Oxygen-vacancy-mediated ferromagnetism in Fe:In2O3 (IFO) and exchange bias effect in IFO/NiO system.

Abstract

Diluted magnetic semiconductor (DMS) with high TC finds significant applications in Spintronics, where both electron charge and electron spin can be manipulated. Exchange bias (EB) effect plays a very important role in many magnetic devices (e.g. spin valve) by pinning or stabilizing a ferromagnetic layer. However, the origin of ferromagnetism in many DMSs and the physics of EB effect are still not clear.

In this final year project, both the ferromagnetism in IFO (Fe:In2O3) and the EB effect in IFO/NiO system are studied through a series of well-designed experiments. For the first part, we aim to clarify the roles of carrier and oxygen vacancy in ferromagnetism of IFO. We designed our experiments such that carrier concentration is varied by doping different concentrations of Sn into the usual IFO samples and oxygen vacancy concentration is varied by annealing the samples in air or high vacuum. The structure, magnetic and electrical transport properties of the samples have been characterized by XRD and Physical Property Measurement System. Based on the experimental results, we have concluded that ferromagnetism in IFO should be attributed to oxygen vacancies. To understand the mechanism, we have proposed a modified oxygen-vacancy model, where the oxygen vacancies contribute to both ferromagnetism and conduction. This model is consistent with both our results and experimental data reported by other groups.

For the EB effect, we have employed our diluted ferromagnetic oxide IFO as the F layer and fabricated a IFO/NiO exchange bias heterostructure by sputtering deposition. The samples have been field cooled (FC) from 350K to 10K and hysteresis loops at various temperatures are collected by PPMS. The results show that we have established exchange bias below the Neel temperature . These results can be explained in our own model and the Spin Galss model.