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A crystal does not grow as a single whole but like a city of neighborhoods — grains. In each neighborhood, atoms are arranged in their own way, and the boundaries between them are like streets. In a new experiment, physicists looked into such a crystal using a microscope sensitive to magnetic fields and saw that the magnetic regions inside it exactly follow the grain boundaries.

The material is a thin film of bismuth-antimony telluride doped with vanadium. At low temperatures, it conducts electricity without loss along the edge while remaining an insulator inside. This property is called the quantum anomalous Hall effect. Magnetism here acts like glue that holds the edge current together. But how exactly the grains are magnetized has long remained a mystery. Some experiments hinted that the entire crystal is one big magnet, while others suggested it consists of many small, independent ones.

The scientists varied the external magnetic field and watched how the magnetization regions rearranged. They found that these regions do not appear at random spots but grow from existing ones, like a stain spreading, not like new splotches. This is typical of ferromagnets, where neighboring atoms tend to align in the same direction. But the size of the regions matched the size of the crystal grains. This means that short-range forces act within a grain, while long-range coupling between grains unites them into a single magnet.

Previously, a similar material with chromium behaved differently: there, magnetic regions did not coincide with grain boundaries. Now it is clear that different dopants affect magnetic order in different ways. This is important for future electronic devices, where the quantum Hall effect could be used for precise measurements or data storage.