  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.
