Hugo Bonette de Carvalho

The magnetization of Zn1-xCoxO (0.0055 < x < 0.073) nanoparticles has been measured as a function of temperature T (1.7 K < T , 10 K) and for magnetic field up to 65 kOe using a SQUID magnetometer. Samples were synthesized by three different growth methods: microwave-assisted hydrothermal, combustion reaction and sol-gel. For all studied samples, the magnetic properties derive from the antiferromagnetic (AF) spin clustering due to the Co2+ nearest neighbors. At T >= 6 K, the magnetization of the Co2+ ions has a Brillouin-type behavior, but below 6 K, it shows a notable deviation. We have shown that the observed deviation may be derived from single-ion anisotropy (SIA) with uniaxial symmetry. Results of fits show that the axial-SIA parameter D (typically D = 4.4 K) is slightly larger that the bulk value D = 3.97 K. No significant change of D has been observed as a function of the Co concentration or the growth process. For each sample, the SIA fit gave also the effective concentration (x) corresponding to the technical saturation value of the magnetization. Comparison of the concentration dependence of x with predictions based on cluster models shows an enhancement of the AF spin clustering independent of the growth method. This is ascribed to a clamped non-random distribution of the cobalt ions in the nanoparticles. The approach of the local concentration (xL) has been used to quantify the observed deviation from randomicity. Assuming a ZnO core/ Zn1-xCoxO shell nanoparticle, the thickness of the shell has been determined from the ratio xL/x.

In this report we present a systematic study on the preparation of Co:ZnO ceramics via standard solid-state route from different Co precursors (Co3O4, CoO and metallic Co) and atmospheres (O2 and Ar). Particular emphasis was done on the defect chemistry engineering and on the sintering growth kinetics. First-principles calculations based on density functional theory were employed to determine the formation energy of the main point defects in ZnO and Co:ZnO systems. Based on the theoretical results a set of chemical reactions was proposed. A detailed microstructural characterization was performed in order to determine the degree of Co incorporation into the ZnO lattice. The samples prepared in Ar atmosphere and from metallic Co presents the highest Co solubility limit (lower apparent Co incorporation activation energy) due to the incongruent ZnO decomposition. The determination of the parameters of the sintering growth kinetics reveals that Co3O4 is the best sintering additive in order to achieve higher densities in both sintering atmospheres. The results give evidences that the sintering in O2 is effective in promoting zinc vacancies in the ZnO structure, while the sintering in Ar promotes zinc interstitial defects. Our findings give valuable contribution to the understanding of the preparation of Co-doped ZnO ceramics and the sintering growth kinetics, what would allow to improve the state of the art on the processing of the material at both bulk and nanometric scales.

In this report we present a systematic structural and magnetic analysis of Co-doped ZnO nanoparticles prepared via a microwave-assisted hydrothermal route. The structural data confirm the incorporation of Co ions into the wurtzite ZnO lattice and a Co concentration mainly near/at the surface of the nanoparticles. This Co spatial distribution is set to passivate the surface of the ZnO nanoparticles, inhibiting the nanoparticle growth and suppressing the observation of a ferromagnetic phase. Based on experimental and theoretical results we propose a kinetic-thermodynamic model for the processes of nucleation and growth of the Co-doped ZnO nanoparticles, and attribute the observed ferromagnetic order to a ferromagnetism associated with specific defects and adsorbed elements at the surface of the nanoparticle. Our findings give valuable contribution to the understanding of both the doping process at the nanoscale and the nature of the magnetic properties of the Co-doped ZnO system.