Strong room temperature spontaneous exchange bias in BiFeO3-CoFe2O4 nanocomposites

BiFeO3-CoFe2O4 nanocomposites were synthesized by hydrothermal method and an enormous spontaneous exchange bias (300Oe) observed during the measurement of our samples in an unmagnetized state without field cooling. The measuring field dependent Hc is monotonic increase, the exchange bias effect decreases as the sweeping range of the measuring field increases. Moreover as the CoFe2O4 proportion increases, the exchange bias effect first increases to the maximum and then decreases. This effect suggests that the magnetic unidirectional anisotropy can be set isothermally (room temperature) during the initial magnetization process due to the interface coupling between ferrimagnetic and antiferromagnetic phases in BiFeO3-CoFe2O4 nanocomposites. A possible model proposed to explain the mechanism underlying our experimental results reasonably.