The recent paper Electrical signature of individual magnetic skyrmions in multilayered systems and its main author Dr. Davide Maccariello (working full-time on MAGicSky until last January) have recently been highlighted by the technical press:
- Magnetic skyrmions could make storage bits for next-generation memories, Nanotechweb.org
- Swirly skyrmions could be the future of data storage, IEEE Spectrum
The content of the article above goes as following:
"About five years ago, researchers at the University of Hamburg demonstrated that tiny, swirling magnetic spin patterns on thin films—known as skyrmions—could be used to store and erase data on magnetic media.
At that time, these spinning magnetic swirls that had been proposed over 60 years ago by British physicist Tony Skyrme—from whom the name derives—had suddenly become a potentially game changing magnetic data storage system. And what a change it represented: skyrmions are 10 times smaller than the magnetic regions used on traditional hard drives. Now a team of researchers from CNRS/Thales joint lab in France with European Funding under the MAGicSky program have taken a critical step in the commercial realization of this technology by electrically detecting for the first time a single small skyrmion at room temperature.
“We believe this is an important advance because it demonstrates one of the unavoidable functions for any type of future concept of devices: electrical detection,” said Vincent Cros, a researcher at CNRS and co-author of the paper published in Nature Nanotechnology.
While the electrical signal of skyrmion lattices or an ensemble of skyrmions has been measured before, mostly at low temperatures, this is the first time that measuring an electrical signal has been demonstrated for a single skyrmion and at room temperature.
As one might imagine, the electrical signals associated with these 100 nanometer skyrmions remain relatively small. These signals are so small, according to Cros, that they had to be sure that the measured electrical signal is actually associated with the presence of a skyrmion. “That is exactly what we demonstrate here by a concomitant electrical measurement and magnetic imaging on the very same devices,” said Cros.
FIGURE: The reproducible annihilation or creation of a single magnetic skyrmion can be electrically detected by recording the variation of the transversal voltage to an applied electrical current. The values that the voltage assumes for the configuration with skyrmion or without might be used as state "I" or "0", respectively, for new generation of memories or logic devices
While it was necessary to use magnetic imaging to ensure that they were measuring a skyrmion for their research, in future memory devices the only possible reading procedure will be through electrical measurement and not by imaging the magnetic configuration of the skyrmion.
Nonetheless, the ability to make electrical measurements of the sample while imaging it magnetically at the same time using magnetic force microscopy is extremely significant, according to Cros, and had never been done before.
As for the actual device, this goes back to work Cros and his colleagues did in 2013 that suggested the best memory device for exploiting skyrmions would be what’s known as “racetrack memory.” Nearly a decade ago, Stuart Parkin and his colleagues at IBM Almaden Research demonstrated a three-bit version of so-called “racetrack memory,” which is a solid-state non-volatile memory that promises much higher storage density than conventional solid-state memory devices.
When Parkin first envisioned racetrack memory, they were based on magnetic features known as domain walls, which essentially separate the magnetic direction of a material into different areas. Electric currents could push those domain walls around the track and a sensor could detect the changes, leading to the “0” and “1” of digital memory.
What Cros and his colleagues suggested five years ago was that the skyrmions could replace the domain walls and they could move along the track and their presence or absence could be detected electrically, leading to a digital memory device. By using a basic skyrmion-race-track memory, the researchers designed electrical contacts on both ends of the tracks. In order to detect the electrical skyrmion signal, they also designed lateral contacts. The electrical signal is simply detected by measuring the associated electrical voltage using a commercial voltmeter.
It sounds all pretty straight forward, however, controlling the position and the density of the skyrmions remained a challenge. The main obstacle revolved around the creation of the skyrmions in the material where prior to their formation it had all been in a uniform magnetized state. The traditional method for producing the skyrmions was based on the use of a magnetic field.
“In the present work, we have employed a new approach in which we inject short current pulses into the materials, which allows us to create isolated skyrmions located in a strip (or track) designed by electron-beam lithography,” explained Cros. The result is that Cros and his colleagues can now adjust the total number of nucleated skyrmions by tuning different parameters, such as the current pulse width or the intensity of the external magnetic field.
While all of this will certainly go down as a significant step towards using skyrmions in memory devices, Cros concedes that commericialization is still a ways down the road.
“We are not yet at the stage where skyrmion devices can be used and implemented as a real new electronic device,” said Cros. “The standard and reasonable time scale between fundamental discoveries and consumer electronics is often between 10 and 15 years.” To realize this 15 year time line, Cros believes that more efforts are needed to further decrease the skyrmion size, targeting sub 10-nm diameter, to increase the skyrmion speed, better understand and control the interaction of skyrmions with material grains (typically of the same sizes) and to increase the electrical signal."
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Vincent Cros, Kirsten von Bergmann & Roland Wiesendanger (who are respectively Scientific Coordinator and participants of MAGicSky), as well as Christian Pfleiderer, Stuart Parkin, Axel Hoffmann, Christos Panagopoulos, Claudia Felser and Mathias Kläui were all interviewed by Science news.
