Microstructural and magnetic properties of ZnO:TM (TM=Co,Mn) diluted magnetic semiconducting nanoparticles

We have investigated the structural and the magnetic properties of 3d transition metal (TM) doped Zn1−xTMxO (TM=Co,Mn) diluted magnetic semiconducting nanoparticles for different doping concentrations (0⩽x⩽0.4) synthesized by chemical “pyrophoric reaction process.” From x-ray diffraction measurements the solubility limits of Co and Mn in ZnO nanoparticles are found to be strongly dependent on growth (calcinations) temperature (Tg). The highest solubility limit of both Co2+ and Mn2+ in ZnO at Tg∼300°C is found to be ∼30%. High resolution transmission electron microscopy studies show that Zn1−xTMxO particles are single crystalline of high quality with a wide particle size distribution in nanometric regime. The non-mean-field-like very strong concave nature of temperature dependent magnetization curves is observed at very low temperature in both the systems without showing any distinct magnetic transition. The magnetic behaviors of those Mn2+ and Co2+ doped ZnO semiconducting nanoparticles are observed to be quite different. The magnitude of net magnetization at a field of 5000Oe for Zn1−xMnxO system is found to grow with the dopant concentration (x) in sharp contrast to the case for Zn1−xCoxO where it is found to decrease. From mean field the Curie-Weiss fit as well as from the calculated values of effective exchange interaction constants (Jex), which is found to be negative, we can assert that the ground states of both of these systems are antiferromagnetic for the entire series. In the case of Zn1−xMnxO samples the magnitude of Jex is found to decrease with the increase in Mn+2 ion concentration, whereas for Zn1−xCoxO samples the magnitude of Jex is found to increase. These typical variations of Jex with antiferromagnetic interaction have been best explained through the magnetic polaron-polaron interaction model [P. A. Wolf et al., J. Appl. Phys. 79, 5196 (1996)].