Spin spirals (SS) are a special case of noncollinear magnetism, where the magnetic-moment direction rotates along an axis. They have generated interest for novel phenomena, spintronics applications, and their potential formation in monolayers, but the search for monolayers exhibiting SS has not been particularly fruitful. Here, we employ density functional theory calculations to demonstrate that SS form in a recently synthesized monolayer, FeOCl. The SS wavelength and stability can be tuned by doping and uniaxial strain. The SS-state band gap is larger by 0.6 eV compared to the gap of both the ferromagnetic and antiferromagnetic state, enabling bandgap tuning and possibly an unusual formation of quantum wells in a single material via magnetic-field manipulation. The SS-induced out-of-plane ferroelectricity enables switching of the SS chirality by an electric field. Finally, forming heterostructures, for example, with graphene or boron nitride, maintains SS ordering and provides another method of modulation and a potential for magnetoelectric devices.
Keywords: bandgap engineering; ferroelectricity; monolayer; spin spiral; type-II multiferroic.