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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