Dr. Michael Fechner
Doctoral Thesis: Magnetoelectric coupling at multiferroic interfaces
When magnetism and ferroelectricity coexist in the same crystalline phase of a material it is called multiferroic. Furthermore, the crystal symmetry of a multiferroic material allows for magnetoelectric coupling which is quite useful since the coupling offers magnetization switching by an external electric field or electric polarization switching by an external magnetic field. In this thesis multiferroic interfaces are examined upon the occurrence of magnetoelectric coupling. Therefore first-principle methods based on density functional theory are used. The model system that are investigated consist of different ultrathin ferromagnetic films (Co and Fe) on top of ferroelectric ATiO3 (A=Pb,Ba) perovskites. The calculations show that at the interface a moderate change of the size of the total magnetization due to a change of the electric polarization occurs. Furthermore, the magnetic order of the Fe film is sensitive to its thickness, so an unexpected ferrimagnetic ordering appears for 2ML Fe whereas for all other thicknesses ferromagnetic ordering is preferred. Hybridization and strain effects at the interface explain the observations. Finally it is shown how the magnetic instability can be used to gain control of the magnetic ordering by the electric polarization.
[ more ... ]
Diploma Thesis: Dispersion Relation of Image Potential States
The aim of this work was to investigate theoretically the measurement of the dispersion relation of image potential states. Image potential states are quasi discrete ones which are induced by the field of a tunnel tip above a surface. Electrons can tunnel resonant through these states which makes a spectroscopy of these states with measurements of the distance on voltage characteristic z(V) and differential conductivity voltage characteristic possible.
[ more ... ]
Research Internship: Tunnelling Through Arbitrary Potentials
The aim of this work was to describe the tunnel process in a 1D model. The Tunnelling process is the possibility of electrons/particles to transmit through a potential barrier on which they would be reflected in the classical case. The transmission can be described thereby in quantum-mechanical senses with the transmission probability. This was calculated for rectangular, trapezoidal and also for selected arbitrary potential barriers.
[ more ... ]
Up
