Laser Light Reveals Hidden Magnetic Signals in Gold, Copper and Aluminum

In 1881, Edwin Hall tried to catch a magnetic effect in metal using a beam of light, and failed. He closed his now-famous paper with a note of defeat: if silver's action had been one tenth as strong as iron's, he thought, the effect would have been detected. None was observed. Nearly a century and a half later, a team led from Hebrew University has found a way to see what Hall could not.

The work, published in Nature Communications, was led by Ph.D. candidate Nadav Am Shalom and Prof. Amir Capua of the Institute of Electrical Engineering and Applied Physics at Hebrew University. Collaborators included Prof. Binghai Yan of the Weizmann Institute of Science, researchers at Pennsylvania State University, and Prof. Igor Rozhansky of the University of Manchester. Their target was the optical Hall effect in metals that are not magnetic, such as copper, gold and aluminum.

Some background explains the difficulty. The Hall effect, known for over a century, describes how electric currents bend in a magnetic field. In magnetic materials like iron it is strong and well understood. In ordinary metals it is far weaker. A related optical version should show how electrons behave when light and magnetic fields interact, but at visible wavelengths it has been too subtle to measure. Scientists knew it should be there and lacked the tools to detect it. Capua compared it to trying for decades to hear a whisper in a noisy room without a sensitive enough microphone.

The team's answer was to upgrade the magneto-optical Kerr effect, or MOKE, a method that uses a laser to measure how magnetism alters the reflection of light. They paired a 440-nanometer blue laser with large-amplitude modulation of the external magnetic field, which greatly raised the technique's sensitivity. With it, they picked up magnetic signals in copper, gold, aluminum, tantalum and platinum, something previously considered near-impossible. Capua noted that these metals don't stick to a fridge the way iron does, yet under the right conditions they do respond to magnetic fields, only very subtly.

The practical appeal is considerable. Hall measurements matter in the semiconductor industry and in studying materials at the atomic scale, because they help determine how many electrons a metal contains. Traditionally that means physically attaching tiny wires to the device, which is slow and tricky for nanometer-sized components. The new approach requires only shining a laser on the electrical device.

The researchers also looked closely at what seemed to be random noise in their signal. It was not random. It followed a clear pattern tied to spin-orbit coupling, a quantum property linking how electrons move to how they spin. That connection also bears on how magnetic energy dissipates in materials, which the team says has direct implications for the design of magnetic memory, spintronic devices and quantum systems. Am Shalom likened it to finding that static on a radio is actually someone whispering valuable information, and said the group is now using light to listen to electrons.

The technique is described as non-invasive and highly sensitive, working without massive magnets or cryogenic conditions. The researchers suggest it could help engineers build faster processors, more energy-efficient systems and more accurate sensors. Capua said the work turns a nearly 150-year-old problem into an opportunity. Those applications remain prospective: what has been shown is the measurement itself, and whether it carries into devices outside the lab is the next thing to watch.