Superparamagnetic behavior in chemically synthesized nanocrystalline Zn 0.99 Ni 0.01 O powders Jyoshnarani Mohapatra a , D.K. Mishra b, ⁎, S.K. Singh b a Department of Physics, Institute of Technical Education and Research, Siksha ‘O’ Anusandhana University, Bhubaneswar 751 030, India b Advanced Materials Technology Department, Institute of Minerals and Materials Technology (CSIR), Bhubaneswar 751 013, India abstract article info Article history: Received 18 October 2011 Accepted 2 February 2012 Available online 9 February 2012 Keywords: Semiconductor Nanocrystalline materials Raman Defects Magnetic materials Hexagonal shaped Zn 0.99 Ni 0.01 O nanoparticles having an average size of ~ 50 nm were synthesized by auto- combustion technique. The incorporation of Ni in the Zn site is clearly reflected from the (101) major X-ray diffraction peak shift. The presence/development of oxygen vacancies are evidenced from the micro-Raman spectrum. The M–H curve of Zn 0.99 Ni 0.01 O shows superparamagnetic behavior at room temperature which is well proven from the decreasing order of zero field cooled magnetization with the decrease in tem- perature. The simultaneous competition between the oxygen vacancies induced ferromagnetism and the antiferromagnetic interactions between Ni ions gives rise to the superparamagnetic state in nanocrystalline Ni doped ZnO. © 2012 Elsevier B.V. All rights reserved. 1. Introduction There is a lot of possibility for spin-manipulation in addition to the charge if room temperature ferromagnetism can be practically achieved in dilute magnetic semiconductor (DMS). This will open up new functionalities in the devices made out of DMS. The problem is still open in research as there has been no clear agreement in ex- perimental reports, especially with ZnO [1–3]. The surprising feature is the variation in the observation of magnetic behavior depending on the synthesis technique. Earlier, a lot of attention has been given to Mn doped ZnO. The room temperature ferromagnetism does not occur in nanocrystalline Mn doped ZnO [4]. Similarly, no ferromagne- tism has been observed in Mn (> 0.02) over doped ZnO bulk polycrys- talline samples [5] due to the appearance of secondary phases like Mn 3 O 4 which is ferrimagnetic at a lower temperature of 41 K [6]. In- corporation of a dopant having much higher and smaller cationic radii is impossible to control in nanocrystalline ZnO. Hence it is necessary to choose a dopant atom having atomic radii nearly equal to the host atom. Hence Ni has been chosen as the dopant having atomic radii 0.69 Å, smaller than the atomic radii of Zn 0.74 Å. The room tem- perature superparamagnetic behavior has been observed in nano- crystalline Zn 0.99 Ni 0.01 O. The simultaneous competition between the oxygen vacancies induced ferromagnetism and the antiferromagnetic interaction between Ni ions gives rise to the superparamagnetic state. 2. Experimental detail Appropriate proportion of high pure nitrates of Zn and Ni (Pura- term-99.999%) were used for the synthesis of nanocrystalline ZnO and Zn 0.99 Ni 0.01 O powders by autocombustion technique. The as syn- thesized powders were calcined at 650 °C for 1 h to make carbon free. X-ray diffraction (XRD) and micro-Raman characterizations were car- ried out for structural and defect analysis. The average particle sizes were measured using transmission electron microscopy. Magnetic measurements of the samples were carried out using superconduct- ing quantum interference device (SQUID) magnetometer. 3. Results and discussions The XRD patterns of nanocrystalline ZnO and Zn 0.99 Ni 0.01 O are shown in Fig. 1. No evidence of impurity phases is detected within the limit of X-ray diffractometer. The lattice parameters ‘a’ and ‘c’ de- termined using Rietveld refinement for ZnO are 3.253 and 5.210 re- spectively whereas for Ni doped ZnO, the values of ‘a’ and ‘c’ are 3.258 and 5.218 respectively. There is not much variation in the lat- tice parameter and c/a ratio which indicates the complete incorpora- tion of Ni into the ZnO matrix. It is known that tetrahedral Ni ions have lower ionic radii of 0.69 Å as compared to the tetrahedral Zn ions (0.74 Å). Substituting of Ni ion in the Zn site will show the XRD peak shift towards the higher 2θ angle. However the reduction of crystallite size in Ni doped ZnO puts two times more strain in com- parison to the nanocrystalline ZnO, which is the main reason for showing the peak shift towards the lower 2θ angle. The crystallite size and strain calculations have been carried out using Williamson– Hall equation are shown in Fig. 2. The crystallite size calculated for Materials Letters 75 (2012) 91–94 ⁎ Corresponding author. Tel.: + 91 674 2379456; fax: + 91 674 2581637. E-mail address: dilipiuac@gmail.com (D.K. Mishra). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.02.005 Contents lists available at SciVerse ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet