Influence Of Assistant Ferromagnetic Layer On Exchange Bias In Ferromagnetic/Antiferromagnetic Bilayers

Recently, magnetic nanomaterials have attracted much attention in information technology, especially in the field of information storage. The magnetic spin-valve structure based on giant magnetoresistance becomes a topic of research after the giant magnetoresistance winning the Nobel Prize of 2007. In this paper, a modified Monte Carlo method is used to study the exchange bias in the bilayers attaching an assistant ferromagnetic layer. The origin of exchange bias is understood and the work will guide the experimental studies. The results obained as follows:1. The field-cooling dependence of exchange bias is studied. The interfacial coupling between antiferromagnetic and assistant ferromagnetic layers and the anisotropy of assistant ferromagnetic layer are responsible for the field-cooling dependence of exchange bias. When the interfacial coupling between antiferromagnetic and assistant ferromagnetic layers is ferromagnetic and strong, exchange bias fluctuates around a stable value while is insensitive to cooling field. As this interfacial coupling is not strong, the exchange bias behaviors against cooling field are similar to the results obtained in the pure ferromagnetic/antiferromagnetic bilayers. If this interfacial coupling becomes antiferromagnetic and strong, the opposite trends of exchange bias versus cooling field are obtained compared to those in the pure bilayers.2. The temperature dependence of exchange bias is studied. In the pure ferromagnetic/antiferromagnetic bilayers, exchange bias increases with the decrease of temperature. Then the antiferromagnetic layer is attached by an assistant ferromagnetic layer. The fact that the interfacial coupling between antiferromagnetic and assistant ferromagnetic layers is antiferromagnetic and strong, or this interfacial coupling is ferromagnetic and strong meanwhile the anisotropy of assistant ferromagnetic layer is strong enhances the blocking temperature of the onset of exchange bias. Moreover, exchange bias in such bilayers with an assistant ferromagnetic layer is larger than that in the pure bilayers, even reaches into saturation.3. The dependence of exchange bias on antiferromagnetic exchange coupling is studied. Exchange bias in the pure ferromagnetic/antiferromagnetic bilayers decreases from a saturated value to zero with the increase of antiferromagnetic exchange coupling. When the bilayers attach an assistant ferromagnetic layer via strong antiferromagnetic interfacial coupling, exchange bias is prone to be saturated for weak antiferromagnetic exchange coupling while coercivity is weakly influenced by antiferromagnetic exchange coupling. However, for strong ferromagnetic interfacial coupling, exchange bias and coercity behaviors are different from those in the pure ferromagnetic/antiferromagnetic bilayers. Exchange bias is zero for weak antiferromagnetic exchange coupling, and increases to a maximum value with the increase of antiferromagnetic exchange coupling. With further increase of antiferromagnetic exchange coupling, exchange bias decreases again to zero. Coercivity decreases monotonously and levels off with the increase of antiferromagnetic exchange coupling.4. The dependence of exchange bias on antiferromagnetic anisotropy is studied. With the increase of antiferromagnetic anisotropy, exchange bias increases from zero to a stable value and coercivity exhibits a peak close to the antiferromagnetic anisotropy where exchange bias changes most abruptly. However, the magnetization behaviors of antiferromagnetic layer are dependent on the assistant ferromagnetic layer via different interfacial couplings strong, which also affects indirectly the ferromagnetic magnetization behaviors in the bilayers with antiferromagnetic anisotropy.