  Spin polarized current is the main ingredient of diluted magnetic oxides due
to its potential manipulation in spintronic devices. However, most research has
focused on ferromagnetic properties rather than polarization of electric
current, because direct measurements were difficult and the origin of spin
polarized currents has yet to be fully understood. The method to increase the
spin polarized current percentage is beyond practical consideration at the
present status. To target this problem, we focus on the role of oxygen
vacancies and nano grain size on the spin polarized current, which are
controlled by growing the Co-doped ZnO thin-films at room temperature in a
reducing atmosphere [Ar + (1% ~ 30%) H2]. We found that the conductivity
increases with an increase in oxygen vacancies via two independent channels:
the variable range hopping (VRH) within localized states and the itinerant
transport in the conduction band. The Andreev Reflection measurements prove
that the VRH conduction is equivalent to spin polarized current. Transport
measurements show that the best way to increase the VRH content, or the
percentage of spin polarized current, is to increase oxygen vacancy
concentration and to reduce grain size and lattice constants of the films.
