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We report here on the magnetic properties of ZnO:Mn-and ZnO:Co-doped nanoparticles. We have found that the ferromagnetism of ZnO:Mn can be switched on and off by consecutive low-temperature annealings in O 2 and N 2 , respectively, while the opposite phenomenology was observed for ZnO:Co. These results suggest that different defects ͑presumably n-type for ZnO:Co and p-type for ZnO:Mn͒ are required to induce a ferromagnetic coupling in each case. We will argue that ferromagnetism is likely to be restricted to a very thin, nanometric layer at the grain surface. These findings reveal and give insight into the dramatic relevance of surface effects to the occurrence of ferromagnetism in ZnO-doped oxides.

Mechanical properties of the orthorhombic phase of YBa2Cu3O7−δ (Y123) at room temperature have been investigated at different applied loads: 5, 10, 30 and 100 mN using a nanoindentation technique. This study was carried out for different monodomains on the (001) plane for textured Bridgman samples with dispersed particles of Y211 as pinning centres. Hardness and Young’s modulus values were calculated using the Oliver–Pharr approach. Nanohardness and Young’s modulus for the Y123, Y2BaCuO5 (Y211) and Y 123/Y 211composite (YBCO) were determined using an applied load of 5 and 10 mN. For higher loads, it is not possible to differentiate the matrix (Y123) from the precipitates (Y211). In this case, only hardness and Young’s modulus of the YBCO composite can be determined. Finally, the ultra-low load imprints obtained by nanoindentation have been correlated with parameters obtained by atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM).

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