Intrinsic Room-Temperature Ferromagnetic Properties of Ni-Doped ZnO Thin Films
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magnetic semiconductors (DMSs) are being considered for use in next generation spintronic devices.[1–3] In the past, (III,Mn)V semiconductors, particularly Mn-doped GaAs and InAs, were widely investigated as DMS materials.[3–6] In recent years, there has been an increasing amount of research focused on the development of zinc oxide (ZnO) as a DMS material. This trend is motivated by the fact that zinc oxide is abundant in nature and is considered to be an environmentally benign material. In addition, synthesis of ZnO is more straightforward than that of III-V semiconductors. Room temperature ferromagnetism was reported in ZnO doped with transition elements (e.g., Fe, Mn, Co, and Ni).[7–9] However, previous reports on room temperature ferromagnetism in doped ZnO films have not been consistent. Observation of room temperature ferromagnetism was shown to be dependent on processing and postprocessing details.[7,8] Intrinsic ferromagnetism is an important consideration for the use of DMS materials in spintronic devices. Although many studies suggest the intrinsic nature of ferromagnetic behavior in ZnO-based DMSs, other reports attribute ferromagnetism in ZnO to clustering C. JIN, Research Associate and R.J. NARAYAN, Professor, are with the Joint Department of Biomedical Engineering, University of North Carolina, Chapel Hill, NC 27599. Contact e-mail: roger_ [email protected] R. AGGARWAL, Process Engineer, formerly with the Department of Materials Science and Engineering, North Carolina State University, is now with Intel, Hillsboro, OR 97124. W. WEI, Postdoctoral Researcher, formerly with the Department of Materials Science and Engineering, North Carolina State University, is now with Case Western Reserve University, Cleveland, OH 44106. S. NORI, Postdoctoral Researcher and J. NARAYAN, John C. Fan Distinguished Chair Professor, are with the Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695. D. PONARIN, Postdoctoral Researcher and A.I. SMIRNOV, Professor, are with the Department of Chemistry, North Carolina State University, and D. KUMAR, Associate Professor, is with the Mechanical and Chemical Engineering Department, North Carolina A&T State University, Greensboro, NC 27411. Manuscript submitted March 13, 2010. Article published online December 9, 2010 3250—VOLUME 42A, NOVEMBER 2011
of secondary phases.[10–13] In spite of this controversy over the origin of ferromagnetism in ZnO, there are only a limited number of reports in the literature in which observation of intrinsic or extrinsic ferromagnetism in ZnO was supported with transmission electron microscopy (TEM) results to rule out or demonstrate precipitation of secondary phases.[14] In addition to experimental results, the mechanisms proposed for ferromagnetism are not consistent. Since the magnetic doping concentration is far below the percolation threshold associated with nearest-neighbor coupling, carrier-mediated ferromagnetism is widely accepted for DMSs.[15,16] Dietal et al. proposed that high Curie temperature
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