8000 Fix #357: Msat=0 affects exchange by JonathanMaes · Pull Request #372 · mumax/3 · GitHub
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Fix #357: Msat=0 affects exchange #372

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Merged
merged 6 commits into from
Jun 17, 2025
Merged

Fix #357: Msat=0 affects exchange #372

merged 6 commits into from
Jun 17, 2025

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JonathanMaes
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This PR fixes issue #357, where cells with Msat=0 were reported to affect other cells through the exchange interaction.

Background

As correctly identified by @jplauzie, this bug appeared in mumax3.10 as part of PR #115 (6303d01), which addressed an "increasing-energy" problem related to Eq. (9) of the mumax³ paper. Note that, since 3.10, Eq. (9) in the paper is no longer the actually implemented equation. Originally, it was

$$\vec{\boldsymbol{\mathrm{B}}}_\mathrm{exch} = 2 S \frac{2\frac{A_\mathrm{ex1}}{M_\mathrm{sat1}}\frac{A_\mathrm{ex2}}{M_\mathrm{sat1}}}{\frac{A_\mathrm{ex1}}{M_\mathrm{sat1}}+\frac{A_\mathrm{ex2}}{M_\mathrm{sat1}}} \sum_i \frac{(\vec{\boldsymbol{\mathrm{m}}}_i - \vec{\boldsymbol{\mathrm{m}}})}{\Delta_i^2} \mathrm{,}$$

but this was changed in PR #115 to

$$\vec{\boldsymbol{\mathrm{B}}}_\mathrm{exch} = \frac{2S}{M_\mathrm{sat1}} \frac{2 A_\mathrm{ex1} A_\mathrm{ex2}}{A_\mathrm{ex1}+A_\mathrm{ex2}} \sum_i \frac{(\vec{\boldsymbol{\mathrm{m}}}_i - \vec{\boldsymbol{\mathrm{m}}})}{\Delta_i^2} \mathrm{.}$$

In other words, since then, the exchange interaction of a spin with its neighbours has no longer been scaled by the Msat of each neighbour separately, but only by Msat of the spin itself.

Edge case for Msat=0

While PR #115 solved the "increasing-energy" problem, it also caused magnets with nonzero Msat to be affected by neighbouring Msat=0 cells. This edge case was the underlying reason for issue #358 and has now been fixed.

On a related note, but unrelated to PR #115: magnets with Msat=0 were affected by the demag field generated by other cells. Since this is a rather weak interaction compared to exchange, functions like relax() were very slow if regions with Msat=0 were present. Hence, from now on, magnets with Msat=0 are no longer affected by demag fields.

@JonathanMaes JonathanMaes requested a review from JLeliaert June 17, 2025 12:01
@JonathanMaes JonathanMaes merged commit 9ae4126 into 3.11.1 Jun 17, 2025
@JonathanMaes JonathanMaes deleted the bugfix/MsatZeroExchange branch June 17, 2025 13:49
@JonathanMaes JonathanMaes mentioned this pull request Jun 17, 2025
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@JLeliaert JLeliaert JLeliaert approved these changes

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