A lithium cobalt phosphate crystal resembles a chessboard, where the black and white squares are regions with opposite directions of the magnetic moments of cobalt ions. These regions are called antiferromagnetic domains, and until now they have been hard to see: conventional methods required complex equipment or damaged the sample. A team of researchers found a way to illuminate this 'board' with infrared light.

It turns out that the two types of domains absorb light differently at a specific wavelength — about 1550 nanometers, the very wavelength used in fiber-optic communications. The difference in absorption arises because the cobalt ions in the crystal are surrounded by slightly distorted oxygen octahedra, and this distortion affects electrons differently depending on the direction of the magnetic moment. When light is directed along one axis of the crystal, one type of domain absorbs it more strongly, while the other absorbs it less.

Thanks to this contrast, the scientists were able to map the domains through an ordinary optical microscope, simply by passing light through a thin slice of the crystal. No complex magnetic probes or synchrotron radiation, just a light source, lenses, and a camera. This opens a simple and fast way to observe the magnetic structure of antiferromagnets, which are considered promising for ultra-dense and energy-efficient memory.

The chessboard of magnetic domains is now visible to the naked eye — if you look through an infrared filter. This means engineers can quickly check material quality and tune it for future devices.