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B1: Defect-induced magnetism in oxides

This is a joint project between the experimental physics group of Prof. P. Esquinazi and Dr. M. Ziese and the theoretical physics group of Prof. W. Hergert. Aim of this project is the study of defect-induced magnetic order in oxides with empty or fully occupied d shell. The focus is on the prototypical oxides MgO, SrTiO₃, LaAlO₃, ZnO, TiO₂ and ZrO₂. In stoichiometric bulk form these oxides are diamagnetic, but by the introduction of defects spin moments are induced that show long range order at appropriate concentrations. A significant influence of interfaces on the magnetic properties of superlattices is expected and is investigated. This might be due to particular types of defects induced by the interfaces or by magnetic moment formation at the interfaces themselves due to the broken symmetry.

With a combination of experimental techniques such as SQUID magnetometry, particle induced X-ray emission and positron annihilation spectroscopy in combination with ab initio calculations it was shown that ZnO films grown under N₂ atmosphere show ferromagnetic order due to the creation of structural defects, possibly Zn vacancies. The saturation magnetization depends systematically on the N₂ partial pressure during deposition, is reproducible and is certainly not due to contamination with the ferromagnetic elements Fe, Co and Ni. The Curie temperature is well above room temperature.

The theoretical calculations are based on a so called multi-code approach.  The real structure of bulk materials and layered systems containing vacancies, impurities or more complicated defect complexes are investigated by means of the Vienna Ab Initio Simulation Package (VASP).  An ab initio multiple scattering code based on the KKR (Korringa Kohn Rostoker) Green’s function method is used to study electronic and magnetic properties in detail. Disorder can be treated in the multiple scattering approach using the coherent potential approximation. Correlation effects are included via self-interaction corrections. Information from the ab initio calculations are used to investigate the temperature dependence of the magnetic effects by means of Monte Carlo methods.  The theoretical calculations are performed in strong interaction with the experimental investigations.

Principal Investigators

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