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The doping of a wide band gap insulator offers an opportunity to increase the coupling between free carriers and magnetic impurities under the magnetic polaron model, leading to an enhanced in the Curie temperature of the host compound, critical for the fabrication of devices with magnetic properties. Some rare earth elements have large intrinsic magnetic moments due to unfilled 4d orbitals, and have been readily incorporated in materials for optical applications. Here the rare earths gadolinium and cerium were explored either as dopants or as part of the high-K semiconducting compound for the fabrication of magnetic heterojunction devices with magnetic properties.
This thesis work explores the effects of rare earth gadolinium and cerium as dopants in high-K compounds such as EuO, HfO2 and Gd2O3. The thesis begins with an exemplary tale of a local moment wide band gap system (although not rare earth based), and a success in achieving negative magneto-resistance in a heterojunction structure with chromium-doped hydrogenated diamond-like carbon (Cr-DLC). In the quest for similar results, we explored the rare earth compounds by means of their electronic band structure using photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES) to provide insight into the material functionality and applicability as an electronic device. Rectifying (diode-like) properties were observed in all the heterostructure and each heterojunction device exhibited unique properties that make them suitable for different applications such as neutron detection or spin electronics applications. Remarkable results were observed on the EuO compound with the inclusion of 4% Gd content. The system undergoes a non-metal to metal transition as suggested by the appearance of filled electron pockets. The device properties resemble those of a tunnel junction diode, which might be related to a band bending at the interface of the film, likely due to surface overoxidation.
Adviser: Jerry L. Hudgins