Resilient chiral spin textures in graphene explained
10.12.2024
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Spin texture on top of a graphene layer. Credit: Paolo Perna / Patricia Bondía. |
- Researchers design a great experiment involving two synchrotrons to find evidence of the origin of strong spin-orbit coupling induced in graphene.
- The effect is greater than previously thought and its understanding paves the way to design new memory devices.
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Madrid, December 10th, 2024. Conventional electronics are based on controlling the tiny electrical charge of electrons flowing through electronic circuits. Spintronics, on the other hand, is an emerging technology that incorporates another of the properties of electrons as a variable: spin. Spin angular momentum is a quantum-mechanical property that is already being harnessed to try to create faster and more efficient information flows. In the same way that traditional electronics uses charging to represent information as zeros and ones, the two spin states (up and down) can be used to represent the same binary data in spintronics.
Spintronic circuits can be faster and more efficient because the change in spin state occurs on extremely short timescales, below the nanosecond, which speeds up data transfer. It takes much less power to change the spin state than it does to switch a conventional electronic device, so spintronic devices would consume relatively little power.En la última década se han generado y estudiado diversos fenómenos relacionados con el espín, como son los skyrmiones, es decir texturas quirales de espín, todos ellos muy relevantes para el campo de la espintrónica.
A quantum hurricane
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When an electron moves through a special type of magnetic texture called a skyrmion, its spin magnetic moment twists to fit the skyrmion's local spin structure (ribbon-like pattern). This twist changes the direction of travel of the electron and pushes the electron and skyrmion in opposite directions. Nature 465, 880. |
Hurricanes, which are characterized by strong winds that spiral around the eye of the storm, can be very stable and travel thousands of kilometers. In the field of quantum field theory and solid-state physics, there are stable topological solutions called skyrmions, with properties similar to those of particles and some resemblance to hurricanes. Skyrmions were predicted in 1960 and first observed experimentally in 2010 in certain magnetic materials. In a skyrmion, the magnetization is gently twisted so that it is antiparallel between the center and the edges of each skyrmion. This pattern turns out to be a stable magnetic knot thanks to the interaction between neighboring spins, called Dzyaloshinskii–Moriya. Skyrmions are therefore classified as topologically stable, a feature drawn from the mathematical discipline of topology that classifies geometric configurations according to properties such as their winding number, which is robust to external perturbations such as magnetic and electric fields.
An electron trying to pass through a skyrmion will flip its spin as it passes through it. It was soon seen that this property can be exploited to manipulate spin states in information storage devices. This opens up the possibility of developing quantum computers or information processing systems that operate at speeds significantly higher than those of current systems.
The Spinorbitronics
In the SpinOrbitronics research group at IMDEA Nanociencia, led by Paolo Perna, he explores new spin-based transport concepts in magnetic materials in which spin-orbit coupling plays an important role. It is a step beyond spintronics: they make use of the interaction of spin with its orbit. "To operate with spins, we have to be able to control and manipulate them. One of the most efficient ways to manipulate electron spins is to exploit the interaction between spin and orbital motion: the so-called spin-orbit coupling. This leads to interesting new phenomena and consolidates another field of electronics, called spinorbitronics, with interesting applications in magneto-electrical memories and neuromorphic computing," says Paolo Perna.
The group's goal is to explore materials with low dimensionality, with a few atomic layers, whose combination gives rise to new physical properties that allow, for example, to create skyrmions and manipulate them. In 2018, researchers led by Dr. Perna demonstrated that devices composed of an atomic layer of graphene combined with ferromagnets and heavy metals meet all these requirements. Graphene makes it possible to obtain a homogeneous, flat and protected magnetic layer, which is also atomically perfect. In addition, thanks to graphene, the device manifests a very reinforced magnetic anisotropy, due to the peculiar interaction between the different materials, and a great Dzyaloshinskii-Moriya interaction. A strong magnetic anisotropy is beneficial to increase the stability of data stored in magnetic memories against thermal fluctuations – we want written information to remain stored for dozens of years. In particular, the fact that these devices manifest a large Dzyaloshinskii–Moriya interaction also means that the skyrmions can be stabilized. This last finding was, at the time, a surprising and pioneering result, which prompted the SpinOrbitronics group to continue researching the subject.
