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.

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

Spin pumping and spin-transfer torques are two reciprocal phenomena widely studied in ferromagnetic materials. However, pumping from antiferromagnets and its relation to current-induced torques have not been explored. By calculating how electrons scatter off a normal metal-antiferromagnetic interface, we derive pumped spin and staggered spin currents in terms of the staggered field, the magnetization, and their rates of change. For both compensated and uncompensated interfaces, spin pumping is of a similar magnitude as in ferromagnets with a direction controlled by the polarization of the driving microwave. The pumped currents are connected to current-induced torques via Onsager reciprocity relations.

In a collinear antiferromagnet with easy-axis anisotropy, symmetry guarantees that the spin wave modes are doubly degenerate. The two modes carry opposite spin angular momentum and exhibit opposite chirality. Using a honeycomb antiferromagnet in the presence of the Dzyaloshinskii-Moriya interaction, we show that a longitudinal temperature gradient can drive the two modes to opposite transverse directions, realizing a spin Nernst effect of magnons with vanishing thermal Hall current. We find that magnons around the $\mathrm{\Gamma }$ point and the $K$ point contribute oppositely to the transverse spin transport, and their competition leads to a sign change of the spin Nernst coefficient at finite temperature. Possible material candidates are discussed.

A spin-pumping antiferromagnet: Antiferromagnets have been used in spintronics mainly as a source of the so-called exchange bias. However, they hold promise for a much more active role given that their magnetization dynamics can in principle be much faster than those in ferromagnets. For this promise to materialize, antiferromagnets must learn the tricks that come naturally to ferromagnets. Vaidya et al. observed one such phenomenon called spin pumping (see the Perspective by Hoffmann). The researchers irradiated the antiferromagnet MnF ₂ with circularly polarized subterahertz light, causing the spins in this material to spring into action. These dynamics, in turn, caused the injection of spin current into a layer of platinum adjacent to MnF ₂ . Science , this issue p. 160 ; see also p. 135