Magnetic properties in randomly diluted magnetic systems: Co-doped ZnO polycrystalline ceramics

Abstract

The influence of structural defects on the magnetic properties of Co-doped ZnO polycrystalline ceramics (Zn1−xCoxO) with 0.00 ≤ x ≤ 0.05 sintered by a pressure-less solid state reaction method were investigated in a broad range of temperatures (5 ≤ T ≤ 300 K) for applied magnetic fields up to 85 kOe. The hysteresis loops indicated the presence of diamagnetic behavior for undoped ZnO samples while both antiferromagnetic (AFM) and paramagnetic (PM) ordering for Co-doped ZnO. The saturation magnetization Ms values decrease with increasing Co concentration (x), being ~ 2.5 μB∕Co and ~ 1.6 μB∕Co for x = 0.005 and x = 0.05, respectively, T = 5 K. Also the fraction of the saturation magnetization values were found to be 0.83 (83 % PM), 0.66 (66 % PM) and 0.62 (62 % PM) for samples (x) 0.005, 0.02, and 0.05, respectively. The decrease of PM ordering with x, could be attributed to the shortening the distance between adjacent Co ions which favor the AFM coupling due to super-exchange interaction. In addition, the Jex∕kB values were found to be negative, being − 20, − 30, and − 31 for x = 0.005, 0.02, and 0.05, respectively, indicated the absence of FM coupling for all Co-doped ZnO samples. The reduction in Jex∕kB values are due to the increase of AFM ordering for Co ions homogeneously diluted in the ZnO structure during sintering process at high temperatures (≥1100 °C). Even though the presence of a large number of structural defects in both pure and doped ZnO ceramic samples reveled by Raman and UV-Vis spectroscopy analysis the ferromagnetic (FM) ordering was not present for the broad range of temperatures (5 ≤ T ≤ 300 K) studied.