicon and gallium arsenide are the two most widely used semiconductors, so the challenge is to find spin-polarized materials — i.e. materials in which most of the electron spins are aligned in a particular direction — that can be combined with them. Promising candidates are "dilute magnetic semiconductors" (DMS) — semiconductors that, when doped with impurity atoms, display ferromagnetism.
In 1999 two groups independently injected spin-polarized electrons from a magnetic semiconductor into gallium arsenide. Laurens Molenkamp and colleagues at Würzburg University, Germany, maintained a polarization of 90% during spin injection from a spin-polarized semiconductor material into a gallium-arsenide structure at low temperature, although the semiconductor injector required an external magnetic field to maintain its polarization. Hideo Ohno's group in Tohoku, Japan, in collaboration with David Awschalom's group at the University of California, Santa Barbara, on the other hand, managed the same feat from a "true" DMS that does not require an applied magnetic field, although the researchers only achieved an injected spin polarization of about 1%. Together, these experiments demonstrated that it was possible to inject spin into a semiconductor; to develop a practical device, the next step was to find DMS materials that would allow robust spin injection at room temperature with only modest (or zero) applied fields.
Rising the Curie temperature: Dilute magnetic semiconductors, which exhibit spin polarization when doped with certain elements — are good candidate materials for injecting spin-polarized electrons into a semiconductor. To be practically useful, however, the Curie temperature of such materials — above which the ferromagnetic behavior disappears — must be high. Some of the most promising materials are zinc manganese oxide (ZnMnO), cobalt oxide doped with titanium and tin (Co[Ti, Sn]O), and gallium manganese nitride (GaMnN). However, there is some controversy surrounding the measured Curie temperatures of these materials. In 2001 and 2003 two groups predicted Curie temperatures of about 600 K for titanium- or tin-doped cobalt oxide, but no convincing experimental confirmations have followed. A significant effort has also been made to raise the Curie temperature in gallium arsenide by doping it with manganese, but the highest value reported to date — 250 K — has been called into question following a revised analysis of the data. The green bar indicates the range of Dietl's predicted values, while the white arrows show the range of experimental values for various concentrations. |
In 2000 Thomas Dietl of the Polish Academy of Sciences in Warsaw made an important breakthrough in this regard. He showed that the highest (Curie) temperature at which ferromagnetism occurs in certain DMS materials should increase significantly as they are doped with increasing concentrations of, in particular, the magnetic elements manganese or cobalt. His calculations were based on a concept first proposed by the late US physicist Clarence Zener in the 1950s, in which interactions between the magnetic moments of the localized impurity atoms and those of the delocalized holes in the semiconductor can cause the moments to align as they would in a ferromagnet. Furthermore, this effect should overcome the misaligning effect that is caused by high temperatures. Notably, Dietl's calculations suggested that the commonly used semiconductors zinc oxide and gallium nitride should, with sufficient doping, exhibit ferromagnetism well above room temperature, thus sparking a major worldwide effort to develop practical DMS materials (see "Raising the Curie temperature").
Finding a material that exhibits spin polarization well above room temperature, however, is not the only challenge in developing a practical spin injector. First, it must have a large polarization in order to be able to inject enough spin-polarized electrons into a semiconductor. Second, it must be possible to control the properties of the interface that forms when the injector material is deposited on the semiconductor. While developing magnetic tunnel junctions in the 1990s, researchers learned that the properties of the few atomic layers close to the interface have a critical effect on spin-injection efficiency. This is because very small amounts of chemical intermixing between the layers can scatter the electrons into new states and therefore substantially lower the amount of electrons that make it across the interface while remaining polarized. It is difficult to control the properties of DMS materials in bulk form, and even more so when the material is deposited in a thin film, as is required when fabricating a device. Achieving clean interfaces between DMS materials and semiconductors, therefore, poses a considerable challenge for researchers trying to build DMS-based spintronic devices.
Tunnel vision
There is, however, an alternative and fundamentally different approach to achieving spin injection. While many researchers concentrated on DMS materials, others reasoned that if spin-polarized electrons could be transmitted across an interface between a semiconductor and a ferromagnetic metal, the metal could then be used as a highly effective spin polarizer. Furthermore, since metal interfaces have been studied for decades, it should be much easier to control the interface properties in such device structures.
In the late 1990s several research groups attempted to inject spin-polarized electrons from ferromagnetic metals and alloys deposited directly onto gallium arsenide, but these early studies achieved injected polarizations of just a few per cent. A further blow to the idea was dealt in 2000, when Georg Schmidt and colleagues at Würzburg University used a simple model of a resistor network to show that a spin polarization of nearly 100% would be needed in the ferromagnetic metal in order to inject a useful spin polarization into the semiconductor. Such high polarizations are impossible to achieve in practice, so for a brief period it seemed that semiconductor spin injectors were likely to be the only possible way forward.
