Lasers make magnets behave like fluids

For years, researchers have pursued a strange phenomenon: when you hit an ultra-thin magnet with a laser, it suddenly becomes disarmed. Imagine the magnet of your fridge falling down.

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

According to a study published this week in Nature Communicationszapped magnets really behave like fluids. Their magnetic properties begin to form "droplets", similar to what happens when you shake a pot of oil and water.

To find out, Ezio Iacocca, CU Boulder, Mark Hoefer, and their colleagues used mathematical modeling, numerical simulations, and experiments at the SLAC National Accelerator Laboratory.

"Researchers are working hard to understand what happens when you use a magnet," said Iacocca, lead author of the new study and research associate at the Department of Applied Mathematics. "What interested us was what happens after the explosion, how does it recover?"

In particular, the group focused on a short but critical moment in the life of a magnet, the first 20 billion seconds after a metallic magnetic alloy was touched by a short laser to high energy.

Iacocca explained that magnets are, by their nature, fairly organized. Their atomic building blocks have orientations, or "rotations," that tend to point in the same direction, up or down – think of the magnetic field of the Earth, which always points north.

Except that, it's when you blow them up with a laser. Hit a magnet with a fairly short laser pulse, said Iacocca, and the mess will ensue. Rotations in a magnet will no longer point just up or down, but in all directions, thus nullifying the magnetic properties of the metal.

"The researchers explained what was happening 3 picoseconds after a laser pulse, and then when the magnet is again balanced after a microsecond," said Iacocca, also a visiting researcher at NIST. "Between the two, there are many unknowns."

It is this missing amount of time that Iacocca and his colleagues wanted to fill. To do this, the research team conducted a series of experiments in California, laser projecting fragments of gadolinium-iron-cobalt alloys. They then compared the results to mathematical predictions and computer simulations.

And the group discovered that the situation had become fluid. Hoefer, an associate professor of applied mathematics, urges to point out that the metals themselves have not turned into liquids. But the rotations inside these magnets behaved like fluids, moving and changing orientation like waves breaking in an ocean.

"We used the mathematical equations that model these rotations to show that they behaved like a superfluid at these short time scales," said Hoefer, co-author of the new study.

Wait a bit and these wandering towers begin to calm down, he added, forming small clusters with the same orientation – basically, "droplets" in which the towers are all directed up or down. Wait a little longer, and the researchers calculated that these droplets would become larger and larger, hence the comparison with the oil and water that were splitting into one pot.

"In some places, the magnet starts to point up or down again," Hoefer said. "It's like a seed for these big groups."

Hoefer added that a zapped magnet does not always go back as before. In some cases, a magnet may tilt after a laser pulse by switching up and down.

Engineers are already taking advantage of this behavior to store information on the hard disk of the computer as bits and zeros. Iacocca said that if researchers could find ways to do this more effectively, they might be able to build faster computers.

"That's why we want to understand exactly how this process occurs," said Iacocca, "so we may be able to find a material that will turn around faster."