Tag Archives: #spintronics

Scientists Create World’s Thinnest Magnet (Physics)

A one-atom thin 2D magnet could advance new applications in computing and electronics

The development of an ultrathin magnet that operates at room temperature could lead to new applications in computing and electronics – such as high-density, compact spintronic memory devices – and new tools for the study of quantum physics.

The ultrathin magnet, which was recently reported in the journal Nature Communications , could make big advances in next-gen memories, computing, spintronics, and quantum physics. It was discovered by scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley.

“We’re the first to make a room-temperature 2D magnet that is chemically stable under ambient conditions,” said senior author Jie Yao, a faculty scientist in Berkeley Lab’s Materials Sciences Division and associate professor of materials science and engineering at UC Berkeley.

“This discovery is exciting because it not only makes 2D magnetism possible at room temperature, but it also uncovers a new mechanism to realize 2D magnetic materials,” added Rui Chen, a UC Berkeley graduate student in the Yao Research Group and lead author on the study.”

The magnetic component of today’s memory devices is typically made of magnetic thin films. But at the atomic level, these magnetic films are still three-dimensional – hundreds or thousands of atoms thick. For decades, researchers have searched for ways to make thinner and smaller 2D magnets and thus enable data to be stored at a much higher density.

Previous achievements in the field of 2D magnetic materials have brought promising results. But these early 2D magnets lose their magnetism and become chemically unstable at room temperature.

“State-of-the-art 2D magnets need very low temperatures to function. But for practical reasons, a data center needs to run at room temperature,” Yao said. “Theoretically, we know that the smaller the magnet, the larger the disc’s potential data density. Our 2D magnet is not only the first that operates at room temperature or higher, but it is also the first magnet to reach the true 2D limit: It’s as thin as a single atom!”

The researchers say that their discovery will also enable new opportunities to study quantum physics. “Our atomically thin magnet offers an optimal platform for probing the quantum world,” Yao said. “It opens up every single atom for examination, which may reveal how quantum physics governs each single magnetic atom and the interactions between them. With a conventional bulk magnet where most of the magnetic atoms are deeply buried inside the material, such studies would be quite challenging to do.”

The making of a 2D magnet that can take the heat

The researchers synthesized the new 2D magnet – called a cobalt-doped van der Waals zinc-oxide magnet – from a solution of graphene oxide, zinc, and cobalt. Just a few hours of baking in a conventional lab oven transformed the mixture into a single atomic layer of zinc-oxide with a smattering of cobalt atoms sandwiched between layers of graphene. In a final step, graphene is burned away, leaving behind just a single atomic layer of cobalt-doped zinc-oxide.

“With our material, there are no major obstacles for industry to adopt our solution-based method,” said Yao. “It’s potentially scalable for mass production at lower costs.”

To confirm that the resulting 2D film is just one atom thick, Yao and his team conducted scanning electron microscopy experiments at Berkeley Lab’s Molecular Foundry to identify the material’s morphology, and transmission electron microscopy imaging to probe the material atom by atom.

With proof in hand that their 2D material really is just an atom thick, the researchers went on to the next challenge that had confounded researchers for years: Demonstrating a 2D magnet that successfully operates at room temperature.

X-ray experiments at Berkeley Lab’s Advanced Light Source characterized the 2D material’s magnetic parameters under high temperature. Additional X-ray experiments at SLAC National Accelerator Laboratory’s Stanford Synchrotron Radiation Lightsource verified the electronic and crystal structures of the synthesized 2D magnets. And at Argonne National Laboratory’s Center for Nanoscale Materials, the researchers imaged the 2D material’s crystal structure and chemical composition using transmission electron microscopy.

As a whole, the research team’s lab experiments showed that the graphene-zinc-oxide system becomes weakly magnetic with a 5-6% concentration of cobalt atoms. Increasing the concentration of cobalt atoms to about 12% results in a very strong magnet.

To the researchers’ surprise, a concentration of cobalt atoms exceeding 15% shifts the 2D magnet into an exotic quantum state of “frustration,” whereby different magnetic states within the 2D system are in competition with each other.

And unlike previous 2D magnets, which lose their magnetism at room temperature or above, the researchers found that the new 2D magnet not only works at room temperature but also at 100 degrees Celsius (212 degrees Fahrenheit).

“Our 2D magnetic system shows a distinct mechanism compared to previous 2D magnets,” said Chen. “And we think this unique mechanism is due to the free electrons in zinc oxide.”

True north: Free electrons keep magnetic atoms on track

When you command your computer to save a file, that information is stored as a series of ones and zeroes in the computer’s magnetic memory, such as the magnetic hard drive or a flash memory. And like all magnets, magnetic memory devices contain microscopic magnets with two poles – north and south, the orientations of which follow the direction of an external magnetic field. Data is written or encoded when these tiny magnets are flipped to the desired directions.

According to Chen, zinc oxide’s free electrons could act as an intermediary that ensures the magnetic cobalt atoms in the new 2D device continue pointing in the same direction – and thus stay magnetic – even when the host, in this case the semiconductor zinc oxide, is a nonmagnetic material.

