Recoil Magnetic Permeability

The ratio of the average slope of the recoil line to the magnetic permeability in vacuum m₀
(m₀= 4 x 10^-7 H/m) is termed relative recoil magnetic permeability or recoil magnetic permeability for short, m_r:

If the demagnetization curve is curly, the value of m_r depends on the location of the starting point and is a variable, but typically varies within a small range. Therefore, it is approximated as a constant and is taken as the slop of the tangent to the point (B_r, 0) on the demagnetization curve. In other words, the recoil lines at different starting points are approximated as a family of parallel lines, which are all parallel to the tangent to the point (B_r, 0) on the demagnetization curve.