https://phys.org/news/2018-06-hidden-magnetism-symmetry.html

People have known about magnetism since ancient times but are still learning how it works, especially at the quantum scale. In ferromagnets, atoms and their neighbors have magnetic moments (caused by spin) that all align along the same direction. We can easily control that direction by an external magnetic field. In antiferromagnets, however, the magnetic moments anti-align with their neighbors and alternate one-by-one. This microscopic spin alignment perfectly shields any external magnetic field and is hidden from the outside world. Antiferromagnets were discovered by Louis Néel in 1948, but were described in his 1970 Nobel lecture as being theoretically interesting but technologically useless.

Jian Liu, assistant professor of physics, explained that generally spins in an antiferromagnet can rotate anyway they want as long as the anti-alignment is maintained. But, if the interaction between the atoms is anistropic, "it will give the spin a certain preferential direction." This is the DM (Dzyaloshinskii-Moriya) interaction originating from relativistic effect, and Liu explained that it does two things. First, it tilts (or cants) the spins slightly away from the perfect anti-alignment, which is good because this means that an external magnetic field won't be completely shielded and can couple to the canted spins, even if they're staggered. There's a tradeoff, however, in that while this interaction allows for canting, it pins the direction.