Manipulating coupling state and magnetism of Mn-doped ZnO nanocrystals by changing the coordination environment of Mn via hydrogen annealing

Mn-doped ZnO nanocrystals are synthesized by a wet chemical route and treated in H_2/Ar atmosphere with different H_2/Ar ratios.It is found that hydrogen annealing could change the coordination environment of Mn in ZnO lattice and manipulate the magnetic properties of Mn-doped ZnO.Mn ions initially enter into interstitial sites and a Mn~(3+)O_6 octahedral coordination is produced in the prepared Mn-doped ZnO sample,in which the nearest neighbor Mn~(3+) and O~2 ions could form a Mn~(3+)-O~(2-)-Mn~(3+) complex.After H_2 annealing,interstitial Mn ions can substitute for Zn to generate the Mn~(2+)O_4tetrahedral coordination in the nanocrystals,in which neighboring Mn~(2+) ions and H atoms could form a Mn~(2+)-O~(2-)-Mn~(2+)complex and Mn-H-Mn bridge structure.The magnetic measurement of the as-prepared sample shows room temperature paramagnetic behavior due to the Mn~(3+)-O~(2-)-Mn~(3+) complex,while the annealed samples exhibit their ferromagnetism,which originates from the Mn-H-Mn bridge structure and the Mn-Mn exchange interaction in the Mn~(2+)-O~(2-)-Mn~(2+)complex.