In this thesis, a unique NiI$_2$ monolayer on atomic iodine bilayer (supported by a Au(111) substrate) system was created (by molecular beam epitaxy) to control and engineer NiI$_2$ multiferroicity in the monolayer limit. In this way the balance of intrinsic magnetic interactions already present in the pristine NiI$_2$ material was changed, and symmetry breaking via the Dzyaloshinskii-Moriya interaction (DMI) was induced - all promoted by the large size and spin-orbit coupling (SOC) of subsurface iodine. This strongly interacting system offered access to a rich moir\'e physics: a primary moir\'e system, where the NiI$_2$ monolayer interacts with just one subsurface atomic iodine layer, and a super-moir\'e system, where the NiI$_2$ monolayer feels the interaction with both subsurface iodine layers. The primary moir\'e system spontaneously stabilises three distinct magnetic skyrmions per supercell and yields non-zero average out-of-plane electric polarization. We also found that the super-moir\'e modulates the magnetic interactions induced by the primary moiré around the high symmetry points of the superlattice and imprints its own symmetry onto both spin and polarization textures. Further, the multiferroic properties of NiI$_2$ monolayer were found to be dependent on the super-moir\'e superlattices achieved (as our system gives access to more than one type of super-moir\'e) and are strongly pinned by them. Overall, this study brings the first ever evidence of (super-) moir\'e control over multiferroicity in a 2D material in the monolayer limit. These findings were obtained through a combination of atomically resolved un-polarised and spin-polarised (SP-) scanning tunnelling microscopy and spectroscopy (STM/STS) and X-ray techniques such as X-ray magnetic circular dichroism (XMCD); the latter also elucidated the behaviour in magnetic field of this non-collinear non-coplanar magnetic phase, and the system’s temperature-induced phase transitions. X-ray standing waves (XSW) studied stacking effects in this multi-layer system, supporting the conclusion that NiI$_2$ monolayer grows indeed onto atomic iodine layers; and angle-resolved photoelectron spectroscopy (ARPES) showed how the subsurface atomic iodine layers affected the electronic structure below the Fermi energy. Finally, through the application of electric field pulses delivered via the STM tip we showed the creation of polarons in the NiI$_2$ monolayer and the manipulation of the charge state of multiferroic domain walls. Strong band bending effects were also evidenced at the edges of the NiI$_2$ monolayer and a screening decay length of $\sim$1 nm was determined.
van der Waals 2D systems with spin texture, and their manipulation: (Alternative Format Thesis)
Turnpenny, L. (Author). 24 Jun 2026
Student thesis: Doctoral Thesis › PhD