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978-3-8439-1041-5, Reihe Physik
An ab initio study of multicomponent polar oxide based systems in combination with ferromagnetic metals
211 Seiten, Dissertation Universität Duisburg-Essen (2012), Softcover, A5
Multifunctional composites based on polar oxides and ferromagnetic metals are promising for a wide range of applications, ranging from solar cells and environmental purification to multiferroic memory devices. The understanding of electronic, ferroelectric, magnetic and structural properties and their coupling at interfaces on an atomistic level is incomplete due to the complexity of such materials.
In the present thesis composites based on TiO2, BaTiO3, Fe, Co2MnGe and Ni8Mn5In3 are investigated by means of density functional theory.
As starting point for the understanding of composites the individual properties of the constituents are presented with special focus on the incipient ferroelectric nature of TiO2 (rutile). The stabilization of ferroelectric rutile by means of lattice distortions is discussed based on phonon spectra and atomic relaxation patterns. Ferroelectric states with two different polarization directions can be stabilized by experimentally achievable strain, and the possibility of strain-engineering of the polarization magnitude and direction is likely.
As second step towards the understanding of composite materials the modification of the physical properties in reduced dimensions is discussed. A systematic study on ferroelectric domain walls in BaTiO3 is presented, and atomically sharp 180° domain walls are found to be most favorable.
Studies on isolated TiO2 and BaTiO3 particles with diameters up to 2.4 nm show that the critical size for a ferroelectric polarization is not reached as there neither a macroscopic polarization nor the formation of regular domain patterns. Instead, large local dipole moments exist which cancel each other.
Third, composites based on polar oxides and ferromagnetic metals in a planar geometry are discussed. As example the adsorption of the prototypical transition metal Fe on the TiO2 surface is investigated. The spin-polarization of the transferred electrons induces finite magnetic moments in the TiO2 surface and a magneto-electrical-coupling at the interface is likely. An even larger coupling is found at the BaTiO3-Ni8Mn5In3 interface as the martensitic phase transition induces a rotation of the polarization direction by 90° in BaTiO3.
Besides the planar geometry, especially particle agglomerates possess large interface regions and thus a large interface coupling is likely.
First benchmarking calculations on the pressure-induced sintering of TiO2-TiO2 and TiO2-Fe composites are presented.