A "tunable Heisenberg model" designed by physicists from MIT reveals the effect of magnetic forces at the quantum level, addressing the fundamental nature of magnetism and advancing the human understanding of one of the most common phenomena.

The team, from the Physics Department of the MIT-Harvard Center for Ultracold Atoms and Research Laboratory of Electronics, published their latest findings in the journal Nature, December 16. Their report details their observations from experimenting with ultracold lithium atoms, finding different spinning behaviors among the atoms. The equilibrium orientation spinning atoms return to depend on the magnetic forces between individual Li atoms. Some atoms spin faster than others, while some follow a slower, diffused pattern.

This discovery marked the first time that these varying spin behaviors have been observed, although it has been predicted by the Heisenberg model - a set of mathematical equations by Werner Heisenberg that describes critical points and phase transitions in magnetic systems.

Aside from addressing magnetism's fundamental nature, the discovery could also lead to the development of "spintronic" devices, which transmit, store, and process data using quantum particle spins instead of the conventional electron flow.


Quantum Spin in Magnetism

In the study of magnetism, quantum spin is considered the microscopic unit, with the atom's orientation is determined by whether it spins clockwise or counterclockwise.

For magnetic materials, a group of atoms could spin in alignment for equilibrium states. On the other hand, dynamic behavior in atoms is illustrated by a cascading spin that resembles a wave across multiple atoms.

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Researchers found out that this dynamic behavior is sensitive to individual atoms' magnetic forces on each other. The wavelike pattern appeared to disperse quicker in isotropic magnetic forces compared to anisotropic magnets.

According to a news release from MIT, the MIT group's experiment cooled lithium atoms down to 50 nanokelvins, close to absolute zero and more than 10 million times colder than interstellar space. At extremely low temperatures, almost all activities from the atoms are halted and near-frozen, making any response to magnetic inputs observable without interference from the atoms' thermal motion.

Using lasers, researchers arranged a lattice of about 40,000 atoms, which is then subjected to magnetic pulses that induce a change in the spin of the atoms.


Transitioning Back to Equilibrium Behavior

In the MIT team's "tunable Heisenberg model," they varied the strength of the magnetic forced applied to the lattice to change the width of the "stripes" observed in the atomic spin patterns. Researchers then noted the rate and behavior at which the patterns faded back into equilibrium.

They found a "ballistic behavior," where spins quickly returned to the equilibrium state, and a "diffusive behavior" where spins transition back in a slower, more erratic manner. Surprisingly, this behavior coincides with the expectations drawn from the Heisenberg model. Harvard's theoretical physicists provided the cutting-edge computations of spin dynamics.

"It was interesting to see that there were properties which were easy to measure, but difficult to calculate, and other properties could be calculated, but not measured," said Wen Wei Ho, one of the study's authors.

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