Observation of the Nernst effect driven by longitudinal spin fluctuations

Heat and electricity are usually thought of as partners that move together in the same direction. When you heat one end of a metal bar, the electricity flows from the hot side to the cold side. But there is a special trick in physics where heat and electricity can be forced to move at right angles to each other, like a river flowing sideways when the wind blows across it. This phenomenon, known as the Nernst effect, is a powerful tool for turning waste heat into useful electricity without needing bulky machinery. Scientists have long understood two main ways this happens: one relies on an external magnet pushing on the electrons, and the other relies on the internal magnetic order of the material itself. For decades, the prevailing belief was that the internal version of this effect would vanish the moment a material lost its permanent magnetism, which happens at a specific temperature called the Curie temperature. If the magnetic order disappeared, the effect was expected to disappear with it.

A team of researchers at the National Institute for Materials Science in Japan and the University of Tokyo has challenged this long-held assumption. By studying a specific family of materials made from manganese, chromium, and antimony, they discovered that the Nernst effect does not simply switch off when the material stops being a permanent magnet. Instead, the effect actually becomes strongest right around the temperature where the magnetic order breaks down. The researchers found that while the usual magnetic signals fade away completely at this critical point, a different kind of signal driven by the chaotic, fluctuating motion of the atomic spins remains strong. This discovery suggests that the restless, jiggling motion of spins in a disordered state can drive the generation of electricity just as effectively as a solid, ordered magnetic field.

To find this hidden behavior, the team created a series of polycrystalline samples with the formula Mn1-aCraSb, where the amount of chromium was carefully adjusted to change the temperature at which the material loses its magnetism. They synthesized these materials by melting high-purity elements together, grinding them into powder, and pressing them into solid bars using a technique called spark plasma sintering. The resulting bars were hexagonal in structure and free of unwanted impurities, with Curie temperatures ranging from about 295 Kelvin to 480 Kelvin depending on the chromium content. The researchers then set out to measure the Nernst effect, but they faced a challenge: measuring this effect directly in a solid block is difficult because heat can leak through the material in confusing ways. To solve this, they used a clever indirect method called lock-in thermography. Instead of measuring electricity directly, they passed an electric current through the sample while applying a magnetic field, which caused the sample to heat up in a specific pattern due to a related effect called the Ettingshausen effect. By using an infrared camera to watch the temperature waves on the surface of the sample, they could calculate exactly how strong the Nernst effect was without the interference of other thermal noise.

The results were surprising and clear. As they heated the samples, the researchers watched how the Nernst signal changed. In a typical magnetic material, the signal would drop to zero exactly when the material stopped being magnetic. However, in these manganese-chromium-antimony samples, the Nernst signal did not vanish. It remained finite even as the material crossed from a magnetic state into a non-magnetic state. In fact, the signal showed a distinct peak right around the Curie temperature. At the same time, the researchers measured the anomalous Hall resistivity, a property that usually tracks perfectly with the Nernst effect in magnetic materials. This property, however, did drop to zero exactly at the Curie temperature, just as expected. This mismatch created a puzzle: the Nernst effect was still alive and kicking, but the usual magnetic driver for it was gone.

The team ruled out several common explanations for this anomaly. They considered whether tiny magnetic particles called magnons, which carry heat but not electricity, were dragging the effect along, but calculations showed that magnons lose their influence at these high temperatures. They also looked for exotic magnetic textures or unusual electronic structures that might be responsible, but the data did not support these ideas. Instead, the researchers proposed that the culprit was the longitudinal fluctuation of spins. In a magnetic material, the spins of the electrons usually point in a fixed direction. As the material heats up toward the Curie temperature, these spins begin to wobble and fluctuate. While most theories focus on spins wobbling sideways, the researchers found that the fluctuations happening along the direction of the spin itself were driving the Nernst effect. They built a theoretical model based on this idea of longitudinal spin fluctuations and found that it perfectly reproduced the peak behavior they observed in the experiments.

This finding changes the way scientists think about converting heat into electricity. It reveals that you do not need a perfectly ordered magnetic state to generate a strong transverse thermoelectric effect. The chaotic, fluctuating state of spins near the Curie temperature is not a dead end but a powerful engine for energy conversion. Because this effect works at temperatures well above room temperature, it opens up new possibilities for designing thermoelectric devices that can harvest heat from industrial processes or engines more efficiently. The work demonstrates that the restless motion of atoms and electrons, often seen as a source of disorder, can be harnessed to create useful electrical currents, offering a fresh path for improving energy technologies.