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A7: Determination of the Local Geometric and Magnetic Order in Ferroic Single- and Multi-Layers

Project A7 investigates the microscopic relationship of ordering effects (i.e. geometric and magnetic structure) at interfaces and surfaces, physical properties and functionalities of new ferroic layer systems. The systems are composed of thin layers of ferroelectric and ferromagnetic materials, where the ferromagnetism of one layer couples to the ferroelectricity of the other layer through an interface. This possibly enables to switch the magnetization of the ferromagnetic layer by applying a voltage to the ferroelectric layer.

The growth of single- and multi-layer systems of ferroic materials is investigated in depth and the local geometric and magnetic structure of the resulting layers is determined. First investigations of the magnetoelectric coupling will be performed on single-layer systems of ferromagnetic layers on ferroelectric substrates. Specifically, layers of cobalt ferrite CoFe₂O₄ will be prepared on BaTiO₃ substrates by reactive simultaneous evaporation of Co and Fe in an oxygen atmosphere and characterised by x-ray photoemission (XPS). On medium terms multi-layer systems will also be investigated using photoemission at high kinetic energies (HIKE-PES) as well as the already established x-ray emission spectroscopy (XES), because with these techniques also buried layers and interfaces can be characterized. XES can in addition also be used to obtain magnetic information through dichroism. In addition to the ferrites, also Mn doped layers of ZrO₂ will be prepared and characterized, for which theoretical investigations have already proposed promising ferrromagnetic properties. As a new preparation method, atomic layer deposition (ALD) will be used, initially for ZrO₂, later also for ferrites and perovskites. ALD achieves more precise control of film growth, since at least one of two reactions for the preparation of the oxide from volatile precursors is self limiting.

Dichroism of the x-ray absorption at K-, L-, and M- edges when applying circular (XMCD) and linear (XMLD) polarised synchrotron radiation to samples with magnetic order is used to determine the magnetic behaviour of layers and the resulting magnetoelectric coupling. In a second step, the analysis will be supplemented with angle-resolved photoemission (MCDAD and MLDAD) in the range of the valence band and shallow core levels. Information will be obtained by crossing temperature dependent phase transitions and by applying external magnetic fields. In particular, XMCD is capable to quantitatively determine magnetic moments in an element specific manner, from which the coupling strength can also be deduced; XMLD is sensitive to the axis of magnetic ordering. Photoemission with spin detection in addition permits the direct measurement of the polarisation of electronic states.

The magnetic properties of the layers as well as the magnetoelectric coupling depend in a fundamental way on the geometric structure. The largest influence is expected from the distortions which result from the lattice mismatch at the interface and from changes in the sites and coordination of the ions sourcing the magnetism. The geometric structure will be determined using low-energy electron diffraction (LEED) and photoelectron diffraction. The lateral structure within the layers will be determined through the analysis of LEED spot profiles (SPA-LEED), the vertical structure through the analysis of I(V) curves. The local structure in layers and at interfaces will be determined by photoelectron diffraction in angle- and energy-scan mode. The emphasis will be on investigations, where the element specificity of the method can be utilized, for example in the determination of the position of dopants in ZrO₂ or the position of Co in CoFe₂O₄. Structure and related physical properties will be determined in close contact with DFT and multiple scattering cluster calculations which include the correct treatment of spin polarisation.