Quickstart#

Magpylib treats magnet polarization as fixed. This package adds the material response: mesh the magnets into cells, assign a magnetic susceptibility, and apply_demag computes the self-consistent polarization of every cell — including the demagnetization of the magnets themselves and the response of soft magnetic parts nearby.

The minimal workflow has three steps: mesh → susceptibility → apply_demag. All quantities are SI (meters, Tesla).

import magpylib as magpy

from magpylib_material_response.demag import apply_demag
from magpylib_material_response.meshing import mesh_Cuboid

# a hard magnet with finite susceptibility, SI units (m, T)
magnet = magpy.magnet.Cuboid(polarization=(0, 0, 1), dimension=(1e-3, 1e-3, 1e-3))
magnet.susceptibility = 0.3  # µr = 1.3

# 1. mesh it into cells, 2. apply the material response
mesh = mesh_Cuboid(magnet, target_elems=125)
mesh_demag = apply_demag(mesh)

# the demagnetized collection is a drop-in field source
observer = (0, 0, 1.5e-3)
print("B ignoring material response:", magpy.getB(magnet, observer))
print("B with demagnetization      :", magpy.getB(mesh_demag, observer))

B ignoring material response: [0.         0.         0.04535929]
B with demagnetization      : [ 2.22261445e-18 -8.13151629e-19  4.07574432e-02]

The z-field drops by several percent — that is the magnet demagnetizing itself. Refining the mesh (target_elems) converges the result; the cuboid example compares against FEM.

Setting material properties#

Susceptibility can be attached to objects — searched up the parent Collection tree when not set on the object itself — or passed explicitly to apply_demag, which then takes precedence:

magnet.susceptibility = 0.3  # isotropic
magnet.susceptibility = (0.3, 0.1, 0.0)  # anisotropic, global frame

# explicit values override object attributes: one scalar/vector for all
# cells, or one entry per cell
coll = apply_demag(mesh, susceptibility=0.3)

A uniform external field can be applied through the H_ext attribute, given as flux density in Tesla units (i.e. \(\mu_0 H_\text{ext}\)):

soft = magpy.magnet.Cuboid(polarization=(0, 0, 0), dimension=(1e-3, 1e-3, 1e-3))
soft.susceptibility = 3999  # µr = 4000
soft.H_ext = (0, 0, 0.1)  # 0.1 T applied along z

soft_demag = apply_demag(mesh_Cuboid(soft, target_elems=125))
print("induced polarization of the first cell:")
print(soft_demag.sources_all[0].polarization)

induced polarization of the first cell:
[0.16779392 0.16779392 0.42129489]

Meshing helpers#

  • mesh_Cuboid / slice_Cuboid — uniform grids of cuboid cells (fastest solver path),

  • mesh_Cylinder — cylinder / cylinder-segment cells,

  • mesh_TriangularMesh — tetrahedral cells for arbitrary closed surfaces (optional TetGen dependency, see the tetrahedral example),

  • mesh_all — walk a Collection and mesh every supported child.

Where to go next#