Spintronics is an emerging field of nanoscale electronics which uses not only the charge of electrons but also the spin of electrons. The technology doesn’t require a specialized semiconductor material resulting in reduced manufacturing costs. Other advantages include less energy requirement, as well as low power consumption with competitive data transfer and storage capacity.
Spintronic devices generate little heat and use relatively minuscule amounts of electricity. Spintronic computers would require no energy to maintain data in memory. They would also start instantly and have the potential to be far more powerful than today’s computers. The spintronic devices will find widespread application in civilian and military markets offering new generation of transistors, lasers and integrated magnetic sensors. It has been used in a variety of devices for information processing, memory and storage — in particular, ultra-high density hard drives and non-volatile memories.
Engineers at the University of California, Riverside, have reported advances in so-called “spintronic” devices that will help lead to a new technology for computing and data storage. They have developed methods to detect signals from spintronic components made of low-cost metals and silicon, which overcomes a major barrier to wide application of spintronics. Previously such devices depended on complex structures that used rare and expensive metals such as platinum. The researchers were led by Sandeep Kumar, an assistant professor of mechanical engineering.
Three independent teams of physicists have unveiled devices that could lead to practical spintronics components of the future. Researchers in the Netherlands have created what they call a “magnon transistor”, whereas a group in China has unveiled their “magnon valve”. Meanwhile in Germany, a team has also demonstrated their own version of a magnon valve. All three devices represent important work towards creating practical spintronics devices that use electron spin to transfer and store information.
In two other scientific papers, the researchers demonstrated that they could generate a key property for spintronics materials, called antiferromagnetism, in silicon. The achievement opens an important pathway to commercial spintronics, said the researchers, given that silicon is inexpensive and can be manufactured using a mature technology with a long history of application in electronics.
Researchers have discovered a switch to control the spin current,
Researchers at Tohoku University in Japan have discovered a switch to control the spin current, a mechanism needed for information processing with full spin-based devices. This is significant because although the technology behind detecting and generating the spin current has been established for some time, a long-missing component in the history of spintronics has been a “spin current switch.” It’s the equivalent of the transistor used in electronics to enable and disable the flow of electricity.
Materials have built-in-mechanisms to enable the electrical detection of the spin current, such as the inverse spin hall effect (ISHE). Using the ISHE, spin current generated by other forms of energy like microwaves (spin pumping) and heat (spin Seebeck effect) is transformed into electrical voltage in the material.
Now, Zhiyong Qiu, Dazhi Hou, Eiji Saitoh and collaborators at Tohoku and Mainz Universities, have proved that a newly developed layered structure of materials works as a spin current switch. Using the structure, they were able to control the transmission of spin current at a 500% increase at near room temperature. The tri-layer structure which sandwiches Cr2O3 between yttrium iron garnet (YIG) and platinum (Pt). The YIG/Pt pair is a standard combination of materials used to investigate the spin current flow — both are insulators in which electrons cannot flow. YIG, a ferrimagnet electric insulator, generates spin current in response to RF microwave or temperature gradient and Pt, a paramagnetic metal, detects the spin current as an electric voltage via ISHE.
By placing Cr2O3 between the materials, the voltage signal at Pt reflects how much the Cr2O3 layer can transmit the spin current. The researchers investigated the change of the voltage against the temperature and the applied magnetic field. “We observed a massive reduction in the voltage signal when crossing the temperature at around 300K, at which point Cr2O3 changes its phase from paramagnet to anti-ferromagnet (Neel point),” said Assistant Professor Dazhi Hou. The change of the spin current transmission is a near 500% increase under the application of a magnetic field. This behaviour suggests that the layered structure works as a spin current switch when crossing the Neel point of Cr2O3 or applying a magnetic field.
“Just as the transistor revolutionized electronics by enabling the scalable development of electronic devices, the discovery of a spin current switch is likely to take spintronics in a new direction,” said Professor Eiji Saitoh. “It’s a significant development.”
Magnon transistors could give spintronics a boost
However, creating spintronics based around the electron as the information carrier has its own challenges, so some physicists are keen on exploring alternatives. One possibility is the magnon, which is a collective excitation in a magnetic material. Magnons propagate as waves – flipping spins as they go. They also have particle-like properties, which is why they are called quasiparticles.
Circuits based on magnons have the potential to be much simpler in design than comparable conventional electronics – while at the same time consuming much less energy. But as Andrii Chumak of Germany’s University of Kaiserslautern, who was not involved in the research, explains: “We are still quite far away from realizing this potential”.
In this latest drive to create magnonic devices, a team led by Mathias Kläui of Johannes Gutenberg University of Mainz in Germany and a team led by Xiufeng Han of the Chinese Academy of Sciences in Beijing have showed that a magnon current can be controlled by changing the relative magnetization orientation of two magnetic layers.
Although made from different materials, both devices comprise a sandwich of two magnets separated by a non-magnetic spacer. By aligning the magnetic moments of the “bread” of the sandwich parallel or antiparallel, the researchers managed to increase and decrease the magnon current flowed through their devices – so the devices operated as valves.
“Both [devices] show typical spin valve behavior, and the effects are large so they could in future be used as a non-volatile low-power logic component,” explains Kläui. “But we now need to quantify the modulation of the magnonic spin current transmission in an ideal spin valve geometry.”
Taking a different approach, but still aiming to control magnon current, Bart van Wees of the University of Groningen, the Netherlands, and colleagues altered magnon current using an electrode to change chemical potential in a device they have called a “magnon transistor”. The device consists of a thin rectangle of platinum on top of a larger square of magnetic material. Magnons are generated at one end of the magnet and detected at the other. Then more magnons are pumped into or absorbed from the square depending on the spin polarization of electrons flowing in the platinum strip. By aligning and then oppositely aligning these electron spins with the magnons in the square, the researchers managed to increase and then decrease the magnon current.
This magnon transistor offers two potential benefits compared to the magnon valves: it operates faster than the valves and it should be more useful for creating complex circuits. However, the change in magnon current is much smaller than in the magnon valves. Also, because a spin current is used to modulate the magnon current, the transistor does not offer a low-power advantage over conventional electronics.
The devices could be key steps towards realizing full magnonic devices, but Chumak urges caution for those believing the research signals that magnonic circuits are just around the corner: “My personal feeling is that these papers represent an important step forward, but in fundamental physics only,” he says. “The magnonic signal has to be converted into electric current (in the Dutch device) or to magnetization orientation (in the two other cases) – a serious problem which requires in-depth investigations.”
Electronics of the future: A new energy-efficient mechanism using the Rashba effect
Scientists at Tokyo Institute of Technology proposed new quasi-1D materials for potential spintronic applications, an upcoming technology that exploits the spin of electrons. They performed simulations to demonstrate the spin properties of these materials and explained the mechanisms behind their behavior. Serious improvements in performance and new applications can be attained through “spin currents”.
As promising as spintronics sound, researchers are still trying to find convenient ways of generating spin currents with material structures that possess electrons with desirable spin properties. The Rashba-Bychkov effect (or simply Rashba effect), which involves a splitting of spin-up and spin-down electrons due to breakings in symmetry, could potentially be exploited for this purpose. A pair of researchers from Tokyo Institute of Technology, including Associate Professor Yoshihiro Gohda, have proposed a new mechanism to generate a spin current without energy loss from a series of simulations for new quasi-1D materials based on bismuth-adsorbed indium that exhibit a giant Rashba effect. “Our mechanism is suitable for spintronic applications, having an advantage that it does not require an external magnetic field to generate nondissipative spin current,” explains Gohda. This advantage would simplify potential spintronic devices and would allow for further miniaturization.
The researchers conducted simulations based on these materials to demonstrate that the Rashba effect in them can be large and only requires applying a certain voltage to generate spin currents. By comparing the Rashba properties of multiple variations of these materials, they provided explanations for the observed differences in the materials’ spin properties and a guide for further materials exploration.
This type of research is very important as radically new technologies are required if we intend to further improve electronic devices and go beyond their current physical limits. “Our study should be important for energy-efficient spintronic applications and stimulating further exploration of different 1D Rashba systems,” concludes Gohda. From faster memories to quantum computers, the benefits of better understanding and exploiting Rashba systems will certainly have enormous implications.
In one paper published in the January issue of the scientific journal Applied Physics Letters, Kumar and colleagues reported an efficient technique of detecting the spin currents in a simple two-layer sandwich of silicon and a nickel-iron alloy called Permalloy. All three of the components are both inexpensive and abundant and could provide the basis for commercial spintronic devices. They also operate at room temperature. The layers were created with the widely used electronics manufacturing processes called sputtering. Co-authors of the paper were graduate students Ravindra Bhardwaj and Paul Lou.
In their experiments, the researchers heated one side of the Permalloy-silicon bi-layer sandwich to create a temperature gradient, which generated an electrical voltage in the bi-layer. The voltage was due to phenomenon known as the spin-Seebeck effect. The engineers found that they could detect the resulting “spin current” in the bi-layer due to another phenomenon known as the “inverse spin-Hall effect.”
The researchers said their findings will have application to efficient magnetic switching in computer memories, and “these scientific breakthroughs may give impetus” to development of such devices. More broadly, they concluded, “These results bring the ubiquitous Si (silicon) to forefront of spintronics research and will lay the foundation of energy efficient Si spintronics and Si spin caloritronics devices.”
Ferromagnetism is the property of magnetic materials in which the magnetic poles of the atoms are aligned in the same direction. In contrast, antiferromagnetism is a property in which the neighboring atoms are magnetically oriented in opposite directions. These “magnetic moments” are due to the spin of electrons in the atoms, and is central to the application of the materials in spintronics.
In the two papers, Kumar and Lou reported detecting antiferromagnetism in the two types of silicon—called n-type and p-type—used in transistors and other electronic components. N-type semiconductor silicon is “doped” with substances that cause it to have an abundance of negatively-charged electrons; and p-type silicon is doped to have a large concentration of positively charged “holes.” Combining the two types enables switching of current in such devices as transistors used in computer memories and other electronics.
In the paper in the Journal of Magnetism and Magnetic Materials, Lou and Kumar reported detecting the spin-Hall effect and antiferromagnetism in n-silicon. Their experiments used a multilayer thin film comprising palladium, nickel-iron Permalloy, manganese oxide and n-silicon. And in the second paper, in the scientific journal physica status solidi, they reported detecting in p-silicon spin-driven antiferromagnetism and a transition of silicon between metal and insulator properties. Those experiments used a thin film similar to those with the n-silicon.
The researchers wrote in the latter paper that “The observed emergent antiferromagnetic behavior may lay the foundation of Si (silicon) spintronics and may change every field involving Si thin films. These experiments also present potential electric control of magnetic behavior using simple semiconductor electronics physics. The observed large change in resistance and doping dependence of phase transformation encourages the development of antiferromagnetic and phase change spintronics devices.”
In further studies, Kumar and his colleagues are developing technology to switch spin currents on and off in the materials, with the ultimate goal of creating a spin transistor. They are also working to generate larger, higher-voltage spintronic chips. The result of their work could be extremely low-power, compact transmitters and sensors, as well as energy-efficient data storage and computer memories, said Kumar
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.
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.
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.