Moore’s Law which stated that the number of transistors on a chip will double approximately every two years has been the driver of semiconductor industry in boosting the complexity, computational performance and energy efficiency while reducing cost. It has led to substantial improvements in economic productivity and overall quality of life through proliferation of computers, communication, and other industrial and consumer electronics. Microelectronics and solid state components have also been the backbone of the military systems and were main contributors in advancement of radar, communication and electronic warfare systems.
As dimensions approach nanometer ranges, CMOS transistors are difficult to operate because of rising power dissipation of chips and the fall in power gain of smaller transistors, soaring fabrication plant costs and finally, quantum effects in silicon are predicted to bring about an end to the ongoing miniaturization of CMOS. Electron spin is one such effect that offers the opportunity to continue the gains predicted by Moore’s Law, by taking advantage of the confluence of magnetics and semiconductor electronics in the newly emerging discipline of spin electronics.
Using either the spin in tandem with the charge or alone, spintronics has some advantages over conventional semiconductor electronics, including higher integration density, non-volatility, decreased power dissipation and faster processing speeds. Spin wave based devices, which utilise collective excitations of electronic spins in magnetic materials as a carrier of information, have huge potential as memory devices that are more energy efficient, faster, and higher in capacity.
The spintronic devices will find widespread application in civilian and military markets offering new generation of transistors, lasers and integrated magnetic sensors.
Spin electronics (also called spintronics, magnetoelectronics or magnetronics) is “A branch of physics concerned with the storage and transfer of information by means of electron spins in addition to electron charge as in conventional electronics.” Spin-based electronics focuses on devices whose functionality is based primarily on the spin degree of freedom of the carriers. This is in contrast to conventional electronics, which exploits only the charge of the carriers.
In quantum mechanics, spin is an intrinsic form of angular momentum carried by elementary particles. “Spin is a purely quantum phenomenon roughly akin to the spinning of a child’s top or the directional behavior of a compass needle. The top could spin in the clockwise or counterclockwise direction; electrons have spin of a sort in which their compass needles can point either “up” or “down” in relation to a magnetic field,” explain Sankar Das Sarma. Spin therefore lends itself elegantly to a new kind of binary logic of ones and zeros. The movement of spin, like the flow of charge, can also carry information among devices.
One advantage of spin over charge is that spin can be easily manipulated by externally applied magnetic fields, a property already in use in magnetic storage technology. Less energy is needed to change spin than to generate a current to maintain electron charges in a device, so spintronics devices use less power. Another subtler (but potentially significant) property of spin is its long coherence, or relaxation, time—once created it tends to stay that way for a long time, unlike charge states, which are easily destroyed by scattering or collision with defects, impurities or other charges.
One spintronic device already in use is the giant magnetoresistive, or GMR, sandwich structure, which consists of alternating ferromagnetic (that is, permanently magnetized) and nonmagnetic metal layers. Depending on the relative orientation of the magnetizations in the magnetic layers, the electrical resistance through the layers’ changes from small (parallel magnetizations) to large (antiparallel magnetizations).
Investigators discovered that they could use this change in resistance (called magnetoresistance, and “giant” because of the large magnitude of the effect in this case) to construct exquisitely sensitive detectors of changing magnetic fields, such as those marking the data on a computer hard-disk platter. These disk drive read/write heads have been wildly successful, permitting the storage of tens of gigabytes of data on notebook computer hard drives, and have created a billions of dollar per year industry.
The two significant examples of spintronic devices and technology are non-volatile magnetic memories (MRAMs) and semiconductor spintronics.
MRAMs and STT-RAM
A magnetoresistive random access memory ( MRAM ) chip is a bidimensional array of magnetoresistive devices with stable remanent states (0 and 1), integrated on a silicon complementary metal–oxide semiconductor (CMOS) circuit allowing to separately address each memory element.
Spin Transfer Torque Random Access Memory (STT-MRAM or STT-RAM) stores information in the magnetic state of Nano magnets, but it is electrically written and read. This combination allows fast-access, non-volatile information storage but with better scalability over traditional MRAM.
It relies on the different spin directions of electrons to signal a binary one or zero. The STT is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current.
Silicon Quantum computer
A long-term and ambitious subfield of spintronics is the application of electron and nuclear spins to quantum information processing and quantum computation. Scientists and engineers from the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology (CQC2T), are developing a scalable quantum computer in silicon. They found that a single atom of phosphorus could be used to tightly hold an electron, which also carries a “spin” (like a tiny magnet) that could be used as a quantum bit.
