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

TL;DR

This study investigates how processing steps like magnetic annealing and lower temperature draws influence the nanostructure and magnetic properties of alnico, revealing that phase isolation and nanostructure evolution significantly enhance coercivity.

Contribution

The paper provides detailed insights into the nanostructural changes during processing and their impact on magnetic properties, highlighting mechanisms beyond chemical refinement.

Findings

Increased Fe-Co phase isolation correlates with coercivity enhancement.

Development of Cu-rich nanostructures influences magnetic properties.

Chemical ordering and phase formation contribute to magnetic performance improvements.

Abstract

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 ($\alpha_1$) isolation, development of Cu-rich spheres/rods/blades and additional $\alpha_1$ 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 ($\alpha_2$) and formation of Ni-rich ($\alpha_3$) 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.