Direct observation of hopping induced spin polarization current in oxygen deficient Co-doped ZnO by Andreev reflection technique

Abstract

Oxygen vacancy induced ferromagnetic coupling in diluted magnetic oxide (DMO) semiconductors have been reported in several studies, but technologically more crucial spin-polarized current (SPC) is still under-developed in DMOs. Few studies have claimed that VRH mechanism can originate the SPC, but, how VRH mechanism associated with percolation path, is not clearly understood. We used Point-contact Andreev reflection (PCAR) technique to probe the SPC in Co-doped ZnO (CZO) films. Since the high resistance samples cause broadening in conductance(G)-voltage(V) curves, which may result in an unreliable evaluation of spin polarization, we include two extra parameters, (i) effective temperature and (ii) spreading resistance, for the simulation to avoid the uncertainty in extracting spin polarization. The effective G-V curves and higher spin polarization can be obtained above a certain oxygen vacancy concentration. The number of completed and fragmentary percolation paths is proportional to the concentration of oxygen vacancies. For low oxygen vacancy samples, the Pb-tip has a higher probability of covering fragmentary percolation paths than the complete ones, due to its small contact size. The completed paths may remain independent of one another and get polarized in different directions, resulting in lower spin-polarization value. High oxygen vacancy samples provide a high density of completed path, most of them link to one another by crossing over, and gives rise to high spin-polarization value.