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.