Light-controlled electronics utilising magnetite

Magnetite, the oldest and strongest natural magnet, is used in electronics and possesses unique properties that have made it significant in the field of spintronics – devices that operate on the spin of electrons rather than their flow (which is known as electrical current). Moreover, magnetite has played a crucial role in understanding magnetism, attracting the interest of Einstein and other renowned scientists. Its magnetic and electronic properties are studied in biomagnetism, catalysis, and paleomagnetism.

Recently, research on exploiting its out-of-equilibrium switching properties gained momentum, highlighting its potential for advanced technologies. Magnetite's rich history and multifaceted applications continued to fascinate and drive scientific discovery.

“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, which means you remove energy from it.”

Carbone led a research project to elucidate and control the microscopic structural properties of magnetite during light-induced phase transitions. The study discovered that using specific light wavelengths (colours) for photoexcitation could drive magnetite into distinct non-equilibrium metastable states, referred to as “hidden phases”. These states can change under certain conditions, revealing a new protocol to manipulate material properties at ultrafast timescales.

The findings, which could impact the future of electronics, were published in PNAS.

An “equilibrium state” refers to a stable state where a material’s properties do not change over time because the forces within it are balanced. When this balance is disrupted, the material (or “system” in physics terms) enters a non-equilibrium state, exhibiting properties that can be exotic and unpredictable.

Magnetite’s hidden phases

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 changing from solid ice to liquid or from liquid to gas when it boils.

Phase transitions in materials usually follow predictable pathways under equilibrium conditions. However, when materials are driven out of equilibrium, they can start showing “hidden phases” – intermediate states that are not normally accessible. Observing hidden phases requires advanced techniques to capture rapid and minute changes in the material’s structure.

Magnetite (Fe3O4) is a well-studied material known for its intriguing metal-to-insulator transition at low temperatures – changing from conducting electricity to actively blocking it. This is known as the Verwey transition, which significantly alters magnetite’s electronic and structural properties.

With its complex interplay of crystal structure, charge, and orbital orders, magnetite can undergo this metal-insulator transition at around 125K.

Utilising light to unlock magnetite's hidden properties

“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 near-infrared (800 nm) and visible (400 nm) light. These were then excited with 800 nm light pulses, and 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 observe the structural changes in magnetite caused by laser pulses, the researchers employed ultrafast electron diffraction. This technique can track atomic movements in materials on sub-picosecond timescales (a picosecond is a trillionth of a second).

This method enabled the scientists to examine how different laser light wavelengths affected magnetite's atomic structure.

Magnetite’s crystal structure is known as a “monoclinic lattice”, which resembles a skewed box with three unequal edges. Two angles are 90 degrees, while the third angle is different.

When magnetite was illuminated with 800 nm light, it caused a rapid compression of the monoclinic lattice, pushing it towards a cubic structure. This transformation occurred in three stages over 50 picoseconds, indicating complex dynamic interactions within the material. Conversely, exposure to 400 nm visible light caused the lattice to expand, reinforcing the monoclinic structure and creating a more ordered, stable insulating phase.

Implications and applications

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

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