Controlling skyrmions for better electronics

Aug 08, 2013
The image shows a magnetic skyrmion in a hexagonal atom arrangement (transparant spheres). The cones represent the magnetic moments and point up (red) in the skyrmion center and down (green) around it. The rotational sense of the skyrmion for a given system is always the same, as the other rotational sense has a higher energy due to an anisotropic exchange interaction. The magnetic skyrmion in PdFe/Ir(111) that can be created and annihilated with small currents from a scanning tunneling microscope tip consists of roughly 270 surface atoms. Credit: Prof. R. Wiesendanger, University of Hamburg

Physicists at the University of Hamburg managed for the first time to individually write and delete single skyrmions, a knot-like magnetic entity. Such vortex-shaped magnetic structures exhibit unique properties which make them promising candidates for future data storage devices. Skyrmions have been in the focus of active research for the last years; however, up to now these states have been merely investigated, a controlled manipulation has not been achieved.

Now this has been realized by scientists from the group of Prof. Roland Wiesendanger in Hamburg, Germany. As Science reports online on August 8, 2013, the creation and annihilation of single skyrmions, corresponding to writing and deleting of information on a , has been demonstrated by these researchers. This work solves one of the longstanding technical problems concerning the future use of skyrmions in information technology.

Future electronic devices are expected to become smaller while increasing their data at the same time. This will soon bring classical to their physical limits. In conventional used up to now, magnetic bits consist of many atoms with their magnetic moments aligned parallel to each other like bar magnets. Pointing in defined directions, they can represent the values "1" and "0" which are the basis for information technology. With the continuing miniaturization, the interaction between neighboring bits increases due to magnetic stray fields which can lead to loss of data. In addition, small are less stable against which is also called the superparamagnetic limit.

Using more "robust" like skyrmions could be a way out of this technological dead end. These structures can be imagined as a two-dimensional knot in which the magnetic moments are rotating about 360° in a plane with a unique rotational sense (see Fig. above). These particle-like magnetic knots can be assigned a kind of charge, the topological charge. With this it is possible to assign the bit states "1" and "0" to the existence or non-existence of a skyrmion.

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Sequence of spin-resolved scanning tunneling spectroscopy images of a sample area of a bilayer of Pd/Fe on top of an Ir(111) surface (50 nm x 38 nm). Each of the 17 images took about 4 minutes to measure. In this measurement single magnetic skyrmions appear and disappear due to the injection of tunneling electrons and show up in the images as black dots. Each skyrmion has a preferred position and individual switching frequency depending on the local environment. Since the area is scanned line by line from top to bottom the skyrmions with higher switching frequency may switch not only from image to image but also from line to line, leading to a stripe-like pattern. Measurement parameters: U = +550 mV, I = 2 nA, B = -2.8 T, T = 4.2 K. Credit: Prof. R. Wiesendanger, University of Hamburg

A clever choice of temperature and external magnetic field enabled the scientists of the group of Prof. Roland Wiesendanger to prepare and manipulate single magnetic skyrmions for the first time. They used a two atomic layer thick film of palladium and iron on an iridium crystal. In an external magnetic field this sample exhibits single localized skyrmions with a diameter of a few nanometers that can be imaged in a spin-polarized scanning tunneling microscope. Using small currents from the tip of the microscope these skyrmions can be written and deleted. For the creation the previously parallel are twisted to form a knot-like magnetic state, and for the deletion this knot is unwound again.

Spin-resolved scanning tunneling microscopy (STM) image of single magnetic skyrmions in a bilayer of palladium and iron on top of an iridium crystal, colorized with a difference image to the same area without skyrmions. A magnetic STM tip, which is sensitive mostly to the in-plane magnetization component of the sample as indicated, is used. The cones indicate the magnetization direction in the sample. Skyrmions can be created and annihilated locally by injecting spin-polarized currents from the STM tip. Color scale: green and red correspond to magnetic moments parallel and antiparallel to the tip magnetization, respectively. Measurement parameters: U = +250 mV, I = 1 nA, B = +3.25 T, T = 4.2 K. Credit: Prof. R. Wiesendanger, University of Hamburg

"We finally found a magnetic system in which we can locally switch between ordinary ferromagnetic order and a complex spin configuration", says Dr. Kirsten von Bergmann, senior scientist in the Wiesendanger group. As published in the current issue of the Science magazine, four skyrmions were specifically addressed and could be created and annihilated. "We transferred the idea of tying a knot to memorize something to the field of storage technology so we can now store data in a two-dimensional magnetic knot", explains Ph.D. student Niklas Romming.

Whether skyrmions will be used as units in our computers, tablets, or smartphones, is not foreseeable. The experimentally accomplished writing and deleting of skyrmions, however, has demonstrated the feasibility of this technology and paved the way towards a realization of such devices.

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More information: "Writing and Deleting Single Magnetic Skyrmions" Science 9 August 2013: Vol. 341 no. 6146 pp. 636-639 DOI: 10.1126/science.1240573

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User comments : 6

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1 / 5 (7) Aug 09, 2013
Personal and mathematical essay on this study by notorious Harvard trained Global Warming skeptic Luboš Motl, the string theorist:
5 / 5 (1) Aug 09, 2013
Personal and mathematical essay on this study by notorious Harvard trained Global Warming skeptic Lubo� Motl, the string theorist:

This topic has nothing to do with global warming. Or are you just reminding us that once one has a PhD in some particular field, one can speak with authority on all matters.
not rated yet Aug 09, 2013
For the creation the previously parallel magnetic moments are twisted to form a knot-like magnetic state

Applying topology to real world problems. Elegant. I wonder what the minimum number of atoms for a skyrmion is in a 2D or a 3D configuration (i.e. what the theoretical maximum data density would be).
Seems that in 3D configuration one could form more 'elaborate' knots which could hold several bits of information per knot. But this would be a lot harder to write (though not necessarily harder to read)
1.6 / 5 (7) Aug 09, 2013
...are you just reminding us that once one has a PhD in some particular field, one can speak with authority on all matters.

Lubos, the Harvard trained Ph.D. physicist, and myself, the Columbia/Harvard trained Ph.D. chemist, can speak *very* competently about most any science after a season or two of delving into recent publications.

Physics and chemistry are not just "some particular field" like climatology indeed is. You know this. Everybody knows this.

That billions of dollars are being stolen from physics and chemistry departments to fund agenda driven Global Warming pseudoscience means that very much so does this topic have something to "do" with it!

Because you are a gorebot, you cannot fathom that my single sentence post was an altruistic addition of factual information about the nature of my link!

gorebot: BORG-like, half human, half wind-up toy supporters of Al Gore. See also Demonic kooks.

So shut up, gorebot, you're wasting people's time.
1.7 / 5 (6) Aug 09, 2013
Figure from the supplementary info:


I see self-organizing domains that remind me of Turing reaction-diffusion patterns:


2 / 5 (4) Aug 11, 2013

Lubos, the Harvard trained Ph.D. physicist, and myself, the Columbia/Harvard trained Ph.D. chemist, can speak *very* competently about most any science after a season or two of delving into recent publications.

Keep beating that drum. Someday maybe even you will believe it. Apparently "Ph.D." is, again, a magic "I am right" club to be wielded indiscriminately in defense of personal opinion.

Anyway, yes - this does seem like a significant advancement in possible storage media. The potential data density is very high, but it seems the read-write head would be, currently, an atomic force microscope...

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