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Local atomic structure investigated by X-ray absorption fine structure in Co-doped ZnO diluted magnetic semiconductors prepared by pulsed laser deposition and by melt spinning: comparative study
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Teodorescu, Cristian-Mihail1; Macovei, Dan1; Neamtu, Jenica1; Malaeru, Teodora1; Georgescu, Gabriela1; Socol, Gabriel1; Axente, Emanuel1; Ristoscu, Carmen1; Cristescu, Rodica1; Mihailescu, Ion Nicolae1; Pivin, Jean-Claude2; Mougin, Alexandra2; Ferre, Jacques2
1Romania;
2France |
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We report X-ray absorption fine structure (EXAFS and XANES) measurements at the Co and Zn K-edges on Co-doped ZnO thin films prepared by pulsed laser deposition and by melt spinning. Films prepared by pulsed laser deposition exhibited complete substitution of Zn(2+) ions by Co(2+) ions, with EXAFS functions practically the same at the Co and Zn edges. Also, the pre-edge peak, which is a sign of dipole-forbidden 1s → 3d transition, has similar amplitude with respect to the absorption jump at the Co and Zn edges, indicating the same occupancy of Co 3d band, resulting in the same ionization state of Co as of Zn. In case of samples prepared by melt spinning, the Co K-edge EXAFS function was closer to that of Co in Co(II) oxide, whereas the pre-edge peak showed a decrease in intensity, indicating that the ionization state of Co is sensibly lower than (+2). At the same time, in these samples more refined analysis of the Co K-edge EXAFS evidenced deviations of the number of neighbours with respect to stoichiometric CoO, which - in line with the observed decrease in the pre-edge peak's integral amplitude - allowed one to infer the formation of non-stoichiometric cobalt oxide aggregates with oxygen deficit. The magnetic properties observed for these samples can be correlated with the observed local atomic neighboring of Co atoms: perfectly Co-substituted zinc oxides are ferromagnetic at low temperature (LT = 4 K) and superparamagnetic at room temperature (RT = 300 K), Co-substituted zinc oxides with a weak oxygen deficit are ferromagnetic both at LT and at RT, whereas samples formed by non-stoichiometric Co oxide with considerable oxygen deficit are superparamagnetic both at LT and RT. A weak density of oxygen defects make them acting as acceptor impurities and providing charge carriers which intermediate ferromagnetic ordering between Co atoms, whereas the density increase of these defects together with formation of Co oxide nanoclusters destroy the magnetic interaction between magnetic entities. Nevertheless, the oxygen deficit in case of embedded cobalt oxide clusters contribute to the stabilization of parallel spin orientation of Co inside each nanocluster, while bulk Co(II) oxide is antiferromagnetic. |