Once the systems are scaled up to a large number of qubits, Quantum computers shall be able to execute nontrivial quantum algorithms to simulate quantum systems efficiently, crack modern encryption codes, search through huge databases, as well as solve a wide range of optimization problems.
Research from a team led by North Carolina State University is opening the door to smarter sensors by integrating the smart material vanadium dioxide (VO2) onto a silicon chip and using lasers to make the material magnetic. The advance paves the way for multifunctional spintronic smart sensors for use in military applications and next-generation spintronic devices.
VO2 is currently used to make infrared sensors. By integrating VO2 as a single crystal onto a silicon substrate, the researchers have made it possible to create infrared smart sensors, in which the sensor and computational function are embedded on a single chip. This makes the sensor faster and more energy efficient, since it doesn’t have to send data to another chip to be processed. Smart sensors are also lighter than conventional ones, since separate chips aren’t necessary.
“For military applications, sensor technology needs to be able to sense, manipulate, and respond to data quickly – and this work achieves that,” says Dr. Jay Narayan, John C. Fan Distinguished Chair Professor of Materials Science and Engineering at NC State and senior author of a paper describing the work.
Scientists Develop Novel “Spintronic” Sensors for the Army
U.S. Army Tank Automotive Research, Development and Engineering Center (TARDEC) researchers have made significant contributions to spintronic microwave detector development theory in collaboration with the research group led by Physics Department Professor Andrei Slavin at Oakland University, Rochester, MI.
Through crucial collaboration, we co-developed mathematical models and computer programs for optimization of parameters and geometrical dimensions of future spintronic detectors to achieve maximum sensitivity. Ten spintronic microwave detectors have been built based on theoretical calculations and computer modeling by the research group led by Professor Ilya Krivorotov at the University of California at Irvine.
The authors received a 2013 Army Outstanding Technical Research and Development Achievement Award for the “Spintronic Radar Detectors for Multifunctional Armor.” We are continuing our research, development and integration of the fast and very accurate spintronic sensor system for detection and analysis of radar threats for ground combat vehicles. The system is based on arrays of nano-scale radiation-hard frequency-selective spintronic microwave diodes (SMD). The spintronic radar detectors and planar microwave antennas could be embedded directly into the vehicle’s armor without compromising its structural integrity.
Spintronic Radar Detector receives a microwave signal from a planar antenna. This signal creates a resonance magnetization precession in the “free layer” of a nanoscale SMD. A precession signal is detected by giant magnetoresistance effect. Radar detector (array of SMDs with different inplane shapes and, thus, different resonance frequencies) works as a fast as ~500 nanoseconds (ns).
The fast and reliable detection of radar threats will provide sufficient time to undertake the relevant countermeasures (e.g., active jamming of the enemy radar, reposition of a sacrificial armor component, etc.) which will lead to greatly improved survivability of ground combat vehicles. The characteristic time of frequency determination will be substantially shorter than the return propagation time of a transmitted radar or control pulse that typically is in the order of a microsecond.
The ultrafast detection and spectral analysis of enemy radio transmissions is vital for survivability applications to allow achieving the active interference with these signals on the time scale of the signal propagation time. This problem arises in antiradar defense (to detect incoming radar pulses and jam them or determine the radar position), counterterrorist activity (to detect and jam triggering microwave signals of radiotriggered explosive devices) and military intelligence (to intercept and/or jam radio messages sent using the frequencyhopping spread spectrum method). In all these tasks the detector should be able to determine the frequency of a microwave signal very fast ― on submicrosecond time scale ― to take appropriate counteractions during the time intervals comparable with the time of the pulse propagation.
The next step in developing the novel spintronic radar detectors is the integration of SMD arrays into protective surfaces of ground vehicles. The authors have begun measuring the effects of various protective materials on the SMD detector with CPW antenna.
The objective is to develop prototype spintronic devices or systems and nanoengineered metamaterials for radar detection, signature management and activesmart armor protection systems. Ultimately, these devices and materials will be integrated into ground combat vehicles.
Spintronics realises new terahertz sources
German physicists, along with international collaborators, have realised a new concept for the production of electromagnetic radiation using spintronic emitters.
