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 2 K for all the samples, which was also confirmed by x-ray magnetic circular dichroism (XMCD) measurements at 30 K. The average magnetic moment per Co2+ ion 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. Clear anomalous Hall effect was observed in the highly conducting Co-doped ZnO films at low temperatures up to 100 K.