Effect of annealing on the structural and magnetic properties of sol-gel synthesized Co doped ZnO

Nanocrystalline powders of undoped ZnO and Co (5wt%) doped ZnO are synthesized successfully by employing soft chemical route. The structural, vibrational and magnetic properties as a function of calcination temperatures (400-1100 °C) are studied by using various characterization techniques. The X-ray diffraction study has clearly revealed the evolution of the major wurtzite phase (ZnO) in addition to the secondary nanocrystalline cubic spinel phase of Co
₃
O
₄
is observed at 800 °C which grows upto 1100 °C. FTIR study supports the existence of the metal cation-oxygen bonding due to secondary cubic spinel Co
₃
O
₄
phase. At lower calcination temperature (800 °C), predominant paramagnetic behavior is demonstrated, though secondary cubic spinel phase of Zn
_x
Co
_3-x
O
₄
is present. With the increase in calcination temperature (1100 °), an enhanced paramagnetic behavior is observed as paramagnetic bulk cubic spinel Zn
_1-x
Co
_3-x
O
₄
phase effectively increases the effective magnetic moment. The occurrence of paramagnetic behavior with the increase in annealing temperature is understood on the basis of inverted cubic spinel, exchange interactions and occupancy of cobalt ions in both tetrahedral and octahedral sites. With the presence of the non-magnetic secondary nanocrystalline and bulk Zn
_x
Co
_3-x
O
₄
spinel phase, Co (5wt%) doped ZnO cannot be used as diluted magnetic semiconductor based spintronics devices.