Fix #357: Msat=0 affects exchange#372
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JonathanMaes merged 6 commits into3.11.1from Jun 17, 2025
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Hi in mumax3.11. I cannot get the output of the stray field from a nanomagnet at a certain distance away from the nanomagnet. it is showing always zero |
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Fix bug from #372 that made B_demag==0 where Msat==0
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This PR fixes issue #357, where cells with
Msat=0were 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
but this was changed in PR #115 to
In other words, since then, the exchange interaction of a spin with its neighbours has no longer been scaled by the
Msatof each neighbour separately, but only byMsatof the spin itself.Edge case for
Msat=0While PR #115 solved the "increasing-energy" problem, it also caused magnets with nonzero
Msatto be affected by neighbouringMsat=0cells. 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=0were affected by the demag field generated by other cells. Since this is a rather weak interaction compared to exchange, functions likerelax()were very slow if regions withMsat=0were present. Hence, from now on, magnets withMsat=0are no longer affected by demag fields.