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Controlling magnetite with light
Controlling magnetite with light
by Robert Schreiber
Berlin, Germany (SPX) Jun 25, 2024

"Some time ago, we showed that it is possible to induce an inverse phase transition in magnetite," says physicist Fabrizio Carbone at EPFL. "It's as if you took water and you could turn it into ice by putting energy into it with a laser. This is counterintuitive as normally to freeze water you cool it down, i.e. remove energy from it."

Carbone has led a research project to control the microscopic structural properties of magnetite during light-induced phase transitions. The study found that specific light wavelengths for photoexcitation can drive magnetite into distinct non-equilibrium metastable states called "hidden phases," revealing a new way to manipulate material properties at ultrafast timescales. The findings are published in PNAS.

An "equilibrium state" is a stable state where a material's properties do not change over time because the forces within it are balanced. When disrupted, the material enters a non-equilibrium state, exhibiting properties that can be exotic and unpredictable.

A phase transition is a change in a material's state due to changes in temperature, pressure, or other external conditions. An everyday example is water going from solid ice to liquid or from liquid to gas when it boils.

Phase transitions in materials usually follow predictable pathways under equilibrium conditions. But when materials are driven out of equilibrium, they can show "hidden phases" - intermediate states not normally accessible. Observing hidden phases requires advanced techniques to capture rapid and minute changes in the material's structure.

Magnetite (Fe3O4) is known for its metal-to-insulator transition at low temperatures, known as the Verwey transition, changing its electronic and structural properties significantly. This transition occurs around 125 K.

"To understand this phenomenon better, we did this experiment where we directly looked at the atomic motions happening during such a transformation," says Carbone. "We found out that laser excitation takes the solid into some different phases that don't exist in equilibrium conditions."

The experiments used two different wavelengths of light: near-infrared (800 nm) and visible (400 nm). When excited with 800 nm light pulses, the magnetite's structure was disrupted, creating a mix of metallic and insulating regions. In contrast, 400 nm light pulses made the magnetite a more stable insulator.

To monitor the structural changes in magnetite induced by laser pulses, the researchers used ultrafast electron diffraction, a technique that can "see" the movements of atoms in materials on sub-picosecond timescales.

The technique allowed the scientists to observe how the different wavelengths of laser light affect the structure of the magnetite on an atomic scale.

Magnetite's crystal structure is a "monoclinic lattice," where the unit cell is shaped like a skewed box, with three unequal edges, and two of its angles are 90 degrees while the third is different.

When the 800 nm light shone on the magnetite, it induced a rapid compression of the magnetite's monoclinic lattice, transforming it towards a cubic structure. This takes place in three stages over 50 picoseconds, suggesting complex dynamic interactions within the material. Conversely, the 400 nm light caused the lattice to expand, reinforcing the monoclinic lattice, and creating a more ordered phase - a stable insulator.

The study reveals that the electronic properties of magnetite can be controlled by selectively using different light wavelengths. Understanding these light-induced transitions provides valuable insights into the fundamental physics of strongly correlated systems.

"Our study breaks ground for a novel approach to control matter at ultrafast timescale using tailored photon pulses," write the researchers. Being able to induce and control hidden phases in magnetite could have significant implications for the development of advanced materials and devices. Materials that can switch between different electronic states quickly and efficiently could be used in next-generation computing and memory devices.

Research Report:Ultrafast generation of hidden phases via energy-tuned electronic photoexcitation in magnetite.

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