Taking the form of thin, multi-layered, metal films and using the spin property of the electron – as opposed to the more conventional semiconductor emitters, which use only the electron charge – Physicists from the Fritz Haber Institute in Berlin and Johannes Gutenberg-University in Mainz (JGU) have developed the pioneering method by isolating a single unified source of terahertz (THz) emission that could provide useful terahertz radiation over a range of frequencies.
Professor Matthias Kläui of the JGU said: “Using this property, we were able to show that it is possible to produce broadband emitters fully covering the 1‑to‑30‑THz range which are also cost efficient in terms of their industrial applications.”
The spin of the electron as a quantum property has led to new developments in spintronics in recent years. The optimisation of emitter performance involved Kläui and his team screening numerous materials with varying compositions and geometries, and was augmented by the calculations of theorists at innovative technology development company Forschungszentrum Jülich.
Samridh Jaiswal, an associate of Kläui at Mainz University, added: “The new THz emitter resembles a photodiode or a solar cell: on illuminating the material with an ultrashort laser pulse, an ultrafast spin current is generated. This spin current is then converted to a charge current via the Inverse Spin-Hall effect. Consequently, a transmitter antenna radiates an equivalent electromagnetic pulse with frequencies in the terahertz range
New spintronics breakthrough paves the way to faster computing
Researchers have achieved all-electric control of the spin of electrons in a major breakthrough that brings much faster and more efficient spintronics-based computation closer than ever before. Because a spinning electrically charged particle like an electron has well-known magnetic properties, the most natural way to control electronic spin is to use ferromagnetic materials embedded in spintronic devices. This, however, makes the devices very bulky, which is clearly the opposite of the direction towards which technological progress is pushing.
Led by Dr. Debray, the UC team managed to control the spin of electrons traveling on a wire with an all electrical device for the very first time, reaching a milestone in this new and very promising field that is important mainly because it allows for much smaller spintronic devices to be built.
The team used an indium arsenide “quantum point contact,” a wire only a few hundred nanometers in length whose conductivity can be modified by regulating the voltages at its two ends. The asymmetry that comes from setting two different voltages at the two ends (gates) allows the electrons to become polarized as they enter the wire.
Spintronics breakthrough could lead to single chip for processing and memory
Spintronics is now the preferred technology for manufacturing fast solid state drives (SSDs). In SSDs, so-called “spin valves” harness the magnetic properties of electrons to detect data that is stored in magnetic bits; by contrast, data processing relies on streams of electrons flowing around circuits within a microchip.
Using flexible organic semiconductors, researchers at Queen Mary, University of London and the University of Fribourg have made a discovery that could lead to the simultaneous storing and processing of data on the same computer chip, bringing a dramatic improvement in power efficiency and reduced weight of electronic devices
The Queen Mary/Fribourg team showed that lithium fluoride (LiF), a material with an intrinsic electric field, can modify the spin of electrons transported through the spin valves. This proves that electric fields can manipulate a magnetically polarized current and could lead to computer chips that not only store data, but are capable of manipulating it as well.
The researchers shot muons – unstable subatomic particles – into layers of lithium fluoride and observed their behavior as they decayed, which offered information about the magnetic processes inside the material. Low-energy muons are unique in that they can be placed into a specific layer, making it possible to study the behavior of single layers independently.
“This is especially exciting, as this discovery has been made with flexible organic semiconductors, which are set to be the new generation of displays for mobile devices, TVs and computer monitors, and could offer a step-change in power efficiency and reduced weight of these devices,” said Dr. Alan Drew, who led the research efforts. While devices that combine electron charge and spin are conceptually straightforward, this is the first time researchers have shown it is possible to proactively control the spin of electrons with electric fields.
The spin properties of light could enable spintronics and photonics to co-exist in devices
Researchers at Purdue University have shown that there is a very simple rule that governs light spin and momentum locking. This is a universal property for all optical materials and nanostructures, which makes it potentially very useful for photonic devices. This universality is unique to light and does not occur for electrons. The researchers aim to use these spin properties of light to interface with spintronics so that we might use both photons and electrons in devices.
“In fact, the spintronics dream is a seamless integration of electronic, optoelectronic and magnetoelectronic multifunctionality on a single device that can perform much more than is possible with today’s microelectronic devices,” says Sharma.
Semiconductor Spintronic: Recent Breakthroughs
Researchers are creating numerous breakthroughs in the field including controlling the spin of electrons, manipulating single electrons independently, and the first plastic spintronic computer memory device.
