Tunable, ferroelectricity-inducing, spin-spiral magnetic ordering in monolayer FeOCl

Spin spirals (SS) are a special case of non-collinear magnetic ordering, where the magnetic-moment direction rotates along an axis. The presence of SS ordering reduces the symmetry of the electron density and induces a spontaneous electrical polarization (ferroelectricity, FE) without atomic displacements. Materials with such features have potential applications in spintronics and information technology. The SS have been observed on multilayers/interfaces and artificial metal-atom films/chains, but so far not on monolayer (ML) two-dimensional (2D) materials. Here, we report density-functional-theory calculations and demonstrate that SS form in ML FeOCl, which was recently synthesized. The propagation wavelength and energetic stability of the SS can be tuned by electronic doping and uniaxial strain. Relative to the ferromagnetic state, the spin-spiral state's bandgap increases in both bulk and ML FeOCl by ~0.6 eV, enabling bandgap engineering through magnetism manipulation. A SS-induced out-of-plane FE in ML FeOCl is predicted, whereby the SS chirality can be switched by an electric field. Finally, forming a heterostructure, e.g., with graphene or boron nitride, SS are sustained, thus providing another way of modulation and the potential for magnetoelectric devices.