Presentation Information
[O14-1]Micromagnetic and reduced-order model simulations of the impact of microstructural defects on the coercivity of recycled Nd2Fe14B magnets
*Johann Fischbacher1, Thomas Schrefl1,2 (1. Department for Integrated Sensor Systems, University for Continuing Education Krems (Austria), 2. Christian Doppler laboratory for magnet design through physics-informed machine learning, University for Continuing Education Krems (Austria))
Keywords:
micromagnetic simulations,reduced-order model,Nd2Fe14B
Demagnetizing effects due to nonmagnetic pores and Nd-rich oxides in triple pockets, the presence of oxides at grain surfaces, or imperfect grain boundaries causing exchange coupling between misaligned grains are some of the effects that reduce the performance of recycled Nd2Fe14B magnets. We used micromagnetic energy minimization [1] and a reduced-order model [2] to estimate the impact of such microstructural defects on the coercivity of permanent magnets with micrometer-sized grains. The model size for micromagnetic simulations using the finite element (FE) method is limited to grain sizes of a few hundred nanometers due to the computational capabilities. The reduced-order model requires four basic assumptions: a) the magnetization within a grain is homogeneous, b) the nucleation of a reversed domain starts close to the surface of a grain, c) after nucleation the grain reverses in a single step without any pinning and d) the grains are exchange decoupled. Those simplifications allow us to replace the 3D finite element mesh with nanometer resolution by a coarse 2D mesh of the grain surfaces only plus a small number of evaluation points at a defined distance from those surfaces. The total effective field out of the exchange field, the demagnetization field and the external field is compared against local switching fields defined at each evaluation point. The local switching fields are available via analytical models or classical micromagnetic simulations combined with AI tools. The demagnetization curves calculated with both approaches for a Nd2Fe14B model system match well [3]. Figure 1 shows that a higher degree of texture (DOT) benefits coercivity (up to +32%). This positive effect almost vanishes, when a higher amount of non-magnetic inclusions is present. Oxides at the grain surfaces may distort the first atom layers close to the surface and cause in-plane anisotropy of the Nd atoms. At evaluation points affected by oxides, we reduced the switching field by 17%. When oxides affected 20% of the surface area, the coercivity was reduced by 17% to 25%.
The authors gratefully acknowledge the financial support by REEsilience, co-funded by the European Union under grant agreement number 101058598, and UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee grant number 10038960 as part of the Horizon Europe HORIZON-CL4-2021-RESILIENCE-01-07.
[1] L. Exl et al., Computer Physics Communications 235, pp. 179–186 (2019).
[2] H. Moustafa et al., AIP Advances 14.2, 025001 (2024).
[3] A. Kovacs et al., Front. Mater., 9, 1094055 (2023).
The authors gratefully acknowledge the financial support by REEsilience, co-funded by the European Union under grant agreement number 101058598, and UK Research and Innovation (UKRI) under the UK government’s Horizon Europe funding guarantee grant number 10038960 as part of the Horizon Europe HORIZON-CL4-2021-RESILIENCE-01-07.
[1] L. Exl et al., Computer Physics Communications 235, pp. 179–186 (2019).
[2] H. Moustafa et al., AIP Advances 14.2, 025001 (2024).
[3] A. Kovacs et al., Front. Mater., 9, 1094055 (2023).
