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Structural and magnetic properties of transition metal added ZnO thin-films

Structural and magnetic properties of transition metal added ZnO thin-films
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Spintronics is the science and technology of manipulating the spin degree of freedom of a single carrier (electron or hole) or of an ensemble of such carriers to provide new functionality for microelectronics devices. Because the spin of a charge carrier may be the last remaining degree of freedom to explore and exploit, numerous research groups have pursued comprehensive knowledge of spin dependent phenomena, and potential technological breakthroughs that may derive from their application. One approach to obtaining practical spintronic applications is to exploit novel materials that show ferromagnetic ordering at above room temperature (RT). Since the suggestion that diluted magnetic semiconductors (DMSs) based on GaN and ZnO doped with 5% at % Mn with an adequate hole concentration (1020cm-3) are promising RT ferromagnetic materials, numerous reports have demostrate ferromagnetic ordering in those materials. However, its origin is still under a lively controversy. The recent “impurity band model” in which bound magnetic polarons cooperate with intrinsic or extrinsic defects to help stabilize ferromagnetic ordering in DMSs has encouraged many researchers to continue their study on wide bandgap ferromagnetic semiconductors. In this study, the most promising wide bandgap ferromagnetic semiconductors, ZnO thin films doped with Fe or Co, were fabricated using reactive RF-magnetron sputtering techniques and the relationship between ferromagnetic ordering and microstructure was carefully ascertained. Moreover, a feasible and effective method of growing thin films is proposed, which has never been employed to fabricate DMSs. Ferromagnetism in Fe-doped ZnO films fabricated by reactive RF magnetron sputtering is strongly influences by intrinsic defects, oxygen vacancies, and Fe content. ZnCoO films showed n-type characteristics, and saturation magnetization that increased as carrier concentration increased. This is clear evidence of RT ferromagnetic ordering which is stabilized in part by electrons originating from oxygen vacancies. High-quality Zn1−
xCoxO (ZnCoO) thin-films with different Co concentrations were synthesized on Al2O3 (0001) substrates using a reactive radio-frequency magnetron sputtering. High-resolution X-ray diffraction measurements showed that the Zn1−
xCoxO (x ● 0.07) films had a hexagonal wurzite structure with no extra phase. The lattice constants of Co-doped ZnO thin-films were slightly greater than in undoped ZnO films. Extended X-ray absorption fine structure revealed that Co atoms were randomly substituted for the Zn sites in the films. X-ray photoelectron spectra show incorporation of Co2+ ions inside the ZnO lattice without changing the wurtzite structure. Magnetic measurement reveals that the ZnCoO thin-films are ferromagnetic. The structural and magnetic properties strongly suggest that ZnCoO thin-films can be used in spintronic devices.
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