The illustration shows a permanent magnet magnetised with uniform magnetisation M (or magnetic polarisation J), surrounded by vacuum. The magnetic field lines are shown separately for each field: M, magnetic field strength H and magnetic flux density B, which is the vector sum of M and H.
The demagnetising field Hd points in the opposite direction but it is non-uniform because some magnetic flux closes also through the outside of the body (where M = 0 and J = 0).
As a result, inside the body B is also non-uniform and in terms of magnitude B < J.
Outside the body, the field lines of B and H have the same shape, because in vacuum the two vector quantities differ only by a scalar constant of the vacuum permeability μ0 (both M and J are zero).
This illustration also shows that at the boundary between the two media with different permeability values, for H the tangential component Ht is preserved, and for B the normal component Bt is preserved.
This image is based on (Fig. 1.1, p. 7):
demagnetising_field_m_j_h_hd_b_magnetica.png
You are permitted and indeed encouraged to use this image freely, for any legal purpose including commercial, and with any modifications (the permission is hereby given, so there is no need to ask for it explicitly again), but you MUST always give the following credits:
S. Zurek, Encyclopedia Magnetica, CC-BY-4.0
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