Microscopic picture of Co clustering in ZnO

Abstract

Density functional theory was applied to study the chemical and magnetic interactions between Co atoms doped in ZnO. It was found that the Co impurities tend to form nanoclusters and the interactions between these atoms are antiferromagnetic within the local spin-density approximation ͑LSDA͒ + Hubbard U approach. The extracted interatomic exchange parameters agree reasonably well with recent experimental results. We have analyzed and compared the electronic structure obtained using the LSDA and LSDA+ U approaches and found that the LSDA+ U gives the most reasonable result, highlighting the importance of short-ranged antiferromagnetic interactions due to superexchange.

Determinant influence of surfaces on the Co clustering trend at ZnO

“…Also, we have taken into account the effect of the magnetic exchange energy, exploring ferro-and antiferro-magnetic orderings in all structures modelled. Its value depends on the specific surface under study, but generally lies between 0.03 eV/Co (for neighbouring isolated impurities) and 0.08 eV/Co (for 2D aggregates), in good agreement with previous estimates when available, 6 formation of Co clusters. All results presented here correspond to the ground state magnetic configuration.…”

“…5 Moreover, a clear tendency of substitutional Co to aggregate in bulk ZnO has been found. 6,7 However, little is known about the influence of surfaces or boundaries in the distribution of Co impurities. This is in spite FM in DMO seems to be largely favoured in low dimensional structures and close to interfaces.…”

Study of the Structural Characteristics of 3d Metals Cr, Mn, Fe, Co, Ni, and Cu Implanted in ZnO and TiO2—Experiment and Theory

“…The tendency of Co clustering in dilute magnetic oxides has already been observed in TiO 2 :Co, 4 in this study it was found that the Co atoms prefer to be in a "close" geometry, and thus form metallic clusters. 29,30 In contrast to this, Co in ZnO shows no sign of surface clustering and very closely resembles the Co 2+ spectrum of Co(OH) 2 .…”

Giant stability of substituent Co chains in ZnO:Co dilute magnetic oxides

“…9,10 Furthermore, this tendency was also suggested by a detailed density functional study of the chemical and magnetic interactions between substitutional Co ions in ZnO. 11 Therefore, a universal substitutional clustering via oxygen seems to apply to all these DMO. Since Co dimers in ZnO are known to couple antiferromagnetically via O-mediated superexchange in contrast to paramagnetic isolated Co monomers, the substitutional Co clustering in ZnO can dominate its magnetic properties.…”

“…This finding is in agreement with the theoretical calculation results of Co-doped ZnO with a smaller Co doping level of 5.6% using local spin-density approximation. 11 Furthermore, the tendency towards substitutional clustering was evidenced experimentally by x-ray absorption fine structure (XAFS) in Co-doped ZnO with low Co concentrations of 0.5% and 2%.…”