This view was turned on its head almost immediately, however, when Emmanuel Rashba at MIT realized that creating a tunnel barrier between the ferromagnetic metal and the semiconductor would solve the problem. He predicted that the spin polarization in the conductive metal would be preserved during tunneling, and therefore that the ferromagnetic-metal–barrier spin injector was analogous to a magnetic tunnel junction. Following this development, a concerted effort was made to investigate the injected spin polarization in a ferromagnetic-metal–gallium-arsenide structure. In such samples, electrical charge is redistributed as the junction between the metal and the semiconductor forms, thus creating a "Schottky" tunnel barrier at the interface. It turns out that this type of structure also demonstrates the concept of a spin-LED (see "Electro-optical injection and detection"): when a polarized electron is injected from the ferromagnetic layer into the semiconductor, it recombines with a hole, which results in the emission of circularly polarized light. (In a conventional LED, in contrast, unpolarized electrons and holes combine to produce unpolarized light.) Several research groups are currently trying to exploit this phenomenon to develop a practical spin-LED device.
Electro-optical injection and detection: The principle underlying the spin-LED can be adapted to enable spin detection — which is vital for real spintronic devices. In a spin-LED (top), when a spin-polarized electron is injected from the ferromagnetic layer (blue) into the semiconductor (orange and yellow) via a Schottky barrier (purple), it recombines with a hole (red) and in doing so emits a circularly polarized photon. The degree of circular polarization can be used to estimate the magnitude of the injected spin polarization. This process can also be reversed by shining circularly polarized light onto the semiconductor structure, which generates a population of excited spin-polarized electrons within the semiconductor (bottom). Depending on the relative direction of the magnetization of the ferromagnetic detector with respect to the photon polarization, photo-excited electrons in the "up" (or "down", if the magnetization is reversed) spin state can tunnel across the Schottky barrier into the ferromagnetic layer, where they can be detected as an electrical signal. |
Since the number of spin-polarized electrons that make it across the barrier depends on its properties, however, some researchers tried replacing the Schottky barrier with a thin insulating layer in an attempt to increase the spin-injection signal in ferromagnetic-metal–semiconductor systems. In 2003 Pol Van Dorpe and colleagues at the Interuniversity Microelectronics Centre (IMEC) in Leuven, Belgium, achieved an injected spin polarization of just over 20% at low temperature with an aluminium-oxide insulating layer. Then, two years later, Parkin's group at IBM showed that using magnesium oxide as an insulator improved the performance further, but that the injected polarization is highly sensitive to the crystal structure of the barrier material.
Meanwhile, progress was also being made in the other big challenge that needs to be overcome in order to build a spintronic device: spin detection. One way to do this is to reverse the process that allows a spin-LED to work (see "Electro-optical injection and detection"). By shining polarized light at a ferromagnetic-metal–Schottky-barrier–gallium-arsenide heterostructure, a population of spin-polarized electrons is generated within the gallium-arsenide substrate (via the optical-selection rules for this semiconductor). These electrons can then tunnel back across the Schottky barrier into the ferromagnetic metal where they can be detected electrically, so offering a way to detect electron spins. In 2004 our group in Cambridge used such a structure to show that this effect produces a voltage that depends on the percentage of the electrons in the ferromagnetic metal that are polarized.
Since then, we have found that by replacing the single layer of ferromagnetic metal with a metal GMR spin valve (i.e. two thin layers of ferromagnetic metal separated by a thin layer of non-magnetic metal), the current flowing into the metal can be determined separately from the current flowing in the semiconductor, since the valve acts as a gate that switches the current flowing to the metal on or off according to the magnetic alignment of its layers. Using the spin valve in this way allowed us to quantify the spin-filtering effect of the interface; and hence estimate the polarization of the detected current.
Interface matters
We are now six years on from Dietl's predictions that the Curie temperature of certain DMS materials should increase significantly with ferromagnetic doping. Yet still no-one has found suitable ferromagnetic semiconductor materials that operate at room temperature and can be used in practical semiconductor spintronic devices. While the effort to develop DMS-based spintronics continues, however, the remarkable development of magnetic tunnel junction technology has given great impetus to using ferromagnetic metals in combination with semiconductors. While ferromagnetic transition metals do not offer 100% spin polarization, this may not be necessary for practical devices: theoretical predictions suggest that by controlling the interface structure and composition, and using appropriate barriers, future ferromagnetic-metal systems could yield dramatic increases in spin transmission over the injector/detector materials tried so far.
The successful development of MTJs has already shown that the properties of the few atomic layers close to the interface have a critical effect on spin transmission. In the future it will be important to precisely control the structure of the materials used in semiconductor spintronic devices by matching the crystal orientation of the interface with that of the spin injector/detector material, and it is clear that there are many promising new routes to investigate. While it is not possible to say how long this will take, it looks increasingly likely that the semiconductor spintronics revolution will be kicked off by devices that use the magnetic transition metal films already found in MRAM and MTJ devices.
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