Vacancy-induced magnetism in ZnO thin films and nanowires

Abstract

Extensive calculations based on density functional theory have been carried out to understand the origin of magnetism in undoped ZnO thin films as found in recent experiments. The observed magnetism is confirmed to be due to Zn, instead of O, vacancy. The main source of the magnetic moment, however, arises from the unpaired 2p electrons at O sites surrounding the Zn vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.490 to 0.740 B. Moreover, the study of vacancy-vacancy interactions indicates that in the ground state, the magnetic moments induced by Zn vacancies prefer to ferromagnetically couple with the antiferromagnetic state lying 44 meV higher in energy. Since this is larger than the thermal energy at room temperature, the ferromagnetic state can be stable against thermal fluctuations. Calculations and discussions are also extended to ZnO nanowires that have larger surface to volume ratio. Here, the Zn vacancies are found to lead to the ferromagnetic state too. The present theoretical study not only demonstrates that ZnO samples can be magnetic even without transition-metal doping but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO nanostructures. In addition, vacancy mediated magnetic ZnO nanostructures may have certain advantages over transition-metal doped systems in biomedical applications.

Magnetic properties of Er-doped ZnO films prepared by reactive magnetron sputtering

“…So, it is reasonable to conclude that abundant zinc vacancies and oxygen vacancies may exist in the Er-doped ZnO films deposited at the O 2 flow rates of 35 sccm and 20 sccm, respectively. Our experimental results may be consistent with bound-magnetic-polaron (BMP) model proposed by Coey et al [22], since both zinc and oxygen vacancies favor the formation of FM in DMS [23,24] ions. The estimated magnetic moment per Er atom decreases with the erbium concentration increase.…”

Experimental and theoretical studies of hydroxyl-induced magnetism in TiO nanoclusters

“…3 Similar DFT calculations for ZnO thin lms and nanowires indicate that while O vacancies do not generate any magnetic moments, Zn vacancies give rise to magnetic moments that arise from the O 2p orbitals of neighboring O. 4 Both studies showcase the complex correlation among defect creation, local structural relaxation, charge redistribution, and magnetism. Another feature of the defect ferromagnetism is the preferential location of the vacancies at the surface of the nanostructures and these surface vacancies dominate the magnetic moment while interior vacancies give negligible contributions.…”

“…For ZnO nanowires, the moment is located on neighboring O sites and arises from O 2p orbitals. 4 All share a complicated correlation among defect structure, structural relaxation, charge delocalization, and magnetism. (ii) The moment is predominantly located at the surface of the nanostructures, which involves the rst few monolayers where the contribution dwindles quickly from the surface to the interior.…”

“…Also the defect formation energy is reduced as the dimension of the sample is reduced from two-dimensional thin lms to one-dimensional nanowires. 4 Thus nanostructures favor the defect ferromagnetism because of the higher surface area and defect concentration. The fact that oxygen vacancy (V o ) does not generate magnetic moments in ZnO can be attributed to the unique electronic structure of ZnO.…”

d0Ferromagnetism in Oxide Nanowires: Role of Intrinsic Defects

“…Recently, room temperature ferromagnetism (RTFM) has been reported in the nanostructures of wide band gap pristine and different non-magnetic element doped oxide semiconductors like ZnO, HfO 2 , CeO 2 , SnO 2 , In 2 O 3 Al 2 O 3 and TiO 2 [2][3][4][5][6][7][8]. However the origin of such unexpected FM in different oxides is found to be controversial issue [2][3][4][5][6][7][8].…”