In the latest work of the spin-orbitronics group, published in ACS Nano, the authors elucidate the origin of the interaction of spins with their environment
To establish direct routes to the development of spintronic devices, it is necessary to know the fundamental origin of these unusual properties. In the latest work of the spin-orbitronics group, published in ACS Nano, the authors elucidate the origin of the interaction of spins with their environment (spin-orbit coupling). Through experiments of angle-and-spin-resolved photoemission spectroscopy (spin-ARPES) and functional density theory (DFT), graphene and cobalt layer devices on the heavy metal iridium were studied. The authors observed a splitting of energy consistent with a "Rashba" interaction, which is either the fingerprint or the very origin of the Dzyaloshinskii-Moriya interaction that keeps the skyrmions stable.
This Rashba-like interaction directly affects the arrangement of spins in the motion of electrons. The Rashba effect can be controlled by applying an electric field, which would allow adjusting the magnetic anisotropy and the Dzyaloshinskii-Moriya interaction allowing precise spin manipulation, and promoting complex spin configurations.
Isolated graphene is an ideal material for its exceptional properties but has a negligible spin-orbit interaction. Combined with other ferromagnetic materials in a thin-layer configuration, graphene enhances its spin-orbit interaction and enables electrical control of spin textures. But to answer the question of how graphene can induce a large Dzyaloshinskii-Moriya interfacial interaction, in their latest work the researchers designed a very comprehensive specific experiment. The study involved the BESSY II synchrotrons in Berlin (Germany) and SOLEIL (France) to look for evidence of the origin of this strong interaction. It was found that for graphene-cobalt-iridium multilayer devices the large Dzyaloshinskii-Moriya interaction is manifested by a spin split of graphene's π states, which is consistent with a Rashba-like spin-orbit interaction at the graphene-cobalt interface and which is 2 orders of magnitude larger than expected. The researchers found that the Rashba effect faded for thicknesses greater than 10 monoatomic sheets of cobalt, indicating that the states are electronically decoupled from the heavy metal iridium. In this case, the spin-orbit interaction came from the heavy metal iridium, was mediated by cobalt, and was observed in graphene.
Following these promising results, the Spinorbitronics group at IMDEA Nanoscience continues to investigate how to enhance this great Rashba effect. In other works, thanks to the combination of graphene, cobalt and rare earths, the possibility of strongly modifying the electronic properties of graphene was discovered, allowing the creation of flat and spin-polarized electronic bands, discoveries that open the door to the realization of new quantum technologies and add another ingredient to the interaction between spin and orbit: the crystalline reticulum (papers published in the journals Advanced Materials and Carbon). All this to be able to explore functionalities at ultrashort timescales (work published in the journal Nature Nanotechnology) and enhance spin-charge conversion by engineering the interfaces (result to be published).
The future of data storage technology
Professors Albert Fert and Peter Grünberg, who were awarded the Nobel Prize in Physics in 2007, discovered that it was possible to use the spin of electrons to increase the speed at which information is read from a hard drive and developed a revolutionary technology to take advantage of this feature. Giant magnetoresistance technology drastically reduces resistance, which speeds up data transfer (IBM, 1997). Since then, researchers have been working on introducing spintronic technology into computer memory, with the goal of replacing current-based dynamic random access memory (DRAM) with magnetoresistive memory (MRAM). It is expected that these memories could be replaced in the future by SOT-MRAMs, memories that exploit spin-orbit torque and that are already marketed by some companies such as Samsumg, Everspin or Intel.
The SpinOrbitronics group at IMDEA Nanociencia is working on finding improvements for magnetic data storage devices, studying ways to amplify the spin-orbit coupling phenomenon. Spintronic components will play an important role in ensuring that we enjoy a steady increase in throughput and faster, higher capacity storage with lower consumption and cost.
