Three atoms of iron for every four of oxygen. That’s what it takes to make magnetite, one of the oldest minerals known to humankind, and a material that has been used for ages to learn about magnetism.
Today, magnetite is still schooling modern physicists—Northeastern researchers included—as they look for clues to a long-standing enigma hidden deep within its atomic structure.
Because of fundamental physics controlling the behavior of magnetite’s electrons and the way its atoms are arranged, the material loses its ability to conduct electricity when it is cooled to 125 K (about as cold as it can be in Jupiter’s frozen moon, Europa). At that temperature, the material turns from being a metal, which transports charge, into an insulator, which doesn’t.
For decades, scientists have tried to fully understand the mechanisms within magnetite’s atoms that lead to this elusive transition into insulation. That’s why Gregory Fiete, a professor of physics at Northeastern, and Martin Rodriguez-Vega, a postdoctoral researcher, teamed up with a global group of collaborators to peer into the inner workings of magnetite.
The team shot magnetite crystals with lasers, watching the behavior of the crystals closely as the laser’s light particles stimulated the atoms. For the first time, they observed new ways in which magnetite’s electrons respond to the lasers. Those excitations indicate how magnetite’s electrons, and the way they arrange in the crystal’s structure, change as they turn the material into an insulator.
“The fact that you take the material out of equilibrium [with a laser] opens up a whole new realm of possibilities,” Rodriguez-Vega says.
Fiete, who focuses on probing exotic properties of different materials, says that the team’s findings give them a better idea of how they could one day control magnetite’s properties with just the tickle of a laser.
“It’s taking another step in the direction of having a material and actually changing what it can do for us by shining a laser on it,” Fiete says. “Turning one material into two materials, or even more.”
Published in Nature Physics, the findings are the result of a modern technique that first blasts a material with a laser to excite it and then uses a second, lower-energy laser, to probe its response at ultra-fast speeds of trillionths of a second.
Rodriguez-Vega and Fiete use the data they gather from the laser’s reflection to model magnetite’s electrical conductivity.
“From the experimental data, we can infer the existence of these excitations that were not known before these experiments,” Rodriguez-Vega says. “The most advanced numerical calculations have had a real hard time to deal with this [magnetite] system, because it has just so many atoms in the unit cell, that it becomes intractable.”
In the experiments, the researchers observed specific oscillations in the patterns of trimerons, basic units consisting of three iron ions aligned symmetrically within the structure of magnetite. These oscillations were accompanied by changes to the fundamental interactions of their electrons.
The team’s ability to measure this behavior in magnetite suggests that its electrons work collectively to form trimerons, which work in tandem as conductivity decreases.
“The complex interactions in magnetite lead to the formation of these units, which collectively order together,” Rodriguez-Vega says.
Earlier experiments suggested the existence and arrangement of this trimeron order. The new findings are pushing the limits of what can be done in the lab based on theoretical knowledge.
Because the properties of magnets are essential for generating electricity, completing the puzzle of magnetite’s hidden powers could eventually lead to new ways to manipulate materials and improving electronics by harnessing the behavior of their electrons.
Faster electronics, better communication devices, more efficient ways to store data—those are just some of the outcomes that the researchers can think of.
And, while thinking ahead would mean venturing into a whole different field of physics, Rodriguez-Vega says he is hopeful that shining more lasers on magnetite and other rare materials could improve everything we know about our electronics.
“The end goal would be to come up with properties [of materials] on-demand,” he says. “We want to be able to tune the behavior of a material using lasers with different frequencies and amplitudes for technological applications.”