Magnetization of Zn1-xCoxO nanoparticles: single-ion anisotropy and spin clustering

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 re-action and sol-gel. For all studied samples, the magnetic properties derive from the antiferromagnetic (AF) spin clustering due to the Co2+ nearest-neighbors. The magnetization curves have been fitted to the Brillouin function (BF). We have used the normalized root mean square error (NRMSE) between the ex-perimental and calculated BF curves as goodness of fit indicator. For all samples, the magnetization of the Co2+ ions has a BF behavior at T >= 6 K with NRMSE approximate to 0.004 typically, but below 6 K, it shows a notable deviation. The magnitude of the deviation is found to increase with decreasing temperature and reaches its maximum value at T = 1.7 K with NRMSE approximate to 0.026. The observed deviation as well as the magnetization curve shape have been successfully explained by a model based on single-ion anisotropy (SIA) with uniaxial symmetry for the Co2+ ions and the assumption of randomly oriented nanoparticles. Fits of the magneti-zation curves have been performed by using this model. The axial-SIA parameter D and the effective Co concentration x over bar corresponding to the technical saturation value of the magnetization were used as ad-justable parameters. The quality of the fit using this model is clearly improved with NRMSE around 0.007 at T = 1.7 K. The obtained value of 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. Comparison of the concentration dependence of the obtained values of x over bar with predictions based on the nearest-neighbors cluster model has been made. The result shows an enhancement of the AF spin clustering independent of the growth method which can be ascribed to a clamped non-random dis-tribution of the cobalt ions in the nanoparticles. The approach of the local concentration (xL) has been used to quantify the observed deviation from randomicity. We obtained xL = 1.4 x. Assuming the picture of a ZnO core / Zn1-xCoxO shell nanoparticle, the thickness of the shell has been estimated from the ratio xL/x to be one fourth of the diameter of the core.(c) 2023 Elsevier B.V. All rights reserved.