Giant spin splitting and in-plane multiferroicity in wurtzite monolayer hidden phases

Achieving polarization switching in wurtzite (wz) crystals has long been hindered by substantial energy barriers and high coercive electric fields. Here, we demonstrate that an in-plane ferroelectric (FE) switch can be triggered within the (0001) crystallographic plane, through the discovery of hidden $${Abm}2$$ and $${Pmc}{2}_{1}$$monolayer phases. The structural self-reconstruction, induced by lattice expansion, converts interfacial covalent bonds into van der Waals interactions, enabling facile exfoliation of wz monolayers. These monolayers exhibit multiferroic order and diverse electronic functionalities, including giant spin splittings (~540 meV), transition between half-metal and semiconductor, and wide band gaps (0–4.57 eV). Importantly, the FE transition can be finely tuned via a transient state, leading to significant reductions in the barrier energy (<~3 meV/atom) and coercive field (~0.6–1.0 MV/cm), and yielding fully electric control of 100% spin polarization. Our study provides in-depth insights into the in-plane FE mechanism in wz systems, opening new avenues for the design and discovery of wz-based FE devices, as well as the rich physics in tetrahedral semiconductors.