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A7: Electronic, geometric, and magnetic properties of multiferroic heterostructures

The general focus of the project is to understand the magneto-electric coupling across interfaces of two-component layered multiferroics on a microscopic level. This is achieved through the simultaneous determination of the electronic, geometric and magnetic structures. Based on the previous XMCD, MOKE and SQUID investigations of metal layers (Fe and Co) or oxides (CoFe₂O₄, NiFe₂O₄, (Mn,Zn)Fe₂O₄) on BaTiO₃(001) single crystals, now systems containing thin films of BTO will be studied. The use of PLD (pulsed laser deposition) has proven to yield well-defined magnetic and ferroelectric oxide layers. Utilizing structured systems, a polarization change of the thin ferroelectric film (BaTiO₃) only requires low voltages, enabling in-situ control of the ferroelectric state and, thus, a direct way to influence the magnetic properties of suitable layers.

Starting from the existing knowledge the anisotropies of the magnetization should show results of the interaction, in particular, if the ferroelectric polarization is changed in a defined way. Pt(111) as substrate for BaTiO₃(BTO) films also opens the promising field of a quasicrystalline BTO substrate for magnetic films, where our previous investigations on crystalline BTO serve as an ideal reference for a systematic determination of physical properties. In this context, the methods XAS (x-ray absorption spectroscopy) and PED (photoelectron diffraction) have the particular advantage that they do not require long-range order, but yield element specific information. Layer growth and also domain formation strongly depend on surface morphology, in particular the presence of islands. This requires extending the methods above with spatial resolution, namely by employing XPEEM (photoelectron emission microscopy) investigations.

The element and spin specific experimental methods will be accompanied in the preparation as well as the analysis stage by calculations within the framework of a fully ab-initio relativistic theory (codes like SPR-KKR, ELK) as well as LFM theory. The geometric structure will be determined from PED experiments by multiple-scattering calculations and ab-initio methods (e.g. VASP). The close connection of theory and experiment has already been used successfully on NFO/BTO, CFO/BTO and (Mn,Zn)FO/STO. The new focus in the theoretical calculations will be the determination of the magneto-crystalline anisotropy in ultrathin films with particular attention to the influence of the interface. The first priority is still the development of an effective method beyond ground state theories and multi-code approximations of core level spectroscopy (e.g. Bethe-Salpeter equation or time-dependent DFT).