Perovskite photovoltaics: Slow recombination unveiled.

news & views neighbouring spins are pointing in opposite directions) as a proximity layer is a new approach in the context of magnetic topological insulators. Antiferromagnets and 'artificial antiferromagnets' (engineered heterostructures in which the coupling between ferromagnetic layers can be tuned) are widely used in magnetic memory applications. One of their advantages is the absence of a macroscopic magnetic moment, and thus the absence of a magnetic stray field, which makes them robust against external fields. The work by Qing Lin He and co-workers reports the realization of a superlattice of the antiferromagnet CrSb and the magnetic topological insulator Cr-doped (Bi,Sb) 2 Te 3 , combining the concepts discussed above. The materials are well lattice-matched, leading to abrupt interfaces as shown in Fig. 2a, and this makes CrSb (whose Néel temperature is about 700 K) one of the few antiferromagnets suited to incorporation into functional layer structures. In this system, He and colleagues demonstrate that the magnetic topological insulator can be exchange-coupled to the antiferromagnet, leading to an enhancement of the exchange field up to ~50 mT, an increase in the Curie temperature from ~38 K to ~90 K, and an enhancement of the coercive field to 67 mT (for a bilayer sample, as compared with 47 mT for a single layer). Interestingly, by using the antiferromagnet with its in-plane moments to pin the magnetic topological insulator at the interface, an overall parallel or antiparallel alignment of the (out-of-plane) moments of the ferromagnetic layers in a trilayer structure can be achieved, as illustrated in Fig. 2b. This is a nice example of a functional heterostructure that could be useful in engineering the quantum effects related to magnetic topological insulators. There is no doubt that several materials challenges must be overcome before one will be able to observe the quantum anomalous Hall effect in these proximity-coupled systems at high temperatures; but the first steps have been taken, and there is more to come. In the near future, thorough magneto-transport studies, as well as angle-resolved photoemission spectroscopy, are needed to complete the experimental proof that the Dirac fermion becomes massive. As the topological surface state lives near the interface, we may see a similar surge of fascinating and surprising phenomena, as with complex oxide interfaces in the recent past 8. Emergent states are waiting to be discovered, and it is time for the topological insulators to stack up. b MTI MTI AFM …