The Interpretation of Magnetic Susceptibility and the EEffect in Terms of Domain Processes

General formulae applicable to any model or any domain process are derived for the magnetic susceptibility χ and the magnetically dependent part of Young's modulus Δ E . From these formulae expressions are obtained for χ and Δ E in the initial magnetization regions for domain assemblies where magnetization proceeds by ( a ) rotation of domain magnetization vectors against internal strain anisotropy, ( b ) rotation against magnetocrystