P. Mellado et al., Phys. Rev. Lett. (2012)

Three of a kind. Each rod is a $2$-centimeter-long bar magnet that can rotate freely in the vertical direction. They experience “frustration” at these $3$-magnet vertices because there is no single arrangement that satisfies every magnet’s desire to be oriented oppositely from its neighbors. A square-meter array of these magnets serves as a model of materials called spin ices.