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 H₂/Ar atmosphere with different H₂/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 Mn³⁺O₆ octahedral coordination is produced in the prepared Mn-doped ZnO sample, in which the nearest neighbor Mn³⁺ and O² ions could form a Mn³⁺-O^2--Mn³⁺ complex. After H₂ annealing, interstitial Mn ions can substitute for Zn to generate the Mn²⁺O₄ tetrahedral coordination in the nanocrystals, in which neighboring Mn²⁺ ions and H atoms could form a Mn²⁺-O^2--Mn²⁺ complex and Mn-H-Mn bridge structure. The magnetic measurement of the as-prepared sample shows room temperature paramagnetic behavior due to the Mn³⁺-O^2--Mn³⁺ 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 Mn²⁺-O^2--Mn²⁺ complex.