Advancing Electronics with Light: The Magnetite Innovation

In recent research published in PNAS, Fabrizio Carbone of EPFL investigated and controlled the microscopic structural features of magnetite during light-induced phase transitions.

Some time ago, we showed that it is possible to induce an inverse phase transition in magnetite. It is 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.

Fabrizio Carbone, Associate Professor, EPFL

The study found that the system can drive magnetite into distinct non-equilibrium metastable states—“hidden phases” refers to states in which the state can change under specific conditions—using particular light wavelengths for photoexcitation. This finding opens up a new protocol for manipulating material properties at extremely fast timescales.

What are the “non-equilibrium states”? An “equilibrium state” is a stable condition in which a material's characteristics remain constant throughout time because the forces inside it are balanced. When this is disturbed, the material (or “system” in physics jargon) is said to enter a non-equilibrium state, showing features that can be exotic and unpredictable.

The “Hidden Phases” of Magnetite

A phase transition is a shift in a material’s state caused by changes in temperature, pressure, or other external factors. A common example is water transitioning from solid ice to liquid or liquid to gas as it boils.

Material phase transitions often follow predictable patterns under equilibrium circumstances. However, when materials are forced out of equilibrium, they might begin to exhibit so-called “hidden phases”–intermediate states that are not ordinarily accessible. Detecting hidden phases necessitates specialized approaches that can capture quick and minute changes in the material’s structure.

Magnetite (Fe₃O₄) is a well-studied substance recognized for its fascinating metal-to-insulator transition at low temperatures, which ranges from the ability to carry electricity to actively preventing it. The Verwey transition alters magnetite’s electronic and structural characteristics significantly.

With its intricate interplay of crystal structure, charge, and orbital orders, magnetite could experience the metal-insulator transition at around 125 K.

Ultrafast Lasers Induce Hidden Transitions in Magnetite

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

The experiments employed two light wavelengths: near-infrared (800 nm) and visible (400 nm). When activated with 800 nm laser pulses, the magnetite’s structure was disturbed, resulting in a mixture of metallic and insulating areas. In contrast, 400 nm light pulses increased the magnetite’s stability as an insulator.

To monitor the structural changes in magnetite caused by laser pulses, the researchers utilized ultrafast electron diffraction. This method can “see” the motions of atoms in materials on sub-picosecond timescales.

The method enables scientists to see how different wavelengths of laser light impact the structure of magnetite on an atomic scale.

Magnetite has a “monoclinic lattice” crystal structure, which means that the unit cell is structured like a skewed box with three uneven edges, two of which are 90 degrees, and the third is different.

The monoclinic lattice of the magnetite rapidly compressed under the influence of the 800 nm light, resulting in a transition towards a cubic structure. This occurs over a period of 50 picoseconds in three phases and implies that the material is undergoing intricate dynamic interactions.

On the other hand, the lattice expanded in response to the visible light at 400 nm, strengthening the monoclinic lattice and generating a more ordered phase that is a stable insulator.

Fundamental Implications and Technological Applications

The study demonstrates that multiple light wavelengths can be used to precisely modify magnetite’s electronic properties. Understanding these light-induced transitions reveals important details about the underlying physics of strongly correlated systems.

The researchers concluded, “Our study breaks ground for a novel approach to control matter at ultrafast timescale using tailored photon pulses.”

Being able to induce and manage hidden phases in magnetite might substantially impact the development of innovative materials and devices. For example, materials capable of rapidly and effectively switching between several electronic states might be employed in next-generation computing and memory devices.

Journal Reference:

Truc, B., et. al. (2024) Ultrafast generation of hidden phases via energy-tuned electronic photoexcitation in magnetite. PNAS. doi:10.1073/pnas.2316438121