Exchange bias in Co/CoO/Co3O4 nanostructures

Nanostructures of Co/CoO/Co3O4 were synthesized by a chemical method at different temperatures between 300 and 600 °C. The samples were characterized by TG, XRD, TEM and SQUID magnetometry. The lower temperature sample (300 °C) is composed of Co, CoO and Co3O4, while the higher temperature sample only contains Co3O4. All the samples exhibit exchange bias effect. The exchange bias is observed below 205 K (below the CoO blocking temperature) for the sample prepared at lower temperature (300 °C), while for other samples (350-600 °C), the effect is observed below 35 K (below the Néel temperature of Co3O4, TN=40 K). The roles of CoO and Co3O4 on the magnetic properties and the mechanisms governing exchange bias effect have been discussed.