On the sol–gel synthesis and thermal, structural, and magnetic studies of transition metal (Ni, Co, Mn) containing ZnO powders

Abstract

Transition metal containing ZnO powders (Zn1−xMxO, 0≤x≤0.30; M = Ni, Mn, Co) have been synthesized by a sol–gel process using zinc acetate dihydrate, respective acetate and oxalic acid as precursors with ethanol as a solvent. The process essentially involves gel formation, drying at 80 °C for 24 h to provide the oxalate, and calcination at 500 °C for 2 h to undergo an exothermic reaction and yield Zn1−xMxO powder. Their XRD patterns correspond to a wurtzite hcp structure similar to that of pure ZnO, but with the lattice parameters varying slightly with type and extent of doping. It is shown that the dissolution of nickel and cobalt in ZnO is less than 10 at.%, whereas that of manganese lies between 10 and 15 at.%. Other phases that emerge include NiO (hexagonal, a = 2.954 A, c = 7.236 A), ZnCo2O4 (cubic, a = 8.094 A) and ZnMnO3 (cubic, a = 8.35 A) in the Ni, Co and Mn containing ZnO systems, respectively. Observations of hysteresis loops both at 10 and 320 K and the nature of ESR spectra provide evidence for the ferromagnetic state in nickel containing ZnO powder. Besides, the deviation occurs in the magnetization versus temperature curves in zero field cooled (ZFC) and field cooled (FC) conditions (blocking temperature TB being 32 K for 5 at.% Ni). The magnetic behaviour of manganese and cobalt doped zinc oxide is, however, different, namely, (i) no hysteresis loops, (ii) decrease in magnetization with increase of Mn or Co content, and (iii) identical M–T curves under ZFC and FC conditions. The inverse susceptibility versus temperature curves of Zn1−xMnxO compounds reveal ferrimagnetism with Neel temperature TN of 4 K for x = 0.02, but antiferromagnetism for x = 0.15 and 0.25 with Curie–Weiss temperature of −43 and −30 K, respectively.