Effect of calcination on structural and magnetic properties of Co doped ZnO nanostructur

Abstract

Diluted magnetic semiconductors have gained much interest in spintronics due to the involvement of both spin and charge [1]. Practical devices require simultaneous control on spin state and charge of the electrons in semiconductors having room temperature ferromagnetism [2]. Among various diluted magnetic semiconductors ZnO lightly doped with transition metals (Mn, Fe, Co, Ni, In, V, and Cr) [1, 2] have gain much interest in spintronics due to the presence of room temperature fer-romagnetism. Amongst various dopants, cobalt is a potential candidate as its ionic radius (Co2+=0.58A) is extremely close to zinc (Zn2+=0.6A) [4]. We here report high value of room temperature ferromagnetism in cobalt doped zinc oxide nanostructures prepared by sol-gel method even under as-synthesized conditions. Zinc acetate dihydrate and cobalt nitrate are used as precursor materials. ZnO sol is synthesized by previously optimized route [3]. Nanostructures are synthesized at low temperature of 70C. The dopant concentration is varied as 1, 3, 5, 7 and 9wt. %. XRD results [Fig. 1] indicate the formation of hexagonal wurtzite structure even under as-synthesized conditions. Crystallite size decreases from 22.7nm to 17.8nm as dopant concentration was increased from 0–7wt%. XRD of the samples with 7wt% Co concentration is calcined in the temperature range of 100–500°C. Presence of IR bands at 475cm−1 corresponds to Zn-O stretching mode. High value of room temperature ferromagnetic behavior is observed in cobalt doped ZnO nanostructures even under as-synthesized condition as shown in Fig. 2(a). Saturation magnetization increases from 5.07emu/g to 6.4emu/g as dopant concentration was increased to 7wt%. Whereas, Fig. 2(b) shows a variation in magnetic behavior for nanostructures prepared with 7wt% Co concentration and calcined at various temperatures. Ferromagnetic behavior in cobalt doped ZnO nanoparticle arises due to exchange interaction arises between delocalized carriers present in ZnO lattice and localized d spins of Co2+ ions [5-6]. Nanostructures calcined at 400C showed highest saturation magnetization of 9.9emu/g. SEM images show nanostructures with grain size less than 40nm.