The work is a result of collaboration between researchers at the Madrid Institute for Advanced Studies in Nanoscience (IMDEA Nanociencia), Institute of Condensed Matter Physics (IFIMAC-UAM), Helmholtz-Zentrum Berlin für Materialien und Energie, Elettra Sincrotrone Trieste, Synchrotron SOLEIL and Forschungszentrum Jülich, and has been partially funded by the FLAG-ERA SOgraphMEM PCI2019-111867-2 project coordinated by Paolo Perna, the knowledge generation project PID2021-122980OB-C52 and research consolidation CNS2022-136143 whose PI is Paolo Perna, and the Severo Ochoa Excellence accreditation to IMDEA Nanociencia (CEX2020-001039-S).
Glossary:
- Spin – magnetic angular momentum of electrons. Unlike the classical intuition of a particle spinning on its own axis, spin does not imply that the electron is actually rotating. Spin is a fundamental property that describes a type of angular momentum associated exclusively with subatomic particles, with no direct equivalent in classical physics.
- Spintronics: also known as spin electronics, it is a branch of physics that exploits, in addition to charge, the spin state of electrons to devise data storage or transfer devices that are more efficient than current electronic devices.
- Magnetic anisotropy is the directional dependence of the magnetic properties of a material.
- Skyrmion: stable configuration of the magnetic field that gives rise to nanomagnetic vortex-type structures that can be displaced while maintaining their shape with great robustness.
- Dzyaloshinskii-Moriya interaction: is an interaction between electron spins in certain magnetic materials that favors a non-collilinear arrangement of neighboring spins, that is, that the spins are aligned in an inclined way with respect to each other instead of being parallel or antiparallel. This interaction is a consequence of the spin-orbit interaction, which couples the spin of electrons with their orbital motion in the presence of an asymmetric structure.
- Rashba effect: A phenomenon in which electrons undergo spin separation due to spin-orbit interaction in materials with structural asymmetry. In other words, when an electron moves in a material without spatial reversal symmetry (i.e., where the environment around the atoms is not identical in all directions), its spin is coupled with its linear momentum due to spin-orbit interaction, causing electrons from opposite spins to shift in different directions. This is observed, for example, on material surfaces and at interfaces where there is an electric field perpendicular to the movement of electrons. The Rashba effect arises when there is an external or internal electric field (e.g., at an interface) that breaks the spatial symmetry of the system. While the Rashba effect affects the arrangement of spins in the motion of electrons, the Dzyaloshinskii-Moriya interaction establishes the preferred alignment between neighboring spins.
Reference:
Beatriz Muñiz Cano, Adrián Gudín, Jaime Sánchez-Barriga, Oliver Clark, Alberto Anadón, Jose Manuel Díez, Pablo Olleros-Rodríguez, Fernando Ajejas, Iciar Arnay, Matteo Jugovac, Julien Rault, Patrick Le Fèvre, François Bertran, Donya Mazhjoo, Gustav Bihlmayer, Oliver Rader, Stefan Blügel, Rodolfo Miranda, Julio Camarero, Miguel Angel Valbuena, and Paolo Perna. Rashba-like Spin Textures in Graphene Promoted by Ferromagnet-Mediated Electronic Hybridization with a Heavy Metal. ACS Nano 2024 18 (24), 15716-15728. DOI: 10.1021/acsnano.4c02154
https://repositorio.imdeananociencia.org/handle/20.500.12614/3744
Contact:
Paolo Perna
SpinOrbitronics Group
https://nanociencia.imdea.org/spinorbitronics/group-home
paolo.perna (at)imdea.org
IMDEA Nanociencia Outreach Office
divulgacion.nanociencia [at]imdea.org
Twitter: @imdea_nano
Facebook: @imdeananociencia
Instagram: @imdeananociencia
Source: IMDEA Nanociencia.
IMDEA Nanociencia Institute is a young interdisciplinary research Centre in Madrid (Spain) dedicated to the exploration of nanoscience and the development of applications of nanotechnology in connection with innovative industries.
Related information:
https://nanociencia.imdea.org/home-en/news/item/stable-skyrmions-in-graphene-based-epitaxial-trilayers
https://phys.org/news/2024-09-analysis-heterostructures-spintronics-desired-quantum.html