Since the discovery of graphene¹, the family of two-dimensional materials has grown, displaying a broad range of electronic properties. Recent additions include semiconductors with spin–valley coupling², Ising superconductors^3,4,5 that can be tuned into a quantum metal⁶, possible Mott insulators with tunable charge-density waves⁷, and topological semimetals with edge transport^8,9. However, no two-dimensional crystal with intrinsic magnetism has yet been discovered^{10,11,12,13,14}; such a crystal would be useful in many technologies from sensing to data storage¹⁵. Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin–Wagner theorem¹⁶. Magnetic anisotropy removes this restriction, however, and enables, for instance, the occurrence of two-dimensional Ising ferromagnetism. Here we use magneto-optical Kerr effect microscopy to demonstrate that monolayer chromium triiodide (CrI₃) is an Ising ferromagnet with out-of-plane spin orientation. Its Curie temperature of 45 kelvin is only slightly lower than that of the bulk crystal, 61 kelvin, which is consistent with a weak interlayer coupling. Moreover, our studies suggest a layer-dependent magnetic phase, highlighting thickness-dependent physical properties typical of van der Waals crystals^17,18,19. Remarkably, bilayer CrI₃ displays suppressed magnetization with a metamagnetic effect²⁰, whereas in trilayer CrI₃ the interlayer ferromagnetism observed in the bulk crystal is restored. This work creates opportunities for studying magnetism by harnessing the unusual features of atomically thin materials, such as electrical control for realizing magnetoelectronics¹², and van der Waals engineering to produce interface phenomena¹⁵.

An intrinsic magnetic tunnel junction: An electrical current running through two stacked magnetic layers is larger if their magnetizations point in the same direction than if they point in opposite directions. These so-called magnetic tunnel junctions, used in electronics, must be carefully engineered. Two groups now show that high magnetoresistance intrinsically occurs in samples of the layered material CrI ₃ sandwiched between graphite contacts. By varying the number of layers in the samples, Klein et al. and Song et al. found that the electrical current running perpendicular to the layers was largest in high magnetic fields and smallest near zero field. This observation is consistent with adjacent layers naturally having opposite magnetizations, which align parallel to each other in high magnetic fields. Science , this issue p. 1218 , p. 1214

A van der Waals heterostructure of monolayer WSe ₂ and ferromagnetic CrI ₃ enables exceptional control of valley pseudospin.