Researchers from the University of Tokyo (Japan), Tokyo Institute of Technology (Japan) and Ho Chi Minh University of Pedagogy (Vietnam) have overcome a significant obstacle to the development of next-generation device technologies. The team has become the first to report growing iron-doped ferromagnetic semiconductors working at room temperature—a longstanding physical constraint.
But until now, ferromagnetic semiconductors have only worked under experimental conditions at extremely low, cold temperatures, typically lower than 200 K (-73oC), which is much colder than the freezing point of water, 273.15 K. Here, K (Kelvin) is a temperature scale which, like the Celsius (oC) scale, has 100 degrees between boiling (373.15 K = 100oC) and freezing (273.15 K = 0oC) of water.
Doping is the practice of adding atoms of impurities to a semiconductor lattice to modify electrical structure and properties. Ferromagnetic semiconductors are valued for their potential to enhance device functionality by utilizing the spin degrees of freedom of electrons in semiconductor devices
“Bridging semiconductor and magnetism is desirable because it would provide new opportunities of utilizing spin degrees of freedom in semiconductor devices,” explained research leader Masaaki Tanaka, Ph.D., of the Department of Electrical Engineering & Information Systems, and Center for Spintronics Research Network, University of Tokyo.
Potential applications of ferromagnetic-semiconductors include designing new and improved devices, such as spin transistors. “Spin transistors are expected to be used as the basic element of low-power-consumption, non-volatile and reconfigurable logic circuits,” Tanaka explained.
Breakthrough in spin wave-based information processing technology
While spin wave based devices are one of the most promising alternatives to current semiconductor technology, spin wave signal propagation is anisotropic in nature — its properties vary in different directions — thus posing challenges for practical industrial applications of such devices.
A research team led by Professor Adekunle Adeyeye from the Department of Electrical and Computer Engineering at the NUS Faculty of Engineering, has recently achieved a significant breakthrough in spin wave information processing technology. His team has successfully developed a novel method for the simultaneous propagation of spin wave signals in multiple directions at the same frequency, without the need for any external magnetic field.
Using a novel structure comprising different layers of magnetic materials to generate spin wave signals, this approach allows for ultra-low power operations, making it suitable for device integration as well as energy-efficient operation at room temperature.
“The ability to propagate spin waves signal in arbitrary directions is a key requirement for actual circuitry implementation. Hence, the implication of our invention is far-reaching and addresses a key challenge for the industrial application of spin wave technology. This will pave the way for non-charge based information processing and realisation of such devices,” said Dr Arabinda Haldar, who is the first author of the study and was formerly a Research Fellow with the Department at NUS. Dr Haldar is currently an Assistant Professor at Indian Institute of Technology Hyderabad.
This discovery builds on an earlier study by the team that was published in Nature Nanotechnology in 2016, in which a novel device that could transmit and manipulate spin wave signals without the need for any external magnetic field or current was developed. The research team has filed patents for these two inventions.
“Collectively, both discoveries would make possible the on-demand control of spin waves, as well as the local manipulation of information and reprogramming of magnetic circuits, thus enabling the implementation of spin wave based computing and coherent processing of data,” said Prof Adeyeye.
New method for generating and detecting spin currents
Battiato and fellow TU Wien scientist Karsten Held developed a new method for generating the spin current, which can be performed extremely quickly. In computer simulations, the researchers attached a layer of nickel to silicon, and zapped the nickel with short laser pulses. This excites the electrons in the nickel to move towards the silicon, with some then passing through into it. The key is that spin up electrons can move much more freely in nickel than spin down ones, and so the majority of those that reach the barrier and pass into the silicon are electrons with a spin up current.
“There have been attempts to send an electric current through a combination of magnets and semiconductors”, says Marco Battiato, one of the researchers on the project. “The idea is to create a flux of electrons with uniform spin, which can then be used for spintronic circuits. But the efficiency of this method is very limited.”
In doing this the team has effectively injected silicon with a specific spin current, without creating an electrical charge. The researchers have calculated that the current created is much stronger than those produced through other methods, and can be done extremely quickly – within quadrillionths of a second. “Spintronics has the potential to become a key technology of the next few decades”, says Battiato. “With our spin injection method there is now finally a way to create ultrafast, extremely strong spin currents.”
