Magnetic excitons in Eu-chalcogenide heterostructures

Eu-chalcogenides (EuXs) are semiconductors exhibiting unique magnetic properties. At low temperatures, EuTe, e.g, shows an antiferromagnetic phase while EuSe shows metamagentic behavior. The magnetic properties of EuXs, caused by the seven, strongly localized electrons in the 4f shell of the Eu ions, profoundly affect also the optical properties of these materials. So, "anomalous" red shifts of the absorption edges and the photo luminescence transitions under applied magnetic field have been observed as well as giant Stokes shifts between the luminescence transition energies and the absorption edges. The physical reason for these observations is not clear yet and deserves further experimental as well theoretical investigations. In our institute we grow EuXs layers by molecular beam epitaxy. The resulting layers exhibit a much higher crystalline quality than all other EuX samples available in the world. This allows, e. g., for the first time to observe excitonic luminescence lines in EuTe. These, excitonic lines show a giant tunability in applied magnetic fields, which is very promising for applications in novel spintronic devices. In this project, we aim to enhance the giant tunability of the exciton photoluminescence even further and to use the large tunability of the exciton states for the development of strongly magnetic field dependent optoelectronic devices. Furthermore, we want to induce ferromagnetism in EuXs and we try to manipulate the spin coherence time. For this purposes we will grow and study EuX hetero- and nanostructures like quantum wells, wires and dots. The samples mainly will be characterized by optical spectroscopy. The experimental results will be compared to theoretical models in order to clarify the physical mechanism causing the gigantic magneto-optical response.

In contrast to metals, semiconductors are optical transparent for a part of the light spectrum. The portion of the transparent region is given by the "energy band gap" of the semiconductor, which also determines in which colour the semiconductor appears to the eye. When conventional semiconductors are placed into an external magnetic field, their band gap energy shrinks by about 1/1000 % of its value. In this project we have demonstrated a semiconductor material for which this effect is about 10 000 times larger, so that the band gap energy decreases by approximately 10% of its value, when the semiconductor is placed into a magnetic field. This huge effect is caused by magnetic phase transitions taken place in this material. In contrast to ferromagnets, where all magnetic moments present in the material are added to each other, they are compensated in antiferromagnetic materials. Thus antiferromagnets appear to be completely unmagnetic to the ambiance. This fact maces the detection of antiferromagnetic layers rather difficult, especially when they are buried under the surface of a sample. Recently, with a special kind of X-ray spectroscope the magnetic moments of buried antiferromagnetic layers could be detected, with a thickness resolution as small as the size of an atom. Our group contributed to this experiment by developing a simulation program which allows to predict the expected experimental results with high accuracy. In addition, with the help of our simulations also the roughness of the interface between the buried antiferromagnetic layer and the surrounding nonmagnetic matrix can be determined with high accuracy. This method, which was demonstrated here for europium-telluride layers, is also applicable for a large number of various other antiferromagnetic materials. Usually, magnetic effects are observed only up to certain maximal sample temperatures, which are called the critical temperatures of these materials. For technological applications, these critical temperatures should be at least above room temperature. To obtain such materials, in this project we have chemical synthesised iron-oxide nanoparticles. These nanoparticles, which can be easily processed because they are present in liquids, have the property that their critical temperatures can be varied by their size. Furthermore, our synthesis allows to some extend even to control their shape, which allows to influence their magnetic properties appearing below their critical temperatures.