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A8: Multiferroic 0–3, 2–2, and 1–3 composites

The aim of the project is the preparation of multiferroic interfaces between magnetic and ferroelectric oxides and the investigation of the interplay between ionic and electronic processes during magnetoelectric switching.

Two central questions concern the contribution of ionic processes to the polarization-dependent tunnel resistance at ferroelectric barriers and to the electric conductivity at multiferroic interfaces. On the materials' site we focus on Ba_1-xSr_xTiO₃ or PbZr_1-xTi_xO₃ in combination with ferrite-spinels MFe₂O₄ (M = Co, Mg, Mn, Ni). Composite multiferroics of different dimensionality are being prepared by various soft-chemistry approaches as well as by pulsed laser deposition (PLD). After phase analysis and determination of the chemical constitution as well as the crystallographic structure, ferroic properties and magnetoelectric coupling are examined in dependence on temperature and electric or magnetic fields. Ionic processes during switching caused by high electrical field will be studied on a local scale applying temperature- and field-dependent scanning force microscopy. Chemical doping and adjustment of the oxygen content are used to control such ionic effects.

1–3 composites will be synthesized using oxide matrices with ordered pores. 2–2 composites are being prepared by spin-coating or epitaxial growth both on commercial substrates and single crystals from the floating zone technique. For 0–3 composites solution-based chemical methods followed by sintering steps are applied. In addition, the eutectic crystallization is used, which in the case of BaTiO₃/CoFe₂O₄ is known to result in strong magnetic coupling. For structural investigations X-ray and neutron diffraction is used. Horizontal buried or vertical interfaces are investigated and manipulated applying the electrical field through the tip of a force microscope. The switching of the electrical polarization requires a large electric field, which in oxides can lead to ionic transport processes (e.g. the generation of oxide defects or decomposition of a surface water layer). Recently developed methods in force microscopy comprising electrochemical strain microscopy (ESM) and local current-voltage curves are employed to analyze ionic processes and their effects on the magnetoelectric coupling.