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. Such materials have potential applications in spintronics and information technology. The presence of SS ordering may reduce the symmetry of the electron density and induce a spontaneous electrical polarization (ferroelectricity, FE) without atomic displacements. Research on SS-FE coupling has been on multilayered/bulk materials and artificial metal-atom chains, but not on monolayer (ML) two-dimensional (2D) materials. Using density functional theory, we demonstrate that SS ordering, found in bulk FeOCl, also exists in ML FeOCl whose synthesis was recently reported. The propagation wavelength and energetic stability of the SS can be tuned by electronic doping and uniaxial strain. Relative to the FM state, the spin-spiral state’s bandgap increases in both bulk and ML FeOCl by ~0.6 eV, enabling bandgap engineering through magnetism manipulation. Moreover, the SS induces out-of-plane FE in ML FeOCl, switchable via the spiral’s chirality. Finally, forming a heterostructure, e.g., with graphene or boron nitride, is another way to modulate and tune the SS ordering.