R&D: Future Data Storage Technology – Extremely Small Magnetic Nanostructures With Invisibility Cloak Imaged

From Max Born Institute, Massachusetts Institute of Technology and DESY

In novel concepts of magnetic data storage, it is intended to send small magnetic bits back and forth in a chip structure, store them densely packed and read them out later.

In the future, a magnetic skyrmion could encode a ‘1’ in data storage. The skyrmion is made up by the specific arrangement of the magnetic moments of neighboring atoms, represented by arrows in the images. Shown on the right is a skyrmion where neighboring atoms have approximately opposite magnetization, hence cloaking the resulting net magnetic stray field. In this way, smaller diameter skyrmions are stable. Physicists talk about ‘antiferromagnetic’ (AFM) rather than ‘ferromagnetic’ (FM) order between neighboring moments.
(Copyright: L. Caretta, M. Huang, MIT)

Click to enlarge

The magnetic stray field generates problems when trying to generate particularly tiny bits. Now, researchers at the Max Born Institute (MBI), the Massachusetts Institute of Technology (MIT) and DESY (Deutsches Elektronen-Synchrotron) were able to put an ‘invisibility cloak’ over the magnetic structures. In this fashion, the magnetic stray field can be reduced in a fashion allowing for small yet mobile bits. The results were published in Nature Nanotechnology.

For physicists, magnetism is intimately coupled to rotating motion of electrons in atoms. Orbiting around the atomic nucleus as well as around their own axis, electrons generate the magnetic moment of the atom. The magnetic stray field associated with that magnetic moment is the property we know from e.g. a bar magnet we use to fix notes on pinboard. It is also the magnetic stray field that is used to read the information from a magnetic hard disk drive. In today’s hard disks, a single magnetic bit has a size of about 15×45 nanometers, about 1.000.000.000.000 of those would fit on a stamp.

One vision for a novel concept to store data magnetically is to send the magnetic bits back and forth in a memory chip via current pulses, in order to store them at a suitable place in the chip and retrieve them later. Here, the magnetic stray field is a bit of a curse, as it prevents that the bits can be made smaller for even denser packing of the information. On the other hand, the magnetic moment underlying the stray field is required to be able to move the structures around.

The researchers were now able to put an ‘invisibility cloak’ on the magnetic nanostructures and to observe, how small and how fast such structures can get. To this end, different atomic elements with opposite rotation of the electrons were combined in one material. In this way, the magnetic stray field can be reduced or even completely cancelled – the individual atoms, however, still carry a magnetic moment but together appear cloaked.

In spite of this cloaking, the scientists were able to image the tiny structures. Via x-ray holography, they were able to selectively make only the magnetic moments of one of the constituent elements visible – in this way an image can be recorded in spite of the invisibility cloak.

It became apparent, that fine tuning of the strength of the invisibility cloak allows to simultaneously meet two goals which are of importance for potential applications in data storage.

“In our images, we see very small, disk-like magnetic structures“, says Dr. Bastian Pfau, MBI. “The smallest structures we observed had a diameter of only 10 nanometers The information density of today’s hard disk drives could be significantly increased, if such structures could be used to encode the data. Furthermore, in additional measurements the researchers realized that suitably cloaked bits can be moved particularly fast by short current pulses – an important property for actual use in a memory device. A velocity higher than one kilometer per second was reached in the MIT laboratory.”

“This is possible as a consequence of quantum physics“, explains Prof. Stefan Eisebitt, MBI. “The contribution of the electron’s orbit around the nucleus to the magnetic moment is only half as large as the contribution of the electron’s spin around its own axis.”

When combining different atom types with different direction and strength of this rotation in one material, one can cancel the total rotation – physicists talk about the total angular momentum – of the system, while still retaining a small magnetic moment. As the angular momentum leads to a drag when moving the structures via current pulses, this approach allows for high speed motion. Hence, if the strength of the invisibility cloak is adjusted just right, both small size and high speed of the magnetic bit structures can be achieved – an interesting prospect for novel magnetic data storage concepts.

Original publication: Fast current-driven domain walls and small skyrmions in a compensated ferrimagnet

Nature Nanotechnology has published an article written by Lucas Caretta, Maxwell Mann, Felix Büttner, Kohei Ueda, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA, Bastian Pfau, Max-Born-Institut, Berlin, Germany, Christian M. Günther, Max-Born-Institut, Berlin, Germany, and Institut für Optik und Atomare Physik, Technische Universität Berlin, Berlin, Germany, Piet Hessing, Max-Born-Institut, Berlin, Germany, Alexandra Churikova, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA, Christopher Klose, Michael Schneider, Dieter Engel, Max-Born-Institut, Berlin, Germany, Colin Marcus, David Bono, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA, Kai Bagschik, Deutsches Elektronen-Synchrotron (DESY), FS-PE, Hamburg, Germany, Stefan Eisebitt, Max-Born-Institut, Berlin, Germany, and Institut für Optik und Atomare Physik, Technische Universität Berlin, Berlin, Germany, and Geoffrey S. D. Beach, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA.

Abstract : “Spintronics is a research field that aims to understand and control spins on the nanoscale and should enable next-generation data storage and manipulation. One technological and scientific key challenge is to stabilize small spin textures and to move them efficiently with high velocities. For a long time, research focused on ferromagnetic materials, but ferromagnets show fundamental limits for speed and size. Here, we circumvent these limits using compensated ferrimagnets. Using ferrimagnetic Pt/Gd₄₄Co₅₆/TaO_x films with a sizeable Dzyaloshinskii–Moriya interaction, we realize a current-driven domain wall motion with a speed of 1.3 km s^–1 near the angular momentum compensation temperature (T_A) and room-temperature-stable skyrmions with minimum diameters close to 10nm near the magnetic compensation temperature (T_M). Both the size and dynamics of the ferrimagnet are in excellent agreement with a simplified effective ferromagnet theory. Our work shows that high-speed, high-density spintronics devices based on current-driven spin textures can be realized using materials in which T_A and T_M are close together.“