A team of scientists, led by An-Ping Li at the Department of Energy’s Oak Ridge National Laboratory, has developed an innovative microscopy technique to detect the spin of electrons in topological insulators, a new kind of quantum material that could be used in applications such as spintronics and quantum computing. The new method builds on a four-probe scanning tunneling microscope–an instrument that can pinpoint a material’s atomic activity with four movable probing tips–by adding a component to observe the spin behavior of electrons on the material’s surface.
This approach not only includes spin sensitivity measurements. It also confines the current to a small area on the surface, which helps to keep electrons from escaping beneath the surface, providing high-resolution results.
2D Materials Go Ferromagnetic, Creating a New Scientific Field
Researchers at the Lawrence Berkeley National Laboratory have successfully demonstrated that two-dimensional (2D) layered crystals held together by van der Waal forces—these include graphene and molybdenum disulfide—can exhibit intrinsic ferromagnetism. Not only did the team demonstrate that it exists in these materials, but the researchers also demonstrated a high degree of control over that ferromagnetism. The discovery could have a profound impact for applications including magnetic sensors and the developing use of spintronics for encoding information.
“Thin films of metals like iron, cobalt, and nickel, unlike 2D van der Waals materials, are structurally imperfect and susceptible to various disturbances, which contribute to a huge and unpredictable spurious anisotropy,” said Cheng Gong, a postdoctoral researcher in Zhang’s lab and co-author of the study, in a press release. “In contrast, the highly crystalline and uniformly flat 2D CGT, together with its small intrinsic anisotropy, allows small external magnetic fields to effectively engineer the anisotropy, enabling an unprecedented magnetic field control of ferromagnetic transition temperatures.
“Our discovery of intrinsic ferromagnetism in 2D van der Waals crystals has opened a scientific research field,” said Xiang Zhang, a senior faculty scientist at Berkeley Lab’s Materials Sciences Division and UC Berkeley professor, in an e-mail interview with IEEE Spectrum.
“Sooner or later, people have to address the ferromagnetism issues in 2D materials, when 3D materials shrink down to 2D regime,” said Zhang. “In other words, 3D materials have to be thinned down to 2D in many fields as a result of the constantly-increasing device density.”
Zhang adds: “We hope to engineer and manipulate the magnetic properties of such 2D materials to make them suitable for various application purposes.” “We envision that 2D ferromagnetic van der Waals materials would also have a broad range of potential applications such as nanoscale memories, magnetic sensors, transparent magnets, magneto-optic modulators,” said Zhang.
Czech scientists build non-metal magnet out of carbon
Scientists in the Czech Republic created magnetized carbon by treating graphene layers with non-metallic elements.
For several years, we have suspected that the path to magnetic carbon could involve graphene — a single two-dimensional layer of carbon atoms,” lead researcher Radek Zbořil, director of the Regional Center of Advanced Technologies and Materials at the Palacky University, Olomouc, said in a news release. “Amazingly, by treating it with other non-metallic elements such as fluorine, hydrogen, and oxygen, we were able to create a new source of magnetic moments that communicate with each other even at room temperature. This discovery is seen as a huge advancement in the capabilities of organic magnets.”
“In metallic systems, magnetic phenomena result from the behavior of electrons in the atomic structure of metals,” explained researcher Michal Otyepka. “In the organic magnets that we have developed, the magnetic features emerge from the behavior of non-metallic chemical radicals that carry free electrons.”
“Such magnetic graphene-based materials have potential applications in the fields of spintronics and electronics, but also in medicine for targeted drug delivery and for separating molecules using external magnetic fields,” said scientist Jiri Tucek.
DARPA’s Semiconductor Technology Advanced Research Network (STARnet)
Semiconductor Research Corporation (SRC) and the Defense Advanced Research Projects Agency (DARPA) has launched a $194 million initiative, the Semiconductor Technology Advanced Research Network (STARnet) to help maintain U.S. leadership in semiconductor technology that is vital to U.S. prosperity, security and intelligence.
One of the six academic teams is Center for Spintronic Materials, Interfaces, and Novel Architectures (C_SPIN): C_SPIN hosted at the University of Minnesota focuses on magnetic materials, spin transport, novel spin-transport materials, spintronic devices, circuits and novel architectures and create the fundamental building blocks that allow revolutionary spin-based multi-functional, scalable memory devices and computational architectures to be realized.
“Without the nanoelectronics sector there would be no viable defence sector, and without defence, investment in nanoelectronics would not be feasible”, said Michael Sieber, EDA assisting one roundtable.