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(Color online) (a) Channeling RBS spectra for the as-implanted and postannealed ZnO single crystals. Preimplantation annealing is indicated. The value for every spectrum is given in %. (b) Exemplary random spectra for the nonpreannealed crystal for different postannealings. Fe is visible in the random spectra as a bump. All spectra have been shifted in direction for better clarity.
CEM spectra and least squares fits with Lorentzian lines (see Ref. 15 ) of the Fe implanted and postannealed ZnO single crystals. (a) Nonpreannealed crystal with pronounced magnetic hyperfine field corresponding to . The ratio between and is 48%:52%, respectively. [(b) and (c)] CEM spectra of the preannealed samples. The ratio between the valence states is indicated.
Fe XAS of the nonpreannealed sample (top) and the two preannealed samples (third and fifth from the top) after implantation and postannealing. Several measurements on reference compounds, namely, Fe metal, , , and are also shown for comparison (see Refs. 16 and 18 ). These spectra allow a qualitative determination of the Fe charge states in the ZnO samples. Please note that only the nonpreannealed sample shows pronounced contributions from metallic Fe.
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 in zero field and subsequently annealing it back to in field. The FC curves were obtained during subsequent cooling of the sample down to in a field. For the insets, the diamagnetic background was subtracted. (a) Nonpreannealed sample exhibiting 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 .
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