Onset of exchange bias in NiO/NiFe2O4 bilayers

Exchange coupling in NiFe2O4 / NiO bilayers is observed after cooling the system through the Neel temperature in the presence of a static magnetic field. To understand the coupling mechanism in more details, we have studied the differential susceptibility curves partial derivativeM/partial derivativeH measured along the hysteresis loops for a set of samples with different NiO layer thicknesses. These curves are well described using lorentzian distribution functions, allowing us to extract both the center and the width of the switching field distribution for both branches of the hysteresis loops. Upon increasing the NiO layer thickness, the onset of exchange bias is associated with a significant increase of coercivity and the broadening of the switching field distribution. While the loops are strongly biased at low temperatures (the exchange bias field may be as strong as 2 kOe), the width of the switching field distribution is roughly the same for both branches of the hysteresis loops, Moreover, this width is weakly dependent upon cycling the magnetic field, whereas the exchange bias field is reduced.