From bound magnetic polarons in dilute magnetic ZnO to vortex dynamics in type II superconducting NbN: a magnetotransport perspective

This thesis investigates the magnetotransport properties of dilute magnetic ZnO and type-II superconducting NbN thin films under varying magnetic fields and temperatures. The work focuses on understanding the underlying transport and magnetic mechanisms relevant for spintronic devices, superconducting electronics, and photon detection technologies. For dilute magnetic semiconductors, ZnO thin films doped with Co and Mn were studied to examine the formation of bound magnetic polarons (BMPs) arising from interactions between magnetic ions and oxygen vacancies. Magnetoconductivity measurements were performed on ZnO, ZnCoO, and ZnMnO thin films with different dopant concentrations in the temperature range from 5,K to 200,K. The results show that controlled oxygen vacancy engineering and transition-metal doping enable stable BMP formation and room-temperature ferromagnetism in ZnCoO thin films. Furthermore, the dimensionality of the conduction layer strongly influences sheet resistance and transport behavior, highlighting its importance for transparent spintronic applications. The second part of the thesis examines vortex dynamics in ultrathin NbN type-II superconducting films. Magnetotransport measurements reveal how vortex motion and flux pinning near the critical temperature affect magnetoresistance and dissipation mechanisms under external magnetic fields. These findings are relevant for superconducting nanowire single-photon detectors and quantum technologies, where vortex behavior impacts device performance. ZnO-based films were synthesized using pulsed laser deposition, while NbN thin films were prepared by atomic layer deposition. Structural, magnetic, optical, and electrical characterizations were combined with theoretical modeling to understand the observed transport phenomena. Overall, this work provides insights into BMP stabilization in ZnCoO and vortex dynamics in NbN, contributing to the advancement of spintronic and superconducting materials.