Surface ferromagnetism and exchange bias in vacuum annealed Co3−yZnyO4 films

We report on the structural and the magnetic properties of spin coated Co3−yZnyO4 thin films. The microstructure of high vacuum annealed films exhibit a crystalline Zn:CoO core with a disordered oxygen deficient amorphous Co:ZnO surface. Weak ferromagnetism (m∼15mμB∕Co) and an exchange bias (HE∼500Oe) are observed at 10K despite the absence of any Co metal clusters. HE is smaller for samples with smaller Zn fractions (for Co3O4-HV, HE∼250Oe). Adding Zn significantly enhances HE. The exchange anisotropy in an antiferromagnetic material due to surface disordered weak ferromagnetism and its overall implications to dilute magnetic semiconducting oxides are discussed.