Publications

The present work investigates the crucial issue of oxidative corrosion of cobalt and its influence on the magnetic properties with the support of suitable models. The synthesis of cobalt microstructures with controllable morphologies was accomplished by a facile wet-chemical reduction route without the aid of a magnetic field. The magnetic dipolar interactions drive cobalt microparticles to self-assemble and, forming one-dimensional cobalt microchains. The density of states (DOS) and band structure of cobalt was determined using ab-initio simulations in both HCP and FCC phases that occurs simultaneously. The impact of ageing and the exchange bias phenomenon between Co and CoO are profoundly investigated in the manuscript with the help of magnetic and XPS measurements. These results were supplemented by atomistic simulations, were that was used to probe model the temperature-dependence of magnetic properties in aged cobalt particles. This was made possible by assuming the aged cobalt particles as core-shell systems possessing a cobalt core and a cobalt oxide shell.

γ'-Iron(IV)Nitride is a magnetic alloy with a highly symmetric cubic structure that has been studied for decades and is applied, for instance, as a coating on steel and as a component in magnetic devices. An analysis of the phonon band structure under applied pressure has recently pointed out a structural instability (soft mode), resulting in a phonon-induced phase transition at a few GPa. The analysis in Ref. [1] is ambiguous in some respects, and therefore we performed an exhaustive analysis of all distortions that are consistent with the soft modes. Several of them were found to be stable and have energies that compete with the high-symmetry ground state structure at zero pressure. A scrutiny of these crystals shows that γ'-Iron(IV) Nitride can lower its energy either by displacing some atoms or by modulating the magnetic moments, often opting for a combination of both. The total magnetization of these distorted cells is significantly smaller than that of the stable phase at 0 GPa γ'-Iron(IV)Nitride. In addition, it is suspiciously close to the experimentally observed magnetization. Therefore, we speculate that these reduced-symmetry solutions may be relevant for the experimentally observed Iron(IV)Nitride phase as well.

[1] T. Cheng et al., Journal of Magnetism and Magnetic Materials 451 (2018) 87–95