Coercivity and magnetization reversal mechanism in ferromagnet/antiferromagnet bilayers: Correlation with microstructure of ferromagnetic layers

Abstract

Exchange biasing in ${\mathrm{Co}}_{20}{\mathrm{Ni}}_{80}∕\mathrm{Fe}\mathrm{Mn}$ and ${\mathrm{Co}}_{80}{\mathrm{Cr}}_{20}∕\mathrm{Fe}\mathrm{Mn}$ bilayers has been investigated, where the CoCr layers are of granular structure and the CoNi layers are in the form of a single phase. In the above two series of bilayers, the exchange field is proportional to $1∕{t}_{\mathrm{FM}}$ (${t}_{\mathrm{FM}}$ denotes ferromagnetic layer thickness). For $\mathrm{Co}\mathrm{Ni}∕\mathrm{Fe}\mathrm{Mn}$ bilayers, the coercivity and the uniaxial anisotropic field decrease with increasing ${t}_{\mathrm{FM}}$ with a linear scale of $1∕{t}_{\mathrm{FM}}$. Since they are equal to each other, the magnetization reversal process can be described by magnetization coherent rotation and the coercivity enhancement can be explained in terms of a uniaxial anisotropy model. For $\mathrm{Co}\mathrm{Cr}∕\mathrm{Fe}\mathrm{Mn}$ bilayers, however, the coercivity displays unusual behaviors. First, in comparison with that of single CoCr layer films, the coercivity is reduced instead of enhanced. Secondly, it increases with increasing ${t}_{\mathrm{FM}}$. Finally, the coercivity of the bilayers is not equal to the uniaxial anisotropic field. A noncoherent rotation process is proposed to occur during the magnetization reversal process. The different characteristics of the coercivity and magnetization reversal mechanisms in the two series of bilayers result from the different microstructures in the CoNi and CoCr layers. The present work might be helpful to clarify the mechanism for the coercivity enhancement in ferromagnet/antiferromagnet bilayers.