Two-dimensional magnetic materials have been hailed as building blocks for the next generation of small, fast electronic devices. These materials, made of layers of crystalline sheets just a few atoms thick, gain their unique magnetic properties from the intrinsic compass-needle-like spins of their electrons. The sheets' atomic-scale thinness means that these spins can be manipulated on the finest scales using external electric fields, potentially leading to novel low-energy data storage and information processing systems. But knowing exactly how to design 2D materials with specific magnetic properties that can be precisely manipulated remains a barrier to their application.