Paramagnetism in Co-doped ZnO films

Co-doped ZnO films with various electron concentrations up to 4.61 × 1019 cm−3 at room temperature were prepared by pulsed laser deposition on a-plane sapphire substrates. Only paramagnetism was observed down to 2K for all samples. X-ray magnetic circular dichroism measurements at 30K confirmed the paramagnetic properties of the doped Co2+ ions. The average magnetic moment is significantly smaller than the expected moment for Co2+ ions (L = 1.07, S = 3/2), mainly due to the antiferromagnetic exchange interaction between the neighbouring Co2+ ions in the ZnO matrix. Also clustering instead of a uniform distribution of Co2+ ions may play a role. The formation of Co clusters is hindered at higher substrate temperature during the thin film growth. A clear anomalous Hall effect was observed in the highly conducting Co-doped ZnO films at low temperatures up to 100 K.