Room temperature ferromagnetism in finite sized ZnO nanoparticles Bhagaban Kisan, Perumal Alagarsamy n Department of Physics, Indian Institute of Technology Guwahati, Guwahati 781039, India article info Keywords: Nanoparticles Mechanical alloying Paramagnetism Curie temperature Ferromagnetism Defect density abstract We report systematic investigations on the evolution of nanostructured ZnO, room temperature ferromagnetism, and tunable optical properties of ZnO nanoparticles prepared by a mechanical alloying process. It was observed that both un-milled and as-milled powders exhibited a wurtzite structure, but average crystallite size decreased and the effective strain increased for the initial periods of milling. Paramagnetic nature observed in un-milled ZnO gradually unveils room temperature ferromagnetic ordering with modest moment and coercivity. A maximum moment of 0.013m B /f.u. at 12 kOe applied field and a coercivity of 172 Oe were obtained for 40 h milled ZnO powder. Thermo-magnetization data reveal a clear magnetic phase transition from ferromagnetic to paramagnetic state around 500 1C, which shifts slightly towards higher temperature with an increasing milling period up to 20 h. Annealing of as- milled ZnO powder and UV–vis studies display a drastic reduction in room temperature magnetic moment and blue-shifting of excitonic absorption peak. The observed ferromagnetic properties are intrinsic and discussed on the basis of finite size effect, defect density due to oxygen vacancy. These nanoparticles with tunable magnetic and optical properties are promising to find applications in multifunctional spintronic and photonic devices. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Wide band gap diluted magnetic semiconductors (DMS) have gained considerable attention recently as a promising route to realize semiconductor based spintronics. A successful fabrication of ferromagnetic (FM) DMS can lead to a development of multi- functional spintronic devices. Hence, the development of DMS based on fine transition metal (TM) based oxide nanoparticles (NPs) with Curie temperature (T C ) above room temperature is considered to be one of the key approaches for future spintronic devices [1]. Sharma et al. [2] reported the first experimental evidence of showing room temperature ferromagnetism (RTF) on Mn doped ZnO processed at temperatures around 700 1C. Simi- larly, large scale RTF oxide materials (HfO 2 , MgO, TiO 2 and SnO 2 ) doped with TM [3,4] have been reported. However, besides the issues of stability and reproducibility, it is still controversial whether the RTF is intrinsic or due to extrinsic origins such as the formation of magnetic clusters and precipitation of impurity phases by TM dopants in the oxides. It may be noted that such materials are not suitable for the application of DMS. On the other hand, the recent studies on the ZnO system revealed the realiza- tion of RTF in either pure nanostructured ZnO without any additional TM doping or ZnO doped with non-magnetic ions such as H, C, N, and Mg [5,6], similar to the one observed in TiO 2 , SnO 2 and MgO, which are defined as d 0 FM [7]. However, a careful analysis of the literature reveals contradictory results on the development of RTF in the pure ZnO system. Bartolomé et al. [8] showed that the preparation of ZnO based particles using a ball milling technique resulted in a drastic reduction in the average size of the crystals without any induction of ferromagnetic proper- ties. In addition, Sanyal et al. [9] reported that ball milling processed ZnO did not show induced FM properties despite the creation of Zn vacancy during the ball milling process. On the other hand, Potzger et al. [10] reported an easy mechanical way to create ferromagnetic defective ZnO and correlated to flake like structures in planar compressed pieces of the powder. Xing et al. [11] suggested that the oxygen vacancies induce characteristic photoluminescence and boost the room temperature FM in ZnO nanowires synthesized using a vapor transport method. Further- more, Podila et al. [12] reported the formation of RTF in ZnO films depending on the sample preparation and annealing conditions. This was also supported with ab-initio calculations exploring the role of surface defects on the magnetic behavior of nanoscale ZnO. Banerjee et al. [13] reported the enhancement of FM in pure ZnO powder upon thermal annealing due to the formation of oxygen vacancy clusters. Very recently, Phan et al. [14] and Ghose et al. [15] proposed that mechanical milling can be used to produce induced defect-related ferromagnetism in ZnO nanoparticles from initial diamagnetic ZnO powders. These studies revealed that the development of FM in un-doped ZnO was attributed to the defect Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B http://dx.doi.org/10.1016/j.physb.2014.02.040 0921-4526 & 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ91 361 258 2714; fax: þ91 361 2690762. E-mail address: perumal@iitg.ernet.in (P. Alagarsamy). Please cite this article as: B. Kisan, P. Alagarsamy, Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.02.040i Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