“Free electrons are constituents of electric currents. They move in the same direction to conduct electricity,” Yao added, comparing the movement of free electrons in metals and semiconductors to the flow of water molecules in a stream of water.

The researchers say that new material – which can be bent into almost any shape without breaking, and is 1 millionth the thickness of a single sheet of paper – could help advance the application of spin electronics or spintronics, a new technology that uses the orientation of an electron’s spin rather than its charge to encode data. “Our 2D magnet may enable the formation of ultra-compact spintronic devices to engineer the spins of the electrons,” Chen said.

“I believe that the discovery of this new, robust, truly two-dimensional magnet at room temperature is a genuine breakthrough by Jie Yao and his students,” said co-author Robert Birgeneau, a faculty senior scientist in Berkeley Lab’s Materials Sciences Division and professor of physics at UC Berkeley who co-led the study’s magnetic measurements. “In addition to its obvious significance to spintronic devices, this 2D magnet is fascinating at the atomic level, revealing for the first time how cobalt magnetic atoms interact over ‘long’ distances” through a complex two-dimensional network, he added.

“Our results are even better than what we expected, which is really exciting. Most of the time in science, experiments can be very challenging,” he said. “But when you finally realize something new, it’s always very fulfilling.”

Co-authors on the paper include researchers from Berkeley Lab, including Alpha N’Diaye and Padraic Shafer of the Advanced Light Source; UC Berkeley; UC Riverside; Argonne National Laboratory; and Nanjing University and the University of Electronic Science and Technology of China.

Featured image: Illustration of magnetic coupling in a cobalt-doped zinc-oxide monolayer. Red, blue, and yellow spheres represent cobalt, oxygen, and zinc atoms, respectively. © Berkeley Lab

Reference: Chen, R., Luo, F., Liu, Y. et al. Tunable room-temperature ferromagnetism in Co-doped two-dimensional van der Waals ZnO. Nat Commun 12, 3952 (2021). https://doi.org/10.1038/s41467-021-24247-w

Provided by LBL

Transforming The Layered Ferromagnet Fe5GeTe2 For Future Spintronics (Chemistry)

  • Realizing in-situ magnetic phase transition in metallic van-der-Waals magnet Fe5GeTe2 via ultra-high charge doping

A RMIT-led international collaboration published this week has achieved record-high electron doping in a layered ferromagnet, causing magnetic phase transition with significant promise for future electronics

Control of magnetism (or spin directions) by electric voltage is vital for developing future, low-energy high-speed nano-electronic and spintronic devices, such as spin-orbit torque devices and spin field-effect transistors.

Ultra-high-charge, doping-induced magnetic phase transition in a layered ferromagnet allows promising applications in antiferromagnetic spintronic devices.

The FLEET collaboration of researchers at RMIT, UNSW, the University of Wollongong and FLEET partner High Magnetic Field Laboratory (China) demonstrates for the first time that ultra-high electron doping concentration (above 1021 cm-3) can be induced in the layered van der Waals (vdW) metallic material Fe5GeTe2 by proton intercalation, and can further cause a transition of the magnetic ground state from ferromagnetism to antiferromagnetism.

guolin image
Co-author FLEET Research Fellow Dr Guolin Zheng (RMIT) © FLEET

The emergence of layered, vdW magnetic materials has expedited a growing search for novel vdW spintronic devices.

Compared to itinerant ferromagnets, antiferromagnets (AFMs) have unique advantages as building blocks of such future spintronic devices. Their robustness to stray magnetic fields makes them suitable for memory devices, and the AFM-based spin-orbit torque devices require a lower current density than that in ferromagnets.

However currently vdW itinerant antiferromagnets are still scarce.

Besides directly synthesizing a vdW antiferromagnet, another possible method toward this function is to induce a magnetic phase transition in an existing vdW itinerant ferromagnet.

“We chose to work with newly synthesised vdW itinerant ferromagnet Fe5GeTe(F5GT)” says the study’s first author, FLEET Research Fellow Dr Cheng Tan (RMIT).

Crystal structure and initial characterization of F5GT © FLEET

“Our previous experience on Fe3GeTe(Nature Communication 2018) enabled us to quickly identify and evaluate the material’s magnetic properties, and some studies indicate Fe5GeTeis sensitive to local atomic arrangements and interlayer stacking configurations, meaning it would be possible to induce a phase transition in it by doping,” Cheng says.

The team firstly investigated the magnetic properties in Fe5GeTe2 nanosheets of various thicknesses by electron transport measurements.

However, the initial transport results also show that the electron density in Fe5GeTeis high as expected, indicating that the magnetism is hard to be modulated by traditional gate-voltage due to the electric-screen effect in metal:

“Despite the high charge density in Fe5GeTe2, we knew it was worth trying to tune the material via protonic gating, as we have previously achieved in Fe3GeTe(Physical Review Letters 2020), because protons can easily penetrate into the interlayer and induce large charge doping, without damaging the lattice structure,” says co-author Dr Guolin Zheng (also at RMIT).

A SP-FET transistor, with F5GT flake on a solid proton conductor (SPC) – scale = 10µm © FLEET

Like all classical-computing beyond-CMOS researchers, the team are seeking to build an improved form of the transistor, the switches that provide the binary backbone of modern electronics.

A solid protonic field-effect transistor (SP-FET) is one that switches based on insertion (intercalation) of protons. Unlike traditional proton FETs (which switch by dipping liquid, and are considered promising candidates for bridging between traditional electronics and biological systems. ), the SP-FET is solid, and thus suitable for use in real devices

The SP-FET has been demonstrated to be very powerful in tuning thick metallic materials (ie, it can induce large charge doping level), which are very difficult to modulate via traditional dielectric based or ion liquid gating techniques(because of electric screening effect in metal).

By fabricating a solid protonic field-effect transistor (SP-FET) with Fe5GeTe2, the team were able to dramatically change the carrier density in Fe5GeTeand change its magnetic ground state. Further density functional theory calculation confirmed the experimental results.

“All the samples show that the ferromagnetic state can be gradually supressed by increasing proton intercalation, and finally we see several samples display no hysteresis loops, which indicates the change of the magnetic ground state, the theoretical calculations are consistent with the experimental results,” says Cheng.

“The success of realizing an AFM phase in metallic vdW ferromagnet Fe5GeTe2 nanosheets constitutes an important step towards vdW antiferromagnetic devices and heterostructures that operate at high temperatures,” says co-author A/Prof Lan Wang (also at RMIT).

“Again, this demonstrates that our protonic gate technique is a powerful weapon in electron transport experiments, and probably in other areas well.”

lan wang image
Team leader FLEET Chief Investigator A/Prof Lan Wang in Class 100 clean room, RMIT © Fleet

Gate-controlled magnetic phase transition in a van der Waals magnet Fe5GeTe2” was published in Nano Letters in June 2021. (DOI: 10.1021/acs.nanolett.1c01108)

As well as support from the Australian Research Council, support was also provided by Natural Science Foundation of China, the National Key Research and Development Program of China, the Fundamental Research Funds for the Central Universities, the Collaborative Innovation Program of Hefei Science Center and the High Magnetic Field Laboratory (China).

Experimental research was performed at the RMIT Micro Nano Research Facility (MNRF) in the Victorian Node of the Australian National Fabrication Facility (ANFF) and the RMIT Microscopy and Microanalysis Facility (RMMF), as well as the High Magnetic Field Laboratory (Anhui, China).

Spintronic devices are studied within Enabling technology B at FLEET, an Australian Research Council Centre of Excellence. The Centre for Future Low-Energy Electronics Technologies (FLEET) brings together over a hundred Australian and international experts, with the shared mission to develop a new generation of ultra-low energy electronics. The impetus behind such work is the increasing challenge of energy used in computation, which uses 5–8% of global electricity and is doubling every decade.

Featured image: First-author FLEET Research Fellow Dr Cheng Tan (RMIT) © FLEET

Provided by FLEET

A New Spintronic Phenomenon: Chiral-spin Rotation Found in Non-collinear Antiferromagnet (Engineering)

Researchers at Tohoku University and the Japan Atomic Energy Agency (JAEA) have discovered a new spintronic phenomenon – a persistent rotation of chiral-spin structure.

Their discovery was published in the journal Nature Materials on May 13, 2021.

Tohoku University and JAEA researchers studied the response of chiral-spin structure of a non-collinear antiferromagnet Mn3Sn thin film to electron spin injection and found that the chiral-spin structure shows persistent rotation at zero magnetic field. Moreover, their frequency can be tuned by the applied current.

“The electrical control of magnetic structure has been of paramount interest in the spintronics community for the last quarter of a century. The phenomenon shown here provides a very efficient scheme to manipulate magnetic structures, offering new opportunities for application, such as oscillators, random number generators, and nonvolatile memory,” said Professor Shunsuke Fukami, who spearheaded the project.

A figure of merit, defined as the ratio of critical field HC to critical current density JC to manipulate magnetic structure, as a function of magnetic layer thickness for non-collinear antiferromagnet (NC-AFM) as seen in this study. Also shown here is a previously studied collinear ferromagnet (C-Ferro) and ferrimagnet (C-Ferri). ©S.Fukami

Figure 1 compares the efficiency of manipulating the magnetic structure on a non-collinear antiferromagnet, as seen in the present work, with those reported for other material systems. The current-induced chiral-spin rotation is much more efficient even for thick magnetic layers above 20 nm.

The schematics of chiral-spin rotation as well as the experimental setup are shown in figure 2.

A schematic of experiment and chiral-spin persistent rotation found in this study. ©S.Fukami

The researchers used a high-quality heterostructure consisting of non-collinear antiferromagnet Mn3Sn sandwiched between heavy metals W/Ta and Pt. They revealed that, when a current is applied to the heterostructure, the chiral-spin structure rotates persistently at zero magnetic field because of the torque originating from the spin current generated in the heavy metals. Meanwhile, the rotation frequency, typically above 1 GHz, depends on the applied current.

Spintronics is an interdisciplinary field, where electric and magnetic degrees of freedom of electrons are utilized simultaneously, allowing for an electrical manipulation of magnetic structure. Representative schemes established so far are summarized in figure 3.

Representative examples of electrical control of magnetism. ©S.Fukami

Magnetization switching, magnetic-phase transition, oscillation, and resonance have been observed in ferromagnets, which are promising since they may lead to the realization of functional devices in nonvolatile memory, wireless communication, and so on.

Additionally, in antiferromagnets, the 90-degree rotation of Néel vector in collinear systems and the 180 degree switching of chiral-spin structures in non-collinear systems have been observed recently. The chiral-spin persistent rotation in the current work is totally different from all the previously found phenomena and thus should open a new horizon of the spintronics research.

“The obtained insight is not only interesting in terms of physics and material science but also attractive for functional device applications,” added Dr. Yutaro Takeuchi, the first author of the paper. “We would like to further improve the material and device technique in the near future and demonstrate new functional devices such as tunable oscillator and high-quality true random number generator.”

Publication Details:

  • Title: Chiral-spin rotation of non-collinear antiferromagnet by spin-orbit torque
  • Authors: Yutaro Takeuchi, Yuta Yamane, Ju-Young Yoon, Ryuichi Itoh, Butsurin Jinnai, Shun Kanai, Jun’ichi Ieda, Shunsuke Fukami, and Hideo Ohno
  • Journal: Nature Materials
  • DOI10.1038/s41563-021-01005-3

Provided by Tohoku University

Wireless and Battery-free Spintronic Energy Harvester (Engineering)

Researchers at the National University of Singapore (NUS) and Tohoku University have demonstrated that an array of electrically connected spintronic devices can harvest a 2.4 GHz wireless signal, which can be used to power and charge small electronic devices and sensors.

The researchers from NUS and Tohoku University have successfully synchronized the four electrically connected magnetic tunnel junction (MTJ), for the signal transmission at 2.4 GHz. Furthermore, the eight MTJs array was integrated with the conventional battery-free electronics to harvest a wireless signal of 2.4 GHz to a DC signal, which is used to power light emitting diodes (LED).

“This breakthrough has proven the potential of an on-chip array of the MTJs towards high-frequency applications such as wireless transmission and energy harvesting,” said Professor Hyunsoo Yang of NUS, who spearheaded the project.

With the increase of Wi-Fi sources everywhere in smart cities, the radio-frequency signal primarily at 2.4 GHz becomes an abundant source of energy harvesting. The radio-frequency energy harvester captures electromagnetic waves from the wireless sources and converts them into a usable DC signal, which can be utilized for wireless charging and self-sustained smart wireless sensors.

MTJs, a representative functional device of spintronics exploiting the spin degree of freedom of electrons, has already had an enormous impact on magnetic sensors and computing memories. The MTJ is expected to find application in wireless communication systems in the form of radio-frequency generation and high-frequency rectification. However, the commercial viability of high-frequency MTJs is hindered due to a typically low output power (nW) and broad linewidth (MHz). Mutual synchronization of multiple MTJs is one way to overcome this problem. However, an effective pathway to synchronize the MTJs at the Wi-Fi bandwidth has not been clear until now.

Schematic of the proof-of-concept demonstration of harvesting a wireless signal of 2.4 GHz using electrically-connected eight magnetic tunnel junctions ©️S.Fukami

The researchers overcame this challenge by developing MTJs with canted anisotropy that can be electrically synchronized at a GHz-range using a single DC source. The MTJs were fabricated by Shunsuke Fukami’s team at Tohoku University and the integration and measurement were performed by Hyunsoo Yang’s team at the NUS. They found an efficient scheme to convert rf signal to DC at zero bias and zero magnetic field. Using this advantage, the rectified response of eight connected STOs in series is integrated with the conventional battery-free electronics to light up a 1.6 V LED.

This work opens a new avenue to realize wireless and battery-free sensors and processors. Enhancing the MTJ performance and increasing the number of MTJs are expected to make this technology useful in the new paradigm of IoT societies, where many more “things” can communicate via the Internet.

Publication Details:

  • Title: Electrically connected spin-torque oscillators for 2.4 GHz WiFi band transmission and energy harvesting
  • Authors: Raghav Sharma, Rahul Mishra, Tung Ngo, Yong-Xin Guo, Shunsuke Fukami, Hideo Sato, Hideo Ohno, and Hyunsoo Yang
  • Journal: Nature Communications
  • DOI: 10.1038/ s41467-021-23181-1

Provided by Tohoku University

A Breakthrough That Enables Practical Semiconductor Spintronics (Physics)

It may be possible in the future to use information technology where electron spin is used to process information in quantum computers. It has long been the goal of scientists to be able to use spin-based quantum information technology at room temperature. Researchers from Sweden, Finland and Japan have now constructed a semiconductor component in which information can be efficiently exchanged between electron spin and light – at room temperature and above.

