Nat. Nanotechnol., 13, 549–553 (2018)
Spin ordering in a semiconductor has attracted much attention in the community of condensed matter physics. By combining ferromagnetism and the semiconducting nature, systems such as Mn-doped PbSnTe or InAs were found to exhibit tunable magnetic properties responsive to an externally applied electric field. It thus holds great promises for developing novel spintronics with the tuning-knobs such as gate voltage. Conventionally, those systems are often studied in bulk forms and commonly referred to diluted magnetic semiconductor (DMS). In recent years, two-dimensional (2D) magnetism has become popular as spin interactions can prevail down to the monolayer limit in a number of gapped van der Waals materials, including the family of ferromagnetic CrX3 (X = I, Br, Cl) and anti-ferromagnetic MPS3 (M = Ni, Fe, Mn). It is of great importance to re-consider now the hunting for DMS by alternatively searching for an intrinsic magnetic semiconductor in the 2D magnetic systems.
Recently, researchers from Cornell University demonstrated that the 2D magnetic semiconductor CrI3 exhibits strong tunability in its magnetic properties when subjected to an externally applied electrical field. It is known that bulk CrI3 has a band gap of ~ 1.2 eV, and its bulk ferromagnetic Curie temperature is reported to be 64 K. When thinned down to the 2D limit, it turns into an anti-ferromagnetic coupling between layers, while spins are ferromagnetically coupled in an Ising manner in each layer. As a result, the net spin alignments become layer-dependent, i.e., ferromagnetic (FM) and anti-ferromagnetic (AFM) for odd and even number of layers, respectively. Strikingly, bilayered CrI3 manifests a transform of magnetic hysteresis loop from AFM at hole doping to FM at electron doping. In other words, when zero external magnetic fields are applied, the magnetic ordering in bilayered CrI3 can be shifted from AFM to FM, purely by electrostatic fields. The 2D CrI3 has thus proven to be a platform for electrical control of spin, which opens up possibilities for future investigations of 2D intrinsic magnetic semiconductors.
In general, 2D magnetic semiconductors, being either ferromagnetic or antiferromagnetic, are building blocks for novel gate-tunable spintronic devices, such as magnetic pn junctions, interfacial topological magnetic ordering, flexible magnetic sensors/memories, and etc. Yet it is noticed that, so far, room temperature spin ordered semiconductor at the 2D limit are still rare, requiring ceaseless efforts from both experimental and theoretical sides.
Zheng Han (Institute of Metal Research, CAS, Shenyang, China)