Chaotic 'spin vortices' could lead to new computer memories

Jan 10, 2013 by Jared Sagoff
In this artist rendering, tiny magnetic vortices form on nanodisks, with each disk having a diameter of about 100 nanometers. Each vortex is directed either upwards or downwards. The Argonne study looked at the interaction between pairs of these nanodisks. Credit: Sander Munster, Dresden University of Technology

(Phys.org)—In science, just like in life, sometimes creating the most effective organization depends on being able to handle just a bit of chaos first.

Scientists at the U.S. Department of Energy's Argonne National Laboratory have used alternating magnetic fields to control the behavior of "spin vortices" trapped in small dots made from iron and nickel that can be magnetized in two separate ways. While the majority of these structures are magnetized in-plane either clockwise or counter-clockwise, a tiny region at their centers – the vortex core – is magnetized out of plane, either up or down.

"If you were able to visualize it, it would look like a funnel," said Argonne materials scientist Valentyn Novosad.

Novosad and postdoctoral researcher Shikha Jain wanted to find a way to control the of pairs or even of these dots that interacted with each other in a lattice.

In the resting state, the cores of the dots are randomly polarized. After applying an oscillating to the pairs of dots, the researchers observed that the central cores began to switch back and forth repeatedly between up- and down-magnetizations – which Jain and Novosad characterized as "chaos."

This chaotic system's behavior is dictated by the fact that the magnetic field, when applied, oscillates at a particular frequency that can be tuned to match the "" – that is, the natural frequency of vibration – for a specific polarity combination in a dot-pair. Each pair of dots has two resonance frequencies, corresponding to parallel (up-up or down-down) or antiparallel (up-down or down-up) magnetization states. In the parallel state, the dots' centers are magnetized in the same direction, while in the antiparallel state they are opposite.

By increasing and then decreasing the strength of the applied field, Argonne's scientists were able to bring the dots' magnetizations into and then out of chaos. When the amplitude of the oscillating field was reduced significantly, the researchers discovered that the new polarizations corresponded to the opposite state of the applied frequency. If the frequency corresponded to the parallel magnetization, an antiparallel arrangement would emerge, and vice versa.

"We were somewhat surprised that the behaviors of multiple dots could be controlled so precisely," Jain said. "There had been a lot of work done before on single-dot systems, but no one had really investigated how magnetic dots interact with each other in this kind of environment."

By achieving consistent control of the central 's direction, the researchers moved one step closer to creating new magnetic devices, including non-volatile random access memories. These devices are the subject of intense research in academia and industry worldwide as they offer energy efficiency, high operating speed and exceptional reliability. Unlike the silicon-based chips in today's computers, their magnetic counterparts employ spin rather than electric charge to store and process information.

According to Novosad, the underlying physics that governs the two-dot interaction should apply as well to a system of many elements. Future studies will seek to explore the collective dynamics in larger three-dimensional crystal-like magnetic structures.

These results were published in the journal Nature Communications. The paper is titled "From chaos to selective ordering of vortex cores in interacting mesomagnets."

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Macksb
1 / 5 (1) Jan 11, 2013
Excellent work. A few comments: These results are consistent with Art Winfree's "law of coupled oscillators," which he postulated circa 1967. Systems of oscillators (in this case identical oscillators) interact, and when they do they coordinate their behavior in certain exact patterns identified and specified by Winfree. The above article describes a system of oscillators--not a "single dot system," as Jain notes. This is critical.

The simplest pattern predicted by Winfree is exactly synchronous or exactly antisynchronous (180 degrees out of phase). This is the result obtained here. Jain says "we were somewhat surprised that the behaviors of multiple dots could be controlled so precisely." But this is exactly what Winfree predicted. The interactions can only be resolved in extremely precise patterns.

Finally, instead of the word "chaos," I would use any antonym of chaos. "Switching back and forth repeatedly between up and down magnetizations" is a dynamically precise pattern.
Macksb
1 / 5 (1) Jan 11, 2013
A good description of Winfree's work can be found online. Google "Coupled Oscillators and Biological Synchronization" by Steven Strogatz and Ian Stewart. Available on an oregonstate.edu site. Scientific American, December 1993. Art Winfree, a MacArthur Prize winner, was a self-described bio-mathematician. He chose to apply his theory in the field of biology, but his theory is mathematical--and so there is every reason to believe that it can be applied to other systems.

Winfree's theory applies to any "system of limit cycle oscillators." In my judgment, everything in physics is a "system of limit cycle oscillators." I realize that is a sweeping statement. But think about it: Max Planck's quantum is periodic.
Macksb
1 / 5 (1) Jan 13, 2013
Here are some fundamental, sweeping ways in which Winfree's law might apply to physics. Winfree's ingredients are simple: a supply of limit cycle oscillators; the frequencies or periods of those oscillations; and enough proximity among the oscillators so that they can influence each other.

Magnetism is an obvious example, including the experiment described above. Indeed, all phases of matter and their transitions may arise from Winfree's law. Temperature (which affects frequencies) and pressure (which varies proximity) drive phase transitions. So we have all three Winfree ingredients: oscillations (quantum), varying frequencies (temperature) and proximity (pressure).

The periodic table of the elements also comes to mind. How are the elements created? In the stars...created by temperature and pressure. The table of the elements is itself periodic, as are the oscillations of quarks, nuclei and electrons.

The main ingredient in string theory: vibrating (oscillating)strings.

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