The cooling field and the exchange bias in ferromagnet/antiferromagnet bilayers

The effect of the cooling field (hcf) on the exchange bias (hE) has been investigated in ferromagnet/antiferromagnet (FM/AFM) bilayers with the different types of interface exchange coupling. The results show that for the antiferromagnetic interface coupling, hE can change from negative values to positive ones with increasing hcf as the direction of cooling field approaches that of the AFM hard axis. However, hE is commonly positive when the cooling field is applied near along the AFM easy axis. On the other hand, for the ferromagnetic interface coupling, the system always exhibits a negative exchange bias while its magnitude depends slowly on hcf. Moreover, it is found that the sign of hE is determined by the interfacial spin configuration between the FM and the AFM after field cooling procedure. The cooling field may play a decisive role in the creation of the interfacial spin configuration for the exchange bias, not only by the interaction between the Zeeman energy of the spins in the external magnetic field and the antiferromagnetic coupling at the FM/AFM interface, but also by the effect of the uniaxial anisotropy of AFM. Particularly, our results highlight the important role of the direction of the hcf with respect to easy axis of the AFM.