Engineering of the atomic structure of hetero-interfaces enables tuning of electronic properties of the heterostructure such as band alignment, Schottky barrier height, interface conductivity and magnetism. Hence it is the aim of continuous experimental and theoretical research to control chemical intermixing arising due to interdiffusion, as well as interface strain, which are the main parameters that control the structural and chemical quality of a given heterostructure. However, tailoring the atomic structure of ferromagnet/semiconductor interfaces, of crucial importance for spintronic applications, has been shown to be a rather elusive goal despite the intensive research efforts over the past years [1,2].In this work, in the case of half metallic Co 2 FeAl 0.5 Si 0.5 (CFAS) and Ge we demonstrate that atomically sharp half metal/semiconductor interfaces are achievable with almost no strain due to the excellent lattice match between CFAS and Ge. We show that the film has the desirable B2 ordering which provides high spin polarisation and it does not form any secondary phases in the interface region. Based on state-of-the-art aberration-corrected high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging we have determined that the atomic structure at the interface CFAS/Ge(111) interface is realised via Ge-Co bonds, Fig. 1.Based on atomistic models derived from HAADF-STEM we construct a realistic interface model. Density functional theory calculations show that this interface atomic structure preserves both the high spin-polarization of the CFAS film as well as its magnetic moment in the interface vicinity, making this system an excellent platform for spin-based device applications, Fig. 2.The atomic level spectroscopic studies were employed to investigate the chemical abruptness of the studied interface. Atomic resolution electron energy loss spectroscopy chemical mapping reveal small and selective out-diffusion of Ge within a ~1 nm region of the interface [3]. This atomic plane selective diffusion process does not change the structural integrity and spin-electronic structure of the CFAS since the outdiffused Ge selectively substitutes only Fe and Si/Al atoms, which in turn does not affect the film's half-metallicity.Finally we report on the annealing effect on the film structures and overall magnetic properties of the CFAS thin films on Ge. Ferromagnetic resonance and vibrational magnetometry shows that mild annealing temperatures improve the magnetic properties i.e. increase the saturation magnetisation and decrease the Gilbert damping constant. Annealing above 450 o C significantly promotes Ge outdiffusion into the film, hence both structural integrity of the hetero-interface and overall magnetic properties of the film deteriorate.

Twin defects, prevalent in face centred cubic stacked materials, are observed across a wide range of natural and synthetic specimens. Such defects are essential to the mechanical behaviour of materials e.g. shape memory alloys and mediate stress and strain in both functional and mechanical materials. Fe 3 O 4 is a prototype material for spinel metal oxide structure systems, particularly the ferrite spinels. Recently magnetite has attarcted a lot of attention due to its 100% spin polarisation at the Fermi level, hence large intestet for spintronic applications. Antiphase domain boundaries are the most studied defects in magnetite and theior correlatin to magnetic properties is reported extensively [1,2]. However twins defects magnetite has not been studied on atomic level, and in particular their effect on spin polarisation and overall magnetic properties is not known. In this we have observed twin defects in Fe 3 O 4 (111) thin films grown on Ytria stablized ZrO 2 , determined their atomic structure, and modelled their electronic properties by using Density Functional Theory (DFT).The twin boundary is on (111) growth planes and it is formed by the breaking of symmetry of a Fe AFe B -Fe A layer. Aberration-corrected STEM-HAADF imaging shows that the boundary is nonstoichiometric with a missing Fe B plane. Electron energy loss spectroscopy shows changes in Fe and O core edges as well as depletion of the Fe at the boundary, which compliments the HAADF results (Fig.1). The DFT calculation of this non-stoichiometric boundary structure was modelled by introducing electron holes as a charge-compensation mechanism to realise the ionic nature of Fe 3 O 4 , Fig. 2. The electronic calculations show that majority band gap is significantly reduced with a presence of the interface states. Atomic bond counting from the DFT-optimised geometrical coordinates shows no presence of high-angle Fe-O-Fe bonds hence the absence of AFM superexchange interactions at this boundary in comparison to antiphase domain boundaries in ferrite spinels. The ferromagnetic (FM) coupling between the twin grains was also confirmed by the DFT calculation which found that AFM coupling is less energetically favourable compared to FM coupling. This work clearly shows that the (111) oriented non-stoichiometric boundaries are energetically stable and their effect is more subtle in comparison to the antiphase domain boundaries [3].