BiFeO3 thickness dependence of the exchange bias in polycrystalline BiFeO3/NiFe bilayers

A clear exchange bias has been observed at room temperature in a series of BiFeO3(t)/NiFe(3.6 nm) bilayers, in which the BiFeO3 layers are single-phase polycrystalline with t varied from 8 nm to 240 nm. With increasing t, both the exchange bias field (HE) and the coercivity (HC) increase sharply and approach maxima when t is about 40 nm, which is close to one half the spin cycloidal modulation period (64 nm) of the bulk BiFeO3 material. The oscillatory variations of HE and HC with the BiFeO3 layer thickness suggest that the cycloidal spin structure may exist in polycrystalline BiFeO3 thin films. The angular dependence of the exchange bias exhibits collinear unidirectional and uniaxial anisotropies in the present BiFeO3/NiFe bilayers.