The role of cobalt doping on magnetic and optical properties of indium oxide nanostructured thin ﬁlm prepared by sol–gel method H. Baqiah a , N.B. Ibrahim a, *, S.A. Halim b , Moayad Husein Flaifel a , M.H. Abdi a a School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia b Superconductors and Thin ﬁlm laboratory, Department of Physics, Faculty of Science, University Putra Malaysia 43400 UPM Serdang, Selangor, Malaysia A R T I C L E I N F O Article history: Received 1 July 2014 Received in revised form 23 October 2014 Accepted 2 December 2014 Available online 4 December 2014 Keywords: Nanostructures Sol–gel chemistry Magnetic properties Thin ﬁlms Defects A B S T R A C T The effect of Co doping concentration, (x = 0.025–0.2), in In 2x Co x O 3 thin ﬁlm was investigated by X-rays diffraction (XRD), transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), Ultraviolet visible spectrophotometer (UV–vis) and vibrating sample magnetometer (VSM). All ﬁlms were prepared by sol–gel technique followed by spin coating process. The XRD and XPS measurements indicate that Co +2 has been successfully substituted in In +3 site. The TEM measurement shows nanostructure morphology of the ﬁlms. The doping of Co in indium oxide resulted in a decrease in the lattice parameters and grain size while the band gap increased with increasing Co concentration. Further, by comparing VSM and XPS results, the magnetic behaviour of the ﬁlms were found to be sensitive to Co concentrations, oxygen vacancies and ratio of oxygen defects to Co concentrations. The magnetic behaviour of the prepared ﬁlms was explained using bound magnetic polaron (BMP) model. ã 2014 Elsevier Ltd. All rights reserved. 1. Introduction Dilute magnetic semiconductors have received great attention in scientiﬁc community because of the possibility of using them in spintronic devices [1]. The research in this ﬁeld has been ignited after theoretical prediction of room temperature ferromagnetic behaviour (RTFM) in Mn doped ZnO as reported by Dietl et al. [2] and the discovery of ferromagnetic behaviour in Co doped TiO 2 found by Matsumoto et al. [2,3]. Several researches have reported on the existence of room temperature ferromagnetic behaviour in transitions metals doped TiO 2 , ZnO, SnO 2 and In 2 O 3 [4–9]. The origin of ferromagnetic behaviour in dilute magnetic oxides is still under debate. According to literature, two essential mechanisms were reported to explain RTFM behaviour in doped oxides, namely; carrier mediate mechanism and bound magnetic polaron [10,11]. Carrier mediate model is applicable to TM doped semiconductors with high carrier concentration [12]. On the other hand, bound magnetic polaron model can be used to explain the magnetic behaviour in high resistive systems [13]. The transparent conducting oxides (TCO) tend to form defects like oxygen vacancies which are associated with electrons that interact with TM ions to form polaron. In this model, two important parameters can determine the magnetic phase in these doped TCO systems, one of which is the effect of oxygen vacancies as studied by many researchers [6,14,15]. The second one is the increase in the TM dopant stepwise from low to high concentration. There are various researches that have reported on the ferromagnetic behaviour of Co doped In 2 O 3 thin ﬁlm which was prepared by laser ablation [16] and magnetron sputtering [17]. Laser ablation is considered a non-equilibrium method and requires expensive equipment while magnetron sputtering could lead to the formation of Co metals [17]. Stankiewicz et al. have observed the ferromagnetic properties in Co doped ITO ﬁlms prepared by magnetron sputtering at a deposition temperature of 200  C [17]. On the other hand, Khatoon et al. found that the Co doped In 2 O 3 nanoparticles prepared by solvothermal method exhibit paramagnetic behaviour with weak antiferromagnetic interaction [18]. To avoid the disadvantages of the above reported preparation techniques, sol–gel chemical method was chosen to prepare Co doped In 2 O 3 as it can produce highly homogeneous materials with ruling out the possibility of forming impurities that can be introduced by physical methods. In addition, it requires cheap apparatus and offers precise control of materials' composition [19]. Xiuqing Meng and Jingbo have reported on the existence of ferromagnetic property in Co doped In 2 O 3 nanocrystals for only one Co concentration. The samples were prepared using a sol–gel method. According to them, the origin of magnetic property was related to strong d–d coupling between the magnetic ions [20]. * Corresponding author. Tel.: +60 389215911; fax: +60 389213777. E-mail address: baayah@ukm.my (N.B. Ibrahim). http://dx.doi.org/10.1016/j.materresbull.2014.12.006 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved. Materials Research Bulletin 63 (2015) 147–154 Contents lists available at ScienceDirect Materials Research Bulletin journal homepage: www.else vie r.com/locat e/mat resbu