Magnetic field effect on capacitance and photocurrent properties of metal oxides

This dissertation focuses on the study of the effect of magnetic field on capacitance and photocurrent, all of which give some insight into the magneto-electric/magneto-optical coupling in dielectric and photovoltaic materials and devices. The main study of this dissertation focuses on magnetic enhancement of capacitance and photocurrent.

(i) The magnetic field effect on the capacitance of GaFeO3-ionic liquid-GaFeO3 composite material was examined. Up to 2.35 folds tunability could be achieved with voltage 10 V and magnetic field strength 1542 Oe. A mechanism which takes account of the coupling between the ionic liquid layer and the dielectric layer was presented. The effect shows a possible route to use the ionic liquid integrated with the dielectric material and suggests a simple way to introduce tunable dielectric permittivity.

(ii) A model that charge gradient would result in the coexisting magneto-resistance and -capacitance tunability in material systems is proposed. Coexisting of tunable magneto-resistance and -capacitance in Sm2Ga2Fe 2O9 is observed. The model agrees with the experimental result.

(iii) Two lithium based dielectric materials Li2ZnSiO 4 and Li4SiO4 were chosen for the investigation of photocurrent and photovoltage under a magnetic field. It is demonstrated that high magnetic field enhancement of photovoltage and photocurrent could be achieved in these two dielectric lossy materials, Li2ZnSiO 4 and Li4SiO4. 3850% of photovoltage tunability with magnetic field 0.4T and 3841% of photocurrent tunability with magnetic field 0.4 T in Li2ZnSiO4, 132.8% of photovoltage tunability and 132.5% of photocurrent tunability in Li4SiO4 are reported. A mechanism which considers the effects of electron/nuclear spin mixing and interaction under the magnetic field was proposed. This introduces large tunability and makes them suitable for practical application in optoelectronic technologies.

(iv) Multi-bands in the magneto-photocurrent spectrum of GaI3O 9 are reported. Their formation is considered to be due to the effect of quantum state transitions and quansi-resonant electronic polarization.

These results establish that the magneto-electro and magneto-opto coupling are importance for practical application of the material system and the application of the magnetic field may be useful for modification of the capacitance and photocurrent properties in the metal oxides. The current studies have shown some interesting features of the use of the magneto-electro and magneto-opto coupling in several metal oxides, which makes this research outcome could provide possible application on the performance improvement of energy harvester, solar cell and magnetometer. Nevertheless, the main attraction for studies of magneto-electro and magneto-opto coupling in the metal oxide might exist because the metal oxides are resource-abundant, non-toxic and easily-synthesized, and their mechanisms of magneto-capacitance/magneto-photocurrent are still need to be explored. Not until a clear physical/chemical picture of these materials is established, the work on this subject can be continued.