Nanoscale spin structures at surfaces driven by exchange frustration and higher-order interactions

In this thesis, density functional theory (DFT) calculations using the full potential linearized augmented planewave and the projector augmented wave method in combination with an atomistic spin model are employed to explore the effect of higher-order exchange interactions on the magnetic ground state of ultrathin transition metal (TM) films on surfaces. First, a systematic study of higher-order interactions (HOI) beyond the pairwise Heisenberg exchange in magnetic trilayers comprising a single hexagonal Fe or Co layer sandwiched between 4d and 5d TM layers is presented. While for Fe based trilayers HOI terms can range on the same order of magnitude as the Heisenberg exchange interaction, they turn out relatively small for Co based systems. The trends obtained for freestanding trilayers serve as a basis to understand HOI in ultrathin films on surfaces that are amenable to experiments. Further DFT calculations show that HOI can induce spontaneous nanoscale two-dimensional multi-Q states with collinear spin structure in hexagonal Fe/Rh films with different stacking sequence and thickness on the Ir(111) surface studied by experimental collaborators via spin-polarized scanning tunneling microscopy (SP-STM). DFT combined with an atomistic spin model elucidates a competition of frustrated Heisenberg exchange and HOI as the driving mechanism for the formation of the observed complex spin structures while the DMI is weak. The same approach further demonstrates that the interplay of pairwise Heisenberg exchange, HOI and the DMI is responsible for the symmetry and collinearity of spin lattices in Fe monolayers in direct contact with Ir(111): while for fcc-Fe the well-known square nanoskyrmion lattice is obtained as the magnetic ground state, a hexagonal multi-Q state with nearly collinear magnetic moments occurs in hcp-Fe similar to the above-mentioned hexagonal spin structures in Fe/Rh bilayers.