They explained to the magazine how the recent research results on magnetic skyrmions makes skyrmion-based data storage devices a plausible breakthrough in the near future.
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Several sientific leaders from the consortium were interviewed for the December 2017 edition of the French dissemination journal La Recherche. Here is the article:
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The amount of information produced by modern society has grown massively in the last years, attracting increasing attention to information storage technology, because of the constant need for more powerful and efficient devices. The state-of-the-art technology includes Hard Disk Drives (HDD), Solid State Drives (SSD), and flash memories, which are based on the detection and movement of electron charges. However, several limitations, among which the speed of writing and reading data from an HDD and the durability of SSD, have slowed down the continuous improvement of these devices, so that reducing their size and power consumption has become a serious technological issue. For this reason, it has become crucial to search for new prototypes for data storage devices.
Nowadays technology listed above is based on electronics, meaning it uses the charge of the electrons to transfer information. But besides that, electrons also have a spin (intrinsic magnetic moment) which application has opened the new field of spintronics: the combination of spin and electronics.
Whereas in electronics the charge current can be driven with electric fields, in spintronics the systems are also controlled via magnetic effects.
Spintronics was born in Europe with the discovery of giant magnetoresistance (GMR) in 1988 by Albert Fert and Peter Grünberg (who both received the Nobel Prize in Physics in 2007 for this discovery). The principle of GMR is that the resistance through two magnetic electrodes separated by a non-magnetic metal shows “giant” changes as the relative orientation of their magnetisations changes. The GMR effect has been used in commercial devices since the 90’s but there are many more possibilities; for example using magnetic skyrmions which are at the core of the MAGicSky (MAGnetic Skyrmions for future nano-spintronic devices) project.
The original mathematical concept of a skyrmion was developed by Tony Skyrme within particle-physics. Since its first definition, it was discovered that the concept of skyrmions could be applied to many phenomena outside of its original framework, in lower dimensions. Within the context of condensed matter physics, the term (magnetic) skyrmion now designates a topologically and physically stable magnetic texture arising from the competition of different short range interactions. This physical and topological stability is, of course, of extreme interest for spintronic applications.
What MAGicSky is focusing on are magnetic skyrmions which are localised, particle-like solitons of a few nanometers in size (from 1 to 100 nm as reported up to now), exhibiting a characteristic configuration of the spins in which they show a defined sense of rotation (figure 1). The spins point in all directions wrapping a sphere, an example of which is shown in figure 2.
|Figure 1: numerical simulation of an object covered with spins (arrows) pointing in every possible direction||Figure 2: representation of a spin-covered object as a sphere, also called hedgehog or magnetic knot|
It is now possible to create and shape skyrmions with magnetic field, electric field or temperature control in a controlled environment. To improve the process, our research teams experimentally identify particular combinations of elements in which skyrmions can be stabilised and manipulated. It is the choice of the consortium to create skyrmions in structures which are obtained by stacking at the nanoscale several magnetic and non-magnetic metal layers, and to control skyrmions with applied currents (figure 3).
|Figure 3: numerical simulation of skyrmions moving on a race track (50nm wide) after an electrical current was applied to them|
A large part of this research is to balance properly the amount and stacking order of the elements that we use and to understand the role of materials defects, either of unavoidable or intended nature.
To accomplish this, it is necessary to explore the new materials and preparation techniques offered by recent technological and industrial progress of ultrathin film fabrication.
State- of-the-art microscopy techniques daily used by our research teams (such as spin-polarised scanning tunneling microscopy, magnetic force and Kerr effect microscopy, energy loss electron spectroscopy, etc.) were indeed developed specifically to address the challenge of observing small magnetic structures at the nanoscale.
The final goal of the 3 year-long MAGicSky project is to manipulate skyrmions individually in devices at room-temperature.
The stability and the size of the skyrmions will eventually lead to the creation of the next generation very high density information storage, breaking though the barriers set by current technology. This could eventually allow the industrialisation of storage devices in which the bits could be spaced down a few nanometers, i.e. the order of magnitude of the skyrmion diameter. Compared to recent HDD or SSD storage devices, there is no limitation related to mechanical parts in the state-of-the-art technology. Particle-like properties of these nanoscale structures should highly improve the stability of the bits, as well as the speed and efficiency of the writing and reading process of information. Furthermore, as skyrmions should be created and manipulated with low-density current, skyrmion-based devices will hopefully allow a drop of the energy consumption compared to the nowadays magnetic-based devices.
This text is the result of a collaborative work from: Davide Maccariello, Benoît Pilorget, Marie Böttcher, Louise Desplat, Aurore Finco, Simone Finizio, William Legrand, Stephan von Malottki, Sebastian Meyer, Marco Perini, Myoung-Woo Yoo and Katharina Zeissler.
Figures: courtesy of Dr Myoung-Woo Yoo
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