Magnetic properties of epitaxially grown semiconducting Zn1-xCoxO thin films by pulsed laser deposition

We have characterized Zn1-xCoxO (x=0.25) films grown on sapphire (0001) substrates by pulsed laser deposition using various growth conditions to investigate the growth condition dependence of properties of Co-doped ZnO films. The substrate temperature (T-S) was varied from 300 to 700 degreesC and the O-2 pressure (P-O2) from 10(-6) to 10(-1) Torr. When T-S is relatively low (less than or similar to600 degreesC), homogeneous alloy films with a wurtzite ZnO structure are grown and predominantly paramagnetic, whereas inhomogeneous films of wurtzite ZnO phase mixed with rock-salt CoO and hexagonal Co phases form when T-S is relatively high and P-O2 is fairly low (less than or similar to10(-5) Torr). The presence of Co clusters leads to room temperature ferromagnetism in inhomogeneous films. The temperature dependence of the magnetization for the homogeneous Zn1-xCoxO (x=0.25) films shows spin-glass behavior at low temperature and high temperature Curie-Weiss behavior with a large negative value of the Curie-Weiss temperature, indicating strong antiferromagnetic exchange coupling between Co ions in Zn1-xCoxO. We have found that Co can be dissolved in ZnO over 40% under an optimum growth condition of T-S=600 degreesC and P-O2=10(-5) Torr, where epitaxial homogeneous Zn1-xCoxO (x=0.25) films of the best crystalline quality are obtained. (C) 2002 American Institute of Physics.