Strain-induced effects in the electronic and spin properties of a monolayer of ferromagnetic GdAg2

Geometrical structure, lattice periodicity and atomic arrangement are subtly intertwined with the electronic properties of materials. Sub-angstrom changes in the atomic distance are sufficient to modify the physical and chemical properties, such as the band-structure, carrier mobility and the chemical reactivity. This implies that considerable progress in tailoring the electronic and optical properties can be obtained by strain-engineering two dimensional layers structures. In mesoscopic systems formed by few-layers thick ferromagnetic materials, structure relaxation processes and in-plane strain lead to additional interesting effects in the magnetic properties.

In this collaborative work, carried out by the groups led by Matthieu Verstrate (University of Liege), Bin Xu (University of Arkansas) and Lucia Vitali (CFM, UPV/EHU, Ikerbasque), it has been successfully demonstrated that the electronic and magnetic properties of a monolayer of GdAg₂, on Ag(111) can be modified via a thermal-controlled surface-strain of the intra-layer atomic distance, leading to lateral hetero-structuring. The GdAg₂ layer, which differs in lattice constant from the supporting Ag(111), leads to the formation of a moiré superstructure. In these superstructures, the atoms of the alloy are in a periodically shifting stacking configuration with respect to the one of the supporting layer. Therefore, also the interlayer coupling varies. This induces an out-of-plane atomic buckling of the supporting substrate at a defined position of the moiré structure. This relaxation effect and the variable coupling interaction steer a modulation in the density of states and a local weakening of the ferromagnetic order resulting in a spin-texture across the layer. These results provide perspectives for control of electronic properties and magnetic ordering in atomically-thin layers.

Topographic image showing a lateral-heterostructure formed by two GdAg2 moiré superstructures differing in lattice constants. Image size 20 nm × 20 nm (b) Calculated relaxed geometry for the GdAg2/Ag(111) in three different stacking configurations (hcp, fcc and top) and inducing variable atomic relaxations. Electron density of states (c) and conductance maps (380 meV and 700 meV) (d–e), showing the localization of the electronic peaks either of the two moiré superstructures.