April 13, 2024

Magnetic avalanche triggered by quantum effects

Iron screws and other materials called ferromagnetic are composed of atoms with electrons that act like small magnets. Typically, magnet orientations are aligned within one region of the material, but are not aligned from one region to the next. Think of groups of tourists in Times Square pointing at different billboards around them. But when a magnetic field is applied, the orientations of the magnets, or spins, in the different regions align and the material becomes fully magnetized. It would be as if all the tourist groups turned to point at the same sign.

The process of aligning the spins, however, does not happen all at once. Instead, when the magnetic field is applied, different regions, or so-called domains, influence other nearby regions, and the changes spread through the material in a grouped fashion. Scientists often compare this effect to a snow avalanche, where a small piece of snow begins to fall, pushing other pieces nearby, until all the snow on the mountainside falls in the same direction.

This avalanche effect was first demonstrated in magnets by physicist Heinrich Barkhausen in 1919. By wrapping a coil around a magnetic material and attaching it to a speaker, he showed that these jumps in magnetism can be heard as a crackling sound, known today as Barkhausen. noise.

Now reporting in the newspaper Annals of the National Academy of Sciences (PNAS), Caltech researchers have shown that Barkhausen noise can be produced not just by traditional or classical means, but through quantum mechanical effects. This is the first time that quantum Barkhausen noise has been detected experimentally. The research represents an advance in fundamental physics that could one day have applications in creating quantum sensors and other electronic devices.

“Barkhausen noise is the collection of small magnets spinning in groups,” says Christopher Simon, lead author of the paper and postdoctoral fellow in the laboratory of Thomas F. Rosenbaum, professor of physics at Caltech, president of the Institute, and the Presidential President Sonja and William Davidow. “We’re doing the same experiment that’s been done many times before, but we’re doing it in a quantum material. We’re seeing that quantum effects can lead to macroscopic changes.”

Typically, these magnetic inversions occur in a classical way, through thermal activation, where particles need to temporarily gain enough energy to jump an energy barrier. However, the new study shows that these reversals can also occur quantum mechanically through a process called quantum tunneling.

In tunneling, particles can jump to the other side of an energy barrier without having to actually go over the barrier. If we could extend this effect to everyday objects like golf balls, it would be like a golf ball going straight over a hill, rather than having to climb it to get to the other side.

“In the quantum world, the ball doesn’t need to go over a hill because the ball, or rather the particle, is actually a wave, and part of it is already on the other side of the hill,” says Simon.

In addition to quantum tunneling, the new research shows a co-tunneling effect, in which groups of tunneling electrons communicate with each other to cause the electrons’ spins to rotate in the same direction.

“Classically, each of the mini avalanches, where groups of gyres spin, would happen on its own,” says co-author Daniel Silevitch, a research professor of physics at Caltech. “But we discovered that through quantum tunneling, two avalanches happen in sync with each other. This is the result of two large sets of electrons talking to each other, and through their interactions, they make these changes. This effect of co- tunneling was a surprise.”

For their experiments, team members used a pink crystalline material called lithium holmium yttrium fluoride, cooled to temperatures close to absolute zero (equivalent to minus 273.15 degrees Celsius). They wrapped a coil around it, applied a magnetic field, and then measured brief jumps in voltage, not unlike what Barkhausen did in 1919 in his more simplified experiment. The observed voltage spikes indicate when groups of electron spins change their magnetic orientations. As the groups of spins change, one after the other, a series of voltage spikes are observed, i.e., Barkhausen noise.

By analyzing this noise, the researchers were able to show that a magnetic avalanche occurred even without the presence of classical effects. Specifically, they showed that these effects were insensitive to changes in the material’s temperature. This and other analytical steps led us to conclude that quantum effects were responsible for the radical changes.

According to scientists, these inverted regions can contain up to 1 million billion spins, compared to the entire crystal containing approximately 1 billion trillion spins.

“We’re seeing this quantum behavior in materials with up to trillions of spins. Sets of microscopic objects are all behaving coherently,” says Rosenbaum. “This work represents the focus of our lab: isolating quantum mechanical effects where we can quantitatively understand what is happening.”

Another recent PNAS paper from Rosenbaum’s lab similarly looks at how small quantum effects can lead to larger-scale changes. In this previous study, researchers studied the element chromium and showed that two different types of charge modulation (involving ions in one case and electrons in the other) operating on different length scales can interfere with quantum mechanics. “People have been studying chromium for a long time,” says Rosenbaum, “but it took until now to appreciate this aspect of quantum mechanics. It’s another example of simple systems engineering to reveal quantum behavior that we can study on a macroscopic scale. “

The PNAS study titled “Quantum Barkhausen Noise Induced by Domain Wall Cotunnel” was funded by the U.S. Department of Energy and the National Science and Engineering Research Council of Canada. The list of authors also includes Philip Stamp, visiting associate in physics at Caltech and professor of physics at the University of British Columbia.

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