Characterisation of high-anisotropy nanocrystalline alloys based on magnetic susceptibilities in the remanent state

•Magnetic susceptibilities of nanocrystalline alloys are studied in the remanent state.•Local interactions induce difference in longitudinal and transverse susceptibilities.•The phenomenon is described within the mean field approach for weak exchange coupling.•Effects of polydispersity and magnetostatic are modelled by Monte Carlo technique.•A new method for estimation of intergrain exchange interaction constant is proposed. The macroscopic magnetic properties of nanocrystalline alloys are strongly dependent on microstructural and micromagnetic inhomogeneities along with intergranular interactions. In particular, magnetic susceptibility is sensitive to these factors and frequently used for alloy characterisation. In this work reversible magnetic susceptibility in the remanent state of the nanocrystalline alloy with randomly oriented easy magnetisation axes is considered. Due to intergranular interactions the susceptibilities measured parallel and perpendicular to the remanence can be distinctly different from each other. When the intergrain exchange is the dominant interaction this susceptibility feature has potential for its quantitative evaluation. Here this opportunity is studied by means of two approaches: analytical solutions based on the mean-field approximation and simulations using the kinetic Monte Carlo model. The dependence of the magnetic susceptibility on the exchange coupling strength are investigated taking into account the effects of grain size distribution and magnetostatic interactions. These results lead us to propose an experimental susceptibility-based method for the estimation of both intergrain exchange interaction strength and effective magnetic anisotropy constant. This method can be used for the characterisation of high-anisotropy nanocrystalline alloys with randomly oriented easy magnetisation axes that is a widespread case. The method is applied to rapidly quenched Nd2(Fe0.8Co0.2)14B nanocrystalline alloy giving strong exchange coupling and a reasonable value for the anisotropy constant.