If you hold an iron nail near a magnet, it will be attracted. Inside the magnet, tiny atomic magnetic arrows point in the same direction, like soldiers in formation. But in some crystals, the formation is different: neighboring arrows point in opposite directions. This order is called altermagnetism, and it is hard to see directly.

In a nickel fluoride (NiF₂) crystal, the magnetic arrows of nickel atoms are indeed arranged this way. But there is a nuance: due to a weak effect related to the motion of electrons around the nucleus, the arrows tilt slightly to one side. This gives rise to weak ferromagnetic behavior, as if soldiers in formation, looking alternately left and right, all tilted their heads a bit.

The scientists irradiated the crystal with X-rays and measured how they were absorbed. The absorption depended on the direction of the magnetic field: this is called X-ray magnetic circular dichroism. The signal turned out to be the sum of two contributions: one from the altermagnetic order and one from weak ferromagnetism. To separate them, they used two methods: they varied the external magnetic field below the Néel temperature (the point where the order disappears) and measured the signal above this temperature. Both methods gave the same result.

This method, X-ray magnetic circular dichroism, proved to be a convenient tool for studying altermagnets. It helps separate the weak ferromagnetic contribution from the main order. This is important for creating new materials where magnetic order can be controlled by an electric field.