Demagnetising $H$ field If we have a permanent magnet we know that we have a $H$ field opposite in direction to the residual magnetic field $B_r$. The $H$ field causes a demagnetisation, but how does it do it? We know that the physical field (the field which causes the Lorentz force) is the magnetic field $B$.
 A: The H field is caused by the positive and negative poles that develop at the end of the magnet in the opposite direction of the B field. Whether H or B is the real field is somewhat a matter of taste, for neither is directly measurable in any easy way and both are the result of averaging; some even say that within the matter they are both just mathematical fictions. There is one well-known and often referenced experiment by Rassetti: ‘‘Deflections of mesons in magnetized iron,’’ Phys. Rev. 66, 1–5 1944, (note the date!) that claims to verify that the Lorentz force formula does indeed have B in it and not H. Here is a quote from Brown: Magnetostatic Principles in Ferromagnetism

The safest view to adopt regarding the field vectors Band H, as they have been defined at internal points, is that they have definite mathematical meanings in terms of a given magnetization distribution, but that no physical meaning should be attributed to them. Physical interpretations are possible in special cases; for instance, in the case of a homogeneous isotropic toroidal specimen, with a toroidal magnetizing winding, the field vector H is identical with the flux density of the winding alone; but introduction of a gap, or of inhomogeneity or anisotropy, immediately destroys this identity. When physical interpretations are possible, they should be based on direct demonstrations in the relevant specific
  cases.

