Green IT: New switching process in non-volatile spintronics devices
A magnetic field pulse switches the initial vortex state to “onion state” with two walls. In the subsequent magnetic snapshots the domain wall motion is shown. After 58 ns both walls meet and annihilate, thus completing the switching process into the opposite sense of rotation. © HZB
The movement of the two domain walls is shown in this video (please click here). © HZB
Physicists achieved a robust and reliable magnetization switching process by domain wall displacement without any applied fields. The effect is observed in tiny asymmetric permalloy rings and may pave the way to extremely efficient new memory devices. The results have been published in Physical Review Applied, highlighted as an Editors' Suggestion.
To construct magnetic memories, elements with two stable magnetization states are needed. Promising candidate for such magnetic elements are tiny rings, typically of the order of few micrometers, with clockwise or counterclockwise magnetization as the two states. Unfortunately, switching between those two states directly requires a circular magnetic field which is not easy to achieve.
Switching in asymmetric nanorings
But this problem can be solved, as demonstrated by a team of scientists from several institutions in Germany including Helmholtz-Zentrum Berlin: If the hole in the ring is slightly displaced, thus making the ring thinner on one side, a simple, uniaxial magnetic field pulse of some nanoseconds duration can switches between the two possible “vortex states” used for data storage (clockwise and counterclockwise).
Short magnetic field pulse is sufficient
The scientists recorded the time evolution of the magnetization dynamics of the device at the Maxymus-Beamline at BESSY II employing time-resolved x-ray microscopy during and after the short magnetic field pulse was applied. They observed how the magnetic field pulse leads in a first step to an intermediate “onion state” in the ring. This state is characterized by two domain walls, where different magnetization zones meet each other. After the external field pulse has vanished, these domain walls move towards each other and annihilate, which results in a stable opposite magnetization of the ring “vortex state”.
Very fast process for spintronics
“Our measurements show domain wall automotion with an average velocity of about 60 m/s. This is very fast for spintronic devices at zero applied field”, Dr. Mohamad-Assaad Mawass, lead author of the publication in Physical Review Applied, points out. Mawass has worked on these experiments already for his PhD at Johannes Gutenberg University Mainz (group of Prof. Kläui) in a joined project with Max Planck Institute for intelligent system at Stuttgart (Schütz-Department). He then continued his research as a postdoc research Scientist at X-PEEM beamline at HZB.
Details of domain wall motion observed
Another observation concerns the effect of the detailed topological nature of the walls in the annihilation process. According to the results, this effect influence the dynamics only on a local scale where walls experience an attractive or repulsive interaction once they get very close to each other without inhibiting the annihilation of walls through automotion. “The domain wall inertia and the stored energy, in the system, allows the walls to overcome both the local extrinsic pinning and the topological repulsion between DWs carrying the same winding number” said Mawass. “We believe to have identified a robust and reliable switching process by domain wall automotion in ferromagnetic rings”, Mawass states. “This could pave the way for further optimization of these devices.”
To the publication in Physical Review Applied (2017): "Switching by domain wall automotion in asymmetric ferromagnetic rings”, Mohamad-Assaad Mawass, Kornel Richter, Andre Bisig, Robert M. Reeve, Benjamin Krüger, Markus Weigand, Andrea Krone, Hermann Stoll, Florian Kronast, Gisela Schütz, and Mathias Kläui
The results are highlighted as Editors' Suggestion.
DOI: 10.1103/PhysRevApplied.7.044009
- BESSY II: Phosphorous chains – a 1D material with 1D electronic propertiesFor the first time, a team at BESSY II has succeeded in demonstrating the one-dimensional electronic properties of a material through a highly refined experimental process. The samples consisted of short chains of phosphorus atoms that self-organise at specific angles on a silver substrate. Through sophisticated analysis, the team was able to disentangle the contributions of these differently aligned chains. This revealed that the electronic properties of each chain are indeed one-dimensional. Calculations predict an exciting phase transition to be expected as soon as these chains are more closely packed. While material consisting of individual chains with longer distances is semiconducting, a very dense chain structure would be metallic.
- What vibrating molecules might reveal about cell biologyInfrared vibrational spectroscopy at BESSY II can be used to create high-resolution maps of molecules inside live cells and cell organelles in native aqueous environment, according to a new study by a team from HZB and Humboldt University in Berlin. Nano-IR spectroscopy with s-SNOM at the IRIS beamline is now suitable for examining tiny biological samples in liquid medium in the nanometre range and generating infrared images of molecular vibrations with nanometre resolution. It is even possible to obtain 3D information. To test the method, the team grew fibroblasts on a highly transparent SiC membrane and examined them in vivo. This method will provide new insights into cell biology.
- Porous Radical Organic framework improves lithium-sulphur batteriesA team led by Prof. Yan Lu, HZB, and Prof. Arne Thomas, Technical University of Berlin, has developed a material that enhances the capacity and stability of lithium-sulphur batteries. The material is based on polymers that form a framework with open pores (known as radical-cationic covalent organic frameworks or COFs). Catalytically accelerated reactions take place in these pores, firmly trapping polysulphides, which would shorten the battery life. Some of the experimental analyses were conducted at the BAMline at BESSY II.