On spinodal decomposition in alnico - A transmission electron microscopy and atom probe tomography study

Alnico is a prime example of a finely tuned nanostructure whose magnetic properties are intimately connected to magnetic annealing (MA) during spinodal transformation and subsequent lower temperature annealing (draw) cycles. Using a combination of transmission electron microscopy and atom probe tomography, we show how these critical processing steps affect the local composition and nanostructure evolution with impact on magnetic properties. The nearly 2-fold increase of intrinsic coercivity ($H_\text{ci}$) during the draw cycle is not adequately explained by chemical refinement of the spinodal phases. Instead, increased Fe-Co phase ($α_1$) isolation, development of Cu-rich spheres/rods/blades and additional $α_1$ rod precipitation that occurs during the MA and draw, likely play a key role in $H_\text{ci}$ enhancement. Chemical ordering of the Al-Ni-phase ($α_2$) and formation of Ni-rich ($α_3$) may also contribute. Unraveling of the subtle effect of these nano-scaled features is crucial to understanding on how to improve shape anisotropy in alnico magnets.