The spin-transfer inverse Faraday effect in Au/YIG thin film bilayers

Light carries more than just energy; it carries a twist. When light waves spin as they travel, a property known as circular polarization, they possess angular momentum, much like a spinning top. For decades, scientists have known that if you shine this spinning light onto certain materials, it can nudge the tiny magnetic spins inside them, creating a magnetic field without the need for electric currents. This phenomenon, called the inverse Faraday effect, offers a way to control magnetism with light, potentially leading to computers that are faster and cooler than today's machines. However, this control has been difficult to achieve in magnetic insulators, materials that do not conduct electricity but are excellent at carrying magnetic information. These materials are crucial for future technology because they waste very little energy, but they are also stubborn; they do not absorb light easily, making it hard to use light to change their magnetic state directly.

Researchers at The Ohio State University and Florida International University have found a clever workaround to this problem by building a sandwich of two different materials. They placed a thin layer of gold, a metal that interacts strongly with light, directly on top of a thin layer of yttrium iron garnet, a magnetic insulator. By shining a pulse of infrared light onto the gold, they were able to transfer the light's spinning energy into the gold, which then passed that energy across the boundary into the magnetic insulator. This process, which they call spin-transfer inverse Faraday effect, allowed them to manipulate the magnetism of the insulator using light, even though the light's energy was too low to be absorbed by the insulator itself. The team demonstrated that this method works on a nanometer scale, changing the magnetic state of the insulator in just a few trillionths of a second, and did so without heating the material significantly, which is a common problem in other methods.

To understand how this works, one must first look at the materials themselves. The researchers grew extremely thin films of yttrium iron garnet, measuring just ten and fifteen nanometers thick, on a crystal base. They then deposited a five-nanometer layer of gold on top. To test their setup, they used a specialized laser system that fires two beams of light: a strong "pump" beam to excite the material and a weaker "probe" beam to measure what happened. They could change the polarization of the pump beam, making it spin either to the left or to the right, or keep it straight. When they fired a straight, non-spinning beam of light at the gold layer, the material heated up slightly. This heat caused a flow of magnetic information from the insulator to the metal, a known effect called the longitudinal spin Seebeck effect. This served as a baseline, showing how the two materials talk to each other when only heat is involved.

The real discovery came when they switched to spinning light. When they shone light that spun to the right, the gold layer generated a specific type of magnetic push. When they switched the light to spin to the left, the push went in the opposite direction. By comparing the results of these two opposite spins, the team could isolate the effect caused purely by the light's twist, removing the background noise caused by simple heating. They found that this twisting light created a magnetic signal in the gold that was significantly stronger than what they saw in gold alone. More importantly, this signal was not just staying in the gold; it was crossing the boundary and changing the magnetism of the yttrium iron garnet underneath.

The researchers were careful to prove that this change was not just a side effect of the light heating the sample. They measured how the signal changed as they increased the power of the laser. If the effect were purely thermal, the signal would have grown in a specific, predictable way related to how heat spreads. Instead, they found that the signal grew in a pattern that matched the behavior of magnetic waves, known as magnons, within the insulator. This confirmed that the light was directly exciting the magnetic order of the insulator through the gold, rather than just warming it up. They also observed that the magnetic state of the insulator flipped or shifted depending on the direction of the light's spin, a clear sign that the angular momentum from the photons was being transferred to the magnetic spins of the material.

This experiment is significant because it shows a new way to control magnetic insulators, which are the building blocks for future low-power memory and logic devices. Previously, controlling these materials required high-power lasers or thick layers of material, which made them impractical for tiny, fast electronic components. The team showed that they could achieve this control with a much lower power level, using a pulse of light with an energy density of just 6.8 joules per square meter, which is far less than what was needed in earlier studies. They achieved this by using the gold layer as a mediator, a transducer that catches the light's energy and hands it off to the insulator. The entire process happens incredibly fast, with the magnetic changes occurring within a few picoseconds, or trillionths of a second, and the system returning to its original state shortly after.

The study also revealed details about how the two materials interact at the atomic level. By measuring the magnetic response at different moments in time, the team could see the sequence of events: the light hits the gold, the gold's electrons react almost instantly, and then the magnetic influence travels into the insulator. They found that the strength of this interaction depends on the quality of the boundary between the gold and the insulator. If the interface is clean and well-ordered, the transfer of spin is efficient. This suggests that by carefully engineering these layers, scientists could design devices that use light to write and erase magnetic information with high speed and minimal energy consumption.

While the researchers did not see the magnetic waves oscillating back and forth in a rhythmic pattern, as seen in some other experiments with thicker materials, they did observe a clear and lasting change in the magnetic state. This indicates that the spin-transfer inverse Faraday effect is a robust mechanism for manipulating magnetism in thin films. The work opens a path for using infrared light to control magnetic insulators, a capability that could be vital for the next generation of spintronic devices. These devices would rely on the spin of electrons rather than their charge, potentially leading to computers that are faster, more efficient, and capable of processing information in ways that current technology cannot. The findings suggest that the key to unlocking these capabilities lies in understanding and mastering the interface between metals and magnetic insulators, turning a simple layer of gold into a powerful tool for controlling the invisible world of magnetism.