A6: Ferromagnetic and ferroelectric properties of oxide surfaces
The aim of the project is the characterization of the electronic properties of interfaces in multiferroic systems by means of electron spectroscopy. Starting point is the investigation of pure oxides. A second component will then be deposited layer by layer under control of the surface analytical tools. One technique is SPMIES (=Spin Polarized Metastable Induced Electron Spectroscopy) which is ideally suited to detect spontaneous spin polarization at the surface. The influence of electric polarization of ferroelectric oxides on their surface structure shall be studied with this technique for BaTiO3. XPS will be employed to investigate band discontinuities at interfaces between semiconductors and ferroelectrics, as well as variations of band discontinuities upon changes of ferroelectric polarization.
The project is devoted to electronic phenomena at surfaces and interfaces of oxides that can experimentally be studied by means of electron spectroscopy. Making use of different primary beams the information depth can be varied from zero (MIES) up to about 20 nm (XPS at synchrotron radiation facility (HIKE)). Thus, phenomena at the top surface layer, e.g. the theoretically predicted surface magnetism at oxide surfaces, as well as buried phenomena, e.g. band bending at interfaces, are accessible. The variation of the electronic structure on parameters like composition, defect density, crystal structure shall be investigated.
It has been predicted theoretically that surface magnetism may occur at polar surfaces of nonmagnetic oxides. Within this project we will establish an experimental arrangement that allows a spin-resolved measurement of the electronic surface structure by SP-MIES. Spin polarization of the metastable Helium atoms is achieved by optical pumping with circularly polarized light of a laser. Using this setup we hope to give experimental evidence of spontaneous spin polarization on MgO(111) and later maybe also on other oxide surfaces. The goal is to find a systematic relationship between composition, crystalline structure, defect density and spin polarization and therefore the magnetic properties of surfaces. This would be a general contribution to find out the reasons for magnetism in thin layers and on surfaces
A further question which shall be studied with MIES is how the electronic structure at ferroelectric oxide surfaces varies under the influence of electric polarization. The typical sample will be BaTiO3. We expect that MIES allows identifying the termination of samples depending on the preparation method as well as the effect of electric polarization on the electronic surface structure. Information with respect to the influence of polarization onto the growth of hetero structures is scarce up to now. This question shall be studied for the systems metal/ BaTiO3 (e.g. nickel and platinum) and semiconductor/ BaTiO3 (e.g. CuI, SrCu2O2) in a systematic way.
For applications of hetero structures with transport of spin or charge through the interface detailed information about the electronic band diagram is very significant. Important parameters are the discontinuities of conduction and valence bands which are difficult to predict theoretically. For this reason the third key point of the project is the experimental determination of band discontinuities as function of composition, crystallographic orientation, chemical bonds and the ferroelectric polarization by means photoelectron spectroscopy for interfaces that are prepared in UHV. Model systems will be CuI/BaTiO3(001), SrCu2O2/BaTiO3(001), ZnO/BaTiO3(001), BaTiO3/ZnO(0001)/r- sapphire and (Zn,Mg)O/ZnO. The experimental band discontinuities provide valuable information for modelling transport processes at these interfaces which have impact onto the macroscopic properties of the hetero structures. The latter properties are subject of investigation in other projects within the Collaborative Research Centre 762. In particular the influence of the polarization onto the band structure at semiconductor/ferroelectric material is hardly studied up to now. Using SXPS (Synchrotron XPS) it will be possible to study effects at the top surface layer (e.g. the Fermi level induced defect formation for wide band gap semiconductors) as well as effects deeper in the material (e.g. the band bending in the space charge zone) by varying the energy of the photon beam. Depth profiles of relevant properties shall be established. Additionally, barrier height between Pt metal and BaTiO3 will be studied vs. the direction of the electrical polarization.
Principal Investigators
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Prof. Dr. Harald Morgner ⇒
phone: +49 (0) 341/97 36389 fax: +49 (0) 341/97 36090 |
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