Monte Carlo Simulation Study of Atomic Structure of alnico Permanent Magnets

Lattice Monte Carlo simulation based on quinternary cluster expansion energy model is used to investigate nano-scale structure of alnico alloy, which is considered as a candidate material for rare-earth free high performance permanent magnets, especially for high or elevated temperature applications such as electric motor for vehicles. We observe phase decomposition of the master alnico alloy into FeCo-rich magnetic ($\alpha $1) and NiAl-rich matrix ($\alpha $2) phases. Concentrations of Fe and Co in $\alpha $1 phase and Ni and Al in $\alpha $2 phase are higher for lower annealing temperature. Ti is residing mostly in the $\alpha $2 phase. The phase boundary between $\alpha $1 and $\alpha $2 phases are quite sharp with only few atomic layers. The $\alpha $1 phase is in B2 ordering with Fe and Al occupying the $\alpha $-site and Ni and Co occupying the $\beta $-site. The $\alpha $2 phase is in L21 ordering with Al occupying the 4a-site. The phase composition profile again annealing temperature suggests that lower annealing temperature would improve the magnetism of $\alpha $2 and diminish the magnetism of $\alpha $2 phase, hence improve shape anisotropy of $\alpha $1 phase rods and that of alnico.