Thermal phase transformation and perpendicular exchange coupling of Co nanocrystals embedded in ZnO

Recently, due to the potential application in spintronics, ferromagnet-semiconductor hybrid structures have attracted huge research attention [1]. Large magnetoresistance was observed in GaAs consisting MaAs nanocrystals formed by ion implantation [2] and in granular ZnO/Co systems [3]. Very recently, wide-band-gap semiconductors (GaN, ZnO and TiO2) doped with transition metals were reported to be diluted magnetic semiconductors with Curie temperatures above room temperature [4]. However, the origin of the observed ferromagnetism is still controversial, e.g. ferromagnetic clusters, or extrinsic reasons. In this work, Co nanocrystals (NCs) were formed inside ZnO my means of ion beam synthesis. The Co NCs are crystallorgraphically oriented inside ZnO. The magnetic properties, e.g. anisotropy and blocking temperature can be tuned by annealing. In the as-implanted and annealed (823 K) samples, hcp-Co or co-existing of hcp and fcc Co NCs have been found and show superparamagnetism. After annealing at 923 K, the sample exhibits a much higher blocking temperature and shows a perpendicular exchange bias effect, which indicates the formation of a Co/CoO core/shell structure. The system of ZnO semiconductor consisting of crystallorgraphically oriented Co NCs could be a promising hybrid for spin-injection.
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3. A. B. Pakhomov, et al., J. Appl. Phys. 95, 7393 (2004).
4. A. H. MacDonald, et al., Nature Materials 4, 195 (2005).