Experimental setup similar to the one the researchers have used. © Thor Balkhed

It is well known that electrons have a negative charge, and they also have another property, namely spin. The latter may prove instrumental in the advance of information technology. To put it simply, we can imagine the electron rotating around its own axis, similar to the way in which the Earth rotates around its own axis. Spintronics – a promising candidate for future information technology – uses this quantum property of electrons to store, process and transfer information. This brings important benefits, such as higher speed and lower energy consumption than traditional electronics.

Developments in spintronics in recent decades have been based on the use of metals, and these have been highly significant for the possibility of storing large amounts of data. There would, however, be several advantages in using spintronics based on semiconductors, in the same way that semiconductors form the backbone of today’s electronics and photonics.

Weimin Chen © Peter Modin

“One important advantage of spintronics based on semiconductors is the possibility to convert the information that is represented by the spin state and transfer it to light, and vice versa. The technology is known as opto-spintronics. It would make it possible to integrate information processing and storage based on spin with information transfer through light”, says Weimin Chen, professor at Linköping University, Sweden, who led the project.

As electronics used today operates at room temperature and above, a serious problem in the development of spintronics has been that electrons tend to switch and randomise their direction of spin when the temperature rises. This means that the information coded by the electron spin states is lost or becomes ambiguous. It is thus a necessary condition for the development of semiconductor-based spintronics that we can orient essentially all electrons to the same spin state and maintain it, in other words that they are spin polarised, at room temperature and higher temperatures. Previous research has achieved a highest electron spin polarisation of around 60% at room temperature, untenable for large-scale practical applications.

Researchers at Linköping University, Tampere University and Hokkaido University have now achieved an electron spin polarisation at room temperature greater than 90%. The spin polarisation remains at a high level even up to 110 °C. This technological advance, which is described in Nature Photonics, is based on an opto-spintronic nanostructure that the researchers have constructed from layers of different semiconductor materials (see description below the article). It contains nanoscale regions called quantum dots. Each quantum dot is around 10,000 times smaller than the thickness of a human hair. When a spin polarised electron impinges on a quantum dot, it emits light – to be more precise, it emits a single photon with a state (angular momentum) determined by the electron spin. Thus, quantum dots are considered to have a great potential as an interface to transfer information between electron spin and light, as will be necessary in spintronics, photonics and quantum computing. In the newly published study, the scientists show that it is possible to use an adjacent spin filter to control the electron spin of the quantum dots remotely, and at room temperature.

The quantum dots are made from indium arsenide (InAs), and a layer of gallium nitrogen arsenide (GaNAs) functions as a filter of spin. A layer of gallium arsenide (GaAs) is sandwiched between them. Similar structures are already being used in optoelectronic technology based on gallium arsenide, and the researchers believe that this can make it easier to integrate spintronics with existing electronic and photonic components.

“We are very happy that our long-term efforts to increase the expertise required to fabricate highly-controlled N-containing semiconductors is defining a new frontier in spintronics. So far, we have had a good level of success when using such materials for optoelectronics devices, most recently in high-efficiency solar-cells and laser diodes. Now we are looking forward to continuing this work and to unite photonics and spintronics, using a common platform for light-based and spin-based quantum technology”, says Professor Mircea Guina, head of the research team at Tampere University in Finland.

Financial support for the research has been granted by, among other bodies, the Swedish Research Council, the Swedish Foundation for International Cooperation in Research and Higher Education (STINT), the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, the European Research Council ERC, the Academy of Finland, and the Japan Society for the Promotion of Science.

Featured image: The quantum dots in the opto-spintronic nanostructure are made from indium arsenide (InAs). Each quantum dot is around 10,000 times smaller than the thickness of a human hair. © Yuqing Huang

The article: “Room-temperature electron spin polarization exceeding 90% in an opto-spintronic semiconductor nanostructure via remote spin filtering”, Yuqing Huang, Ville Polojärvi, Satoshi Hiura, Pontus Höjer, Arto Aho, Riku Isoaho, Teemu Hakkarainen, Mircea Guina, Shino Sato, Junichi Takayama, Akihiro Murayama, Irina A. Buyanova and Weimin M. Chen, (2021), Nature Photonics, published online on 8 April 2021, doi: 10.1038/s41566-021-00786-y

Provided by Linköping University

The Spintronics Technology Revolution Could Be Just a Hopfion Away (Physics)

Pioneering study co-led by Berkeley Lab has significance for next-gen information technologies

A decade ago, the discovery of quasiparticles called magnetic skyrmions provided important new clues into how microscopic spin textures will enable spintronics, a new class of electronics that use the orientation of an electron’s spin rather than its charge to encode data.

But although scientists have made big advances in this very young field, they still don’t fully understand how to design spintronics materials that would allow for ultrasmall, ultrafast, low-power devices. Skyrmions may seem promising, but scientists have long treated skyrmions as merely 2D objects. Recent studies, however, have suggested that 2D skyrmions could actually be the genesis of a 3D spin pattern called hopfions. But no one had been able to experimentally prove that magnetic hopfions exist on the nanoscale.

