Local Structure Investigation of Cobalt and Manganese Doped ZnO Nanocrystals and Its Correlation with Magnetic Properties

Abstract

Co and Mn doped ZnO nanocrystals have been synthesized by two different routes, viz., a wet chemical method and a microwave-assisted nonaqueous method and it has been found that the samples prepared by the former method are ferromagnetic while those prepared through the later route are paramagnetic. Systematic investigation of these doped ZnO nanocrystals has been carried out by extended X-ray absorption fine structure technique to determine the changes in the local structure at the Zn and dopant sites. Co doped samples prepared by either of the techniques show almost similar behavior, with Co substituting Zn up to a 10% doping concentration, beyond which there is a signature of Co clustering. However, in the case of Mn doped samples, Mn clustering commences at lower values of doping (∼7%) for samples prepared by microwave-assisted method, while for nanocrystals prepared by the wet chemical method, Mn−K edge X-ray absorption near edge spectroscopy measurement reveals the presence of a Mn 2 O 3 phase at lower concentration and clustering at higher concentration (>10%). These findings were supported by the results of optical and magnetic measurements on the samples. The experimental results have been further corroborated by first principle calculations. The findings suggest that the origin of ferromagnetic properties in Co doped ZnO nanoparticles prepared by the wet chemical method may be a consequence of surface modification related to the preparation process and not related to bulk properties. The ferromagnetism in Mn doped samples prepared by the wet chemical method can be explained by the presence of a secondary Mn 2 O 3 phase. The Mn doped samples prepared by the microwave-assisted method, on the other hand, manifest paramagnetism with a signature of antiferromagnetic interaction due to Mn clustering at relatively lower Mn concentration.

In Situ XAS Study on Growth of PVP‐Stabilized Cu Nanoparticles

“…The final spectrum recorded at 3600 s corresponds to a Cu−Cu bond distance of 2.51 Å with coordination number of 9 which also agrees with crystallographic parameters of fcc Cu. It should be noted that the coordination number is less than its bulk value of 12 due to nanoparticle nature of the samples . The intermediate species as predicted by MCR‐ALS analysis gives a Cu−O bond at 1.98 Å with coordination number of 2.4.…”

X-ray absorption spectroscopy of Mn doped ZnO thin films prepared by rf sputtering technique

“…The bond length obtained is in good agreement with other studies. 35 The second peak is fitted with single Zn-Zn shell with coordination number of 12. As can be seen from Table I, for Zn K edge data, though there is no change in coordination number and bond length with increase in Mn doping concentration, σ 2 value is found to increase for Zn-Zn paths manifesting substitution of Mn ions at Zn sites.…”

“…At lower Mn concentration, ferromagnetic interaction dominates which is attributed to indirect interaction among Mn ions through defects (oxygen vacancies) or in other words due to the overlap of unoccupied Mn 3d states with impurity band of oxygen vacancies. 42 However, as Mn concentration increases, Mn clustering takes place which gives rise to AFM ordering 35 and hence the net magnetization of the sample decreases. To further probe the role of oxygen vacancies in determining the magnetic properties of the present thin films, a 4% Mn doped ZnO film has been deposited in a mixed Ar +O 2 (50%+50%) ambient instead of in pure Ar ambient.…”

Investigations on absorption, photoluminescence and magnetic properties of ZnO: Co nanoparticles

“…The experimental results of magnetic property in Codoped ZnO are quite controversial, giving rise to ferromagnetism or paramagnetic behaviour depending upon the preparation condition [8,14]. Ferromagnetism in Co-doped ZnO has been reported by different authors [15,16].…”