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Lasers cause magnets to behave like liquids



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For years, researchers have been following a strange phenomenon: when you hit an ultrathin magnet with a laser, the magnetism suddenly becomes demagnetized. Imagine the magnet on your fridge.

Now, CU Boulder scientists are studying how magnets recover from this change and regain their properties within a fraction of a second.

According to a study published in this week, Nature Communications zapped magnets actually behave like fluids. Its magnetic properties begin to form "droplets," similar to what happens when you shake a jar of oil and water.

To find out, CU Boulder's Ezio Iacocca, Mark Hoefer, and their colleagues drew on mathematical modeling and numerical simulations and experiments conducted at Stanford University's SLAC National Accelerator Laboratory.

"Researchers have been working hard to understand what happens when you blow up a magnet," said Iacocca, lead author of the new study and research associate in the Department of Applied Mathematics. "What interests us is what happens after you shoot it, how does it recover?"

In particular, the group focused on a brief but critical time in the life of a magnet ̵

1; the first 20 trillionths of a second after a magnetic, metallic alloy was hit by a short high-energy laser.

Iacocca explained that magnets are fairly organized in nature. Their atomic building blocks have orientations or "spins" pointing either up or down, in the same direction – think of the Earth's magnetic field, which always points north.

Only when you bombard them with explosive devices does a laser hit a magnet with a short laser pulse, said Iacocca, and it comes to a disruption. The spins within a magnet no longer point up or down, but in all directions, canceling out the magnetic properties of the metal.

"Researchers have addressed what 3 picoseconds after a laser pulse and then when the magnet is back, equilibrium occurs after one microsecond," said Iacocca, also a guest scientist at the National Institute of Standards and Technology (NIST) of the USA. "In between there are a lot of unknowns."

It was this missing window of opportunity that Iacocca and his colleagues wanted to fill. To do this, the research team conducted a series of experiments in California in which minuscule pieces of gadolinium were blasted into iron-cobalt alloys with lasers. Then they compared the results with mathematical predictions and computer simulations.

And the group found that things were flowing. Hoefer, an associate professor of applied mathematics, quickly points out that the metals themselves have not become liquid. But the spins in these magnets behaved like fluids, moving and moving like waves crashing into an ocean.

"We used the mathematical equations that model these rotations to show that they behave like a super fluid at these short timescales." said Hoefer, a co-author of the new study.

Wait a little, and these roving turns begin to settle, he added, forming small clusters with the same orientation – essentially "droplets" in which the turns turn up or down. Wait a bit, and researchers have calculated that these droplets would get bigger and bigger, hence the comparison with oil and water that settles in a vessel.

"At certain points, the magnet again points up or down," Hoefer said. "It's like a seed for these larger groups."

Hoefer added that a zapping magnet does not always return to its original state. In some cases, a magnet can switch after a laser pulse and switch from top to bottom.

Engineers already use this inversion behavior to store information on a computer's hard drive in the form of bits of ones and zeros. Iacocca said if researchers could figure out how to do that more efficiently, they might be able to build faster computers.

"That's why we want to understand exactly how this process works," said Iacocca, finding a material that beats faster. "


The spinflipper turns protons on its head


Further information:
E. Iacocca et al., Spin-current-mediated Fast Magnon Localization and Coalescence After Ultrafast Optical Pumping of Ferrimagnetic Alloys, Nature Communications (2019). DOI: 10.1038 / s41467-019-09577-0

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University of Colorado in Boulder




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Lasers cause magnets to behave like liquids (2019, April 18)
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