arXiv subjects

Iver E. Anderson

Publications and source records attributed to Iver E. Anderson.

3 recordsLinked to original sources

arXiv2018

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.

cond-mat.mtrl-sci↗

arXiv2017

Microstructural and magnetic property evolution with different heat-treatment conditions in an alnico alloy

Further property enhancement of alnico, an attractive near-term, non-rare-earth permanent magnet alloy system, primarily composed of Al, Ni, Co, and Fe, relies on improved morphology control and size refinement of its complex spinodally decomposed nanostructure that forms during heat-treatment. Using a combination of transmission electron microscopy and atom probe tomography techniques, this study evaluates the magnetic properties and microstructures of an isotropic 32.4Fe-38.1Co-12.9Ni-7.3Al-6.4Ti-3.0Cu (wt.$\%$) alloy in terms of processing parameters such as annealing temperature, annealing time, application of an external magnetic field, as well as low-temperature "draw" annealing. Optimal spinodal morphology and spacing is formed within a narrow temperature and time range ($\sim 840 \unicode{x2103}$ and 10 min during thermal-magnetic annealing (MA). The ideal morphology is a mosaic structure consisting of periodically arrayed $\sim 40$ nm diameter (Fe-Co)-rich rods ($α_1$ phase) embedded in an (Al-Ni)-rich ($α_2$ phase) matrix. A Cu-enriched phase with a size of $\sim$ 3-5 nm is located at the corners of two adjacent $\{110\}$ facets of the $α_1$ phase. The MA process significantly increased remanence ($B_\text{r}$) ($\sim$ 40-70 $\%$) of the alloy due to biased elongation of the $α_1$ phase along the $\langle100\rangle$ crystallographic direction, which is closest in orientation to the applied magnetic field. The optimum magnetic properties of the alloy with an intrinsic coercivity ($H_\text{cj}$) of 1845 Oe and a maximum energy product ($BH_\text{max}$) of 5.9 MGOe were attributed to the uniformity of the mosaic structure.

cond-mat.mtrl-sci↗

arXiv2017

Simulation of alnico coercivity

Micromagnetic simulations of alnico show substantial deviations from Stoner-Wohlfarth behavior due to the unique size and spatial distribution of the rod-like Fe-Co phase formed during spinodal decomposition in an external magnetic field. The maximum coercivity is limited by single-rod effects, especially deviations from ellipsoidal shape, and by interactions between the rods. Both the exchange interaction between connected rods and magnetostatic interaction between rods are considered, and the results of our calculations show good agreement with recent experiments. Unlike systems dominated by magnetocrystalline anisotropy, coercivity in alnico is highly dependent on size, shape, and geometric distribution of the Fe-Co phase, all factors that can be tuned with appropriate chemistry and thermal-magnetic annealing.

cond-mat.mtrl-sci↗