Now, a team of researchers co-led by Berkeley Lab has reported in Nature Communications the first demonstration and observation of 3D hopfions emerging from skyrmions at the nanoscale (billionths of a meter) in a magnetic system. The researchers say that their discovery heralds a major step forward in realizing high-density, high-speed, low-power, yet ultrastable magnetic memory devices that exploit the intrinsic power of electron spin.

“We not only proved that complex spin textures like 3D hopfions exist – We also demonstrated how to study and therefore harness them,” said co-senior author Peter Fischer, a senior scientist in Berkeley Lab’s Materials Sciences Division who is also an adjunct professor in physics at UC Santa Cruz. “To understand how hopfions really work, we have to know how to make them and study them. This work was possible only because we have these amazing tools at Berkeley Lab and our collaborative partnerships with scientists around the world,” he said.

According to previous studies, hopfions, unlike skyrmions, don’t drift when they move along a device and are therefore excellent candidates for data technologies. Furthermore, theory collaborators in the United Kingdom had predicted that hopfions could emerge from a multilayered 2D magnetic system.

The current study is the first to put those theories to test, Fischer said.

Using nanofabrication tools at Berkeley Lab’s Molecular Foundry, Noah Kent, a Ph.D. student in physics at UC Santa Cruz and in Fischer’s group at Berkeley Lab, worked with Molecular Foundry staff to carve out magnetic nanopillars from layers of iridium, cobalt, and platinum.

The multilayered materials were prepared by UC Berkeley postdoctoral scholar Neal Reynolds under the supervision of co-senior author Frances Hellman, who holds titles of senior faculty scientist in Berkeley Lab’s Materials Sciences Division, and professor of physics and materials science and engineering at UC Berkeley. She also leads the Department of Energy’s Non-Equilibrium Magnetic Materials (NEMM) program, which supported this study.

Hopfions and skyrmions are known to co-exist in magnetic materials, but they have a characteristic spin pattern in three dimensions. So, to tell them apart, the researchers used a combination of two advanced magnetic X-ray microscopy techniques – X-PEEM (X-ray photoemission electron microscopy) at Berkeley Lab’s synchrotron user facility, the Advanced Light Source; and magnetic soft X-ray transmission microscopy (MTXM) at ALBA, a synchrotron light facility in Barcelona, Spain – to image the distinct spin patterns of hopfions and skyrmions.

To confirm their observations, the researchers then carried out detailed simulations to mimic how 2D skyrmions inside a magnetic device evolve into 3D hopfions in carefully designed multilayer structures, and how these will appear when imaged by polarized X-ray light.

“Simulations are a hugely important part of this process, enabling us to understand the experimental images and to design structures that will support hopfions, skyrmions, or other designed 3D spin structures,” Hellman said.

To understand how hopfions will ultimately function in a device, the researchers plan to employ Berkeley Lab’s unique capabilities and world-class research facilities – which Fischer describes as “essential for carrying out such interdisciplinary work” to further study the quixotic quasiparticles’ dynamical behavior.

“We have known for a long time that spin textures are almost inevitably three dimensional, even in relatively thin films, but direct imaging has been experimentally challenging,” said Hellman. “The evidence here is exciting, and it opens doors to finding and exploring even more exotic and potentially significant 3D spin structures.”

Co-authors with Fischer and Hellman include David Raftrey, Ian T.G. Campbell, Selven Virasawmy, Scott Dhuey, and Rajesh V. Chopdekar of Berkeley Lab; Aurelio Hierro-Rodriguez of the University of Oviedo, and Andrea Sorrentino, Eva Pereiro, and Salvador Ferrer of the ALBA Synchrotron, Spain.

The Advanced Light Source and Molecular Foundry are DOE Office of Science user facilities at Berkeley Lab.

This work was supported by the U.S. Department of Energy Office of Science.

Featured image: Artist’s drawing of characteristic 3D spin texture of a magnetic hopfion. Berkeley Lab scientists have created and observed 3D hopfions. The discovery could advance spintronics memory devices. (Credit: Peter Fischer and Frances Hellman/Berkeley Lab)

Reference: Kent, N., Reynolds, N., Raftrey, D. et al. Creation and observation of Hopfions in magnetic multilayer systems. Nat Commun 12, 1562 (2021). https://www.nature.com/articles/s41467-021-21846-5 https://doi.org/10.1038/s41467-021-21846-5

Provided by LBL

Demonstrating the World’s Fastest Spintronics p-bit (Engineering)

Tohoku University researchers have, for the first time, developed the technology for the nanosecond operation of the spintronics-based probabilistic bit (p-bit) – dubbed the poor man’s quantum bit (q-bit).

The late physicist R.P. Feynman envisioned a probabilistic computer: a computer that is capable of dealing with probabilities at scale to enable efficient computing.

“Using spintronics, our latest technology made the first step in realizing Feynman’s vision,” said Shun Kanai, professor at the Research Institute of Electrical Communication at Tohoku University and lead author of the study.

Magnetic tunnel junctions (MTJs) are the key component of non-volatile memory or MRAM, a mass produced memory technology that uses magnetization to store information. There, thermal fluctuation typically poses a threat to the stable storage of information.

P-bits, on the other hand, function with these thermal fluctuations in thermally unstable (stochastic) MTJs. Prior collaborative research between Tohoku University and Purdue University demonstrated a spintronics-based probabilistic computer at room temperature consisting of stochastic MTJs with millisecond-long relaxation times.

In order to make probabilistic computers a viable technology, it is necessary to develop stochastic MTJs with much shorter relaxation times which reduces the fluctuation timescale of the p-bit. Doing so would effectively increase the computation speed/accuracy.

The Tohoku University research group, comprising Kanai, professor Hideo Ohno (the current Tohoku University president), and professor Shunsuke Fukami, produced a nanoscale MTJ device with an in-plane magnetic easy axis (Fig. 1). The magnetization direction updates every 8 nanoseconds on average – 100 times faster than the previous world record (Fig 2).

A top-view scanning electron microscopy image of a magnetic tunnel junction device. © K. Hayakawa et al.

The group explained the mechanism of this extremely short relaxation time by utilizing entropy – a physical quantity used to represent the stochasticity of systems that had previously not been considered for magnetization dynamics. Deriving a universal equation governing the entropy in magnetization dynamics, they discovered that the entropy rapidly increases in MTJs with in-plane easy axis with larger magnitudes of perpendicular magnetic anisotropy. The group intentionally employed an in-plane magnetic easy axis for achieving shorter relaxation times.

Real time measured transmitted voltage signal which reflects the magnetization state as well as bit state. Relaxation time, defined as a switching time averaging over 100 million times a second, was observed. ©K. Hayakawa et al.

“The developed MTJ is compatible with current semiconductor back-end-of-line processes and shows substantial promise for the future realization of high-performance probabilistic computers,” added Kanai. “Our theoretical framework of magnetization dynamics including entropy also has broad scientific implication, ultimately showing the potential of spintronics to contribute to debatable issues in statistical physics.”

Publication Details:

Title: Nanosecond Random Telegraph Noise in In-Plane Magnetic Tunnel Junctions
Authors: K. Hayakawa, S. Kanai, T. Funatsu, J. Igarashi, B. Jinnai, W.A. Borders, H. Ohno, and S. Fukami
Journal: Physical Review Letters
DOI: 10.1103/PhysRevLett.126.117202 link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.117202

Provided by Tohoku University

Theory Could Accelerate Push for Spintronic Devices (Chemistry)

Rice University models help ID materials for advanced electronics, computer memories

A new theory by Rice University scientists could boost the growing field of spintronics, devices that depend on the state of an electron as much as the brute electrical force required to push it.

Materials theorist Boris Yakobson and graduate student Sunny Gupta at Rice’s Brown School of Engineering describe the mechanism behind Rashba splitting, an effect seen in crystal compounds that can influence their electrons’ “up” or “down” spin states, analogous to “on” or “off” in common transistors.

“Spin” is a misnomer, since quantum physics constrains electrons to only two states. But that’s useful, because it gives them the potential to become essential bits in next-generation quantum computers, as well as more powerful everyday electronic devices that use far less energy.

The image at left shows the crystal structure of a MoTe2|PtS2 heterobilayer with isocharge plots from a model created at Rice University. When the materials are stacked together, mirror symmetry is broken and there is a charge transfer that creates an intrinsic electric field. This field is responsible for Rashba-type spin-splitting shown by the band structure at right, where the spin is perpendicular to momentum. Illustration by Sunny Gupta

However, finding the best materials to read and write these bits is a challenge.

The Rice model characterizes single layers to predict heteropairs — two-dimensional bilayers — that enable large Rashba splitting. These would make it possible to control the spin of enough electrons to make room-temperature spin transistors, a far more advanced version of common transistors that rely on electric current.

“The working principle behind information processing is based on the flow of electrons that can be either off or on,” Gupta said. “But electrons also have a spin degree of freedom that can be used to process information and is the basis behind spintronics. The ability to control electron spin by optimizing the Rashba effect can bring new functionality to electronic devices.

“A cellphone with spin-related memory would be much more powerful and much less energy-consuming than it is now,” he said.

Yakobson and Gupta would like to eliminate the trial and error of finding materials. Their theory, presented in the Journal of the American Chemical Society, aims to do just that.

“Electron spins are tiny magnetic moments that usually require a magnetic field to control,” Gupta said. “However, manipulating such fields on the small scales typical in computing is very difficult. The Rashba effect is the phenomenon that allows us to control the electron spin with an easy-to-apply electric field instead of a magnetic field.”

Yakobson’s group specializes in atom-level computations that predict interactions between materials. In this case, their models helped them understand that calculating the Born effective charge of the individual material components provides a means to predict Rashba splitting in a bilayer.

“Born effective charge characterizes the rate of the bond polarization change under external perturbations of the atoms,” Gupta said. “When two layers are stacked together, it effectively captures the resulting change in lattices and charges, which brings about the overall interlayer polarization and interface field responsible for the Rashba splitting.”

Their models turned up two heterobilayers — lattices of MoTe2|Tl2O or MoTe2|PtS2 — that are good candidates for the manipulation of Rashba spin-orbit coupling, which happens at the interface between two layers held together by the weak van der Waals force. (For the less-chemically inclined, Mo is molybdenum, Te is tellurium, Tl is thallium, O is oxygen, Pt is platinum and S is sulfur.)

Gupta noted the Rashba effect is known to occur in systems with broken inversion symmetry — where the spin of the electron is perpendicular to its momentum — that generates a magnetic field. Its strength can be controlled by an external voltage.

“The difference is that the magnetic field due to the Rashba effect depends on the electron’s momentum, which means the magnetic field experienced by a left-moving and right-moving electron is different,” he said. “Imagine an electron with spin pointing in the z-direction and moving in the x-direction; it will experience a momentum-dependent Rashba magnetic field in the y-direction, which will precess the electron along the y-axis and change its spin orientation.”

Where a traditional field-effect transistor (FET) turns on or off depending on the flow of charge across a barrier with gate voltage, spin transistors control the spin precession length by a gate electric field. If the spin orientation is the same at the transistor’s source and drain, the device is on; if the orientation differs, it’s off. Because a spin transistor does not require the electronic barrier found in FETs, it needs less power.

“That gives spintronic devices an enormous advantage compared to conventional charge-based electronic devices,” Gupta said. “Spin states can be set quickly, which makes transferring data quicker. And spin is nonvolatile. Information sent using spin remains fixed even after a loss of power. Moreover, less energy is needed to change spin than to generate current to maintain electron charges in a device, so spintronics devices use less power.”

“To the chemist in me,” Yakobson said, “the revelation here that spin-splitting strength depends on the Born charge is, in a way, very similar to the bond ionicity versus the electronegativity of the atoms in Pauling’s formula. This parallel is very intriguing and deserves further exploration.”

Yakobson is the Karl F. Hasselmann Professor of Materials Science and NanoEngineering and a professor of chemistry at Rice.

The Office of Naval Research and the Army Research Office supported the study. The National Energy Research Scientific Computing (NERSC) Center, a Department of Energy Office of Science user facility, provided computing resources.

Featured image: Sunny Gupta. (Credit: Rice University)

Reference: Sunny Gupta and Boris I. Yakobson, “What Dictates Rashba Splitting in 2D van der Waals Heterobilayers”, J. Am. Chem. Soc. February 24, 2021.
https://doi.org/10.1021/jacs.0c12809 https://pubs.acs.org/doi/10.1021/jacs.0c12809.

Provided by Rice University

Spintronics: New Manufacturing Process Makes Crystalline Microstructures Universally Usable (Physics)

New storage and information technology requires new higher performance materials. One of these materials is yttrium iron garnet, which has special magnetic properties. Thanks to a new process, it can now be transferred to any material. Developed by physicists at Martin Luther University Halle-Wittenberg (MLU), the method could advance the production of smaller, faster and more energy-efficient components for data storage and information processing. The physicists have published their results in the journal “Applied Physics Letters”.

Magnetic materials play a major role in the development of new storage and information technologies. Magnonics is an emerging field of research that studies spin waves in crystalline layers. Spin is a type of intrinsic angular momentum of a particle that generates a magnetic moment. The deflection of the spin can propagate waves in a solid body. “In magnonic components, electrons would not have to move to process information, which means they would consume much less energy,” explains Professor Georg Schmidt from the Institute of Physics at MLU. This would also make them smaller and faster than previous technologies. 

But until now, it has been very costly to produce the materials needed for this. Yttrium iron garnet (YIG) is often used because it has the right magnetic properties. “The problem so far has been that the very thin, high-quality layers that are required can only be produced on a specific substrate and cannot be detached,” explains Schmidt. The substrate itself has unfavourable electromagnetic properties. 

The physicists have now resolved this issue by getting the material to form bridge-like structures. This enables it to be produced on the ideal substrate and later removed. “Then, in theory, these small platelets can be stuck to any material,” says Schmidt. The method was developed in his laboratory and is based on a manufacturing process that can be conducted at room temperature. In the current study, the scientists glued the platelets, which are only a few square micrometres in size, onto sapphire and then measured their properties. “We have also had good results at low temperatures,” says Schmidt. This is necessary for many high-frequency experiments carried out in quantum magnonics. 

“The yttrium iron garnet platelets could also be glued to silicon, for example,” says Schmidt. This semiconductor is very frequently used in electronics. In addition, other thin-film microstructures of any shape can be produced from YIG. According to Schmidt, this is particularly exciting for hybrid components in which spin waves are coupled with electrical waves or mechanical vibrations. 

The study was funded by the Deutsche Forschungsgemeinschaft (German Research Foundation, DFG) as part of the Collaborative Research Center / Transregio 227.

Featured image: Coloured electron microscopy image (pink: YIG-bridge, green: glue, gray: sapphire) Foto: AIP Applied Physics Letters

About the study: Trempler, P. et al. Integration and characterization of micron-sized YIG structures with very low Gilbert damping on arbitrary substrates. Applied Physics Letters (2019). doi: 10.1063/5.0026120

Provided by Martin Luther University of Halle Wittenburg