Non-uniform external magnetic field

[MiroPol21]

Dear all,
Is it possible to apply a non-uniform external magnetic field in mumax+?

Thank you all for your help.
Sincerely,
Miroslav Polach

[lamoreel]

Hi!

This is definitely possible! You could do this in two different ways:

1. Use numpy arrays of shape (3, nz, ny, nx). For example

import numpy as np
Bext = np.zeros(magnet.bias_magnetic_field.shape)
Bext[0, :, :, :] = 1          # x-comp is 1 everywhere
Bext[1, :, :, :nx//2] = 1   # y-comp is 1 in left half
Bext[1, :, :, nx//2:] = -1  # y-comp is -1 in right half
magnet.bias_magnetic_field = Bext

2. Use a function which takes grid coordinates and returns a vector with 3 components. For example:

def my_Bext(x,y,z):
    return (x*y, 1, 2*z)
magnet.bias_magnetic_field = my_Bext

This is also explained in this tutorial for the case of magnetization setting. The methodology is the same, apart from the fact that the magnetization gets normalized behind the scenes.

Hope this helps!
Lars

[lamoreel]

One small addition: you can do this for any parameter in mumax+ (every grid cell can hold a different parameter value). When using numpy arrays, keep in mind that the axes order is (comp, Nz, Ny, Nx) where comp is the number of components (1 or 3).

[lamoreel]

Okay, one last addition 😄: you could also create inhomogeneities by defining regions and setting material parameters region-wise. In mumax+ v1.1.3 it's also possible to use a function for this.

[MiroPol21]

Ooh, that is awesome! Thank you (also for the additional comments).
Miro