Reversible ferromagnetic switching in ZnO:(Co, Mn) powders

Abstract

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.

Room-Temperature Ferromagnetism in Zn-Mn-O, X-Ray Photoemission Surface Study

“…We believe that ferromagnetic samples have more O terminated surfaces that are not suitable for carbonate growth. This is in accordance with the fact that annealing in oxygen atmosphere switches on the ferromagnetism, while annealing in nitrogen switches it off [9]. These findings are also in accordance with our discussion about O 1s spectrum.…”

Defects induced ferromagnetism in Mn doped ZnO

“…Reports in favor of Zn vacancy (V Zn ) [8,16], O vacancy (V O ) [17,18] or Zn interstitial (I Zn ) [19] mediated ferromagnetism can be found. It is also found that the defect species involved in inducing ferromagnetism in one ZnO based DMS system may not be effective in similar others [4,7,20]. The site occupied by the dopant atom [16], its size, electronic configuration and defect decoration [9], lattice strain [21], band gap [5] etc.…”

Microscopic picture of Co clustering in ZnO

“…Also, by successive annealings in O 2 atmosphere, the ferromagnetism of Co:ZnO nanoparticles could be correlated with the presence of defects. 5 Before any claims of intrinsic ferromagnetism can be made, an extremely careful structural analysis must be done to exclude the presence of secondary phases 6 or nanocrystals 7 that can account for the observed ferromagnetism. This issue becomes even more acute in DMSs with low concentrations of magnetic transition metals ͑typically ϳ5%͒.…”