2D magnetism gets hot

The demonstration of room-temperature ferromagnetism in an atomically thin layer broadens the prospects for device applications of 2D van der Waals materials.

Soon after the first successful exfoliation of graphene in 2004¹, scientists started dreaming of devices made from purely two-dimensional (2D) materials. These dreams are based on appealing properties, such as high flexibility, optical transparency or enormous carrier mobility, of layered van der Waals materials when sliced to the monolayer limit. The ability to produce micron-scale flakes with vanishing defect concentration and to combine different properties in single devices by simply stacking flakes of various materials on top of each other has fuelled these dreams further.

Since then, researchers have established functionalities such as single-photon emission or photon-to-charge conversion. Intrinsic magnetic order in a single 2D layer, however, was still missing from the 2D portfolio until recently, but has been wished for in view of low-power ultra-compact spintronic applications.

In early 2017, two independent experiments showed magnetic order in two different systems at low temperature down to the very limit of 2D. Gong and co-workers showed that in the Heisenberg-type ferromagnet Cr₂Ge₂Te₆ the Curie temperature is suppressed when crossing from 3D to 2D, but ferromagnetic order was detected down to the bilayer system². Meanwhile, Huang et al. showed that in CrI₃, even the monolayer shows Ising-type order and magnetization stable up to 45 K (ref. ³). However, two monolayers of CrI₃ combined into a bilayer lack a net magnetization because of an antiferromagnetic coupling between the two layers. These 2D magnets provide a playground to study new phenomena in low-dimensional magnetism, create new topological phases⁴ or break time-reversal symmetry in 2D stacks for valleytronics⁵. While low-temperature 2D magnets are precious for proof-of-principle devices, most technological applications will demand magnetic order at room temperature. Bonilla et al. have now tackled this challenge. In this issue, the authors demonstrate ferromagnetic order in monolayers of VSe₂ grown on either graphite or MoSe₂ (ref. ⁶). Beside the fact that this transition metal dichalcogenide is paramagnetic in bulk, these results are remarkable because the ferromagnetic order with large magnetic moments persists above room temperature.

But where will we go from here? Recent experiments have demonstrated all-electric reversible control of the magnetization in bilayer CrI₃ (ref. ⁷). In addition, single-layer CrI₃ emits circularly polarized photons when excited with linearly polarized light, and the helicity then depends on the magnetization direction of the layer⁸. It might not take too long until the first room-temperature 2D spin valves or flexible single-photon sources with electric polarization control are demonstrated. We are not there yet and hurdles for industrial-scale applications are substantially higher, but the toolbox for engineers has been updated.