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ZFC-FC magnetization vs temperature measurements and magnetic hysteresis loops (insets) for all Fe implanted and postannealed ZnO single crystals [(a)–(c)]. The ZFC curves were obtained by cooling the sample from 300 down to 5 K in zero field and subsequently annealing it back to 300 K in 50 Oe field. The FC curves were obtained during subsequent cooling of the sample down to 5 K in a 50 Oe field. For the insets, the diamagnetic background was subtracted. (a) Nonpreannealed sample exhibiting α-Fe nanoparticles [(b) and (c)] and vacuum preannealed crystals after postannealing. (d) As-implanted vacuum preannealed crystal (for comparison). The latter three show a weak separation in the ZFC-FC and very low saturation moment in the hysteresis loops, as compared to (a). For (c) and (d), the thermomagnetic irreversibility temperature is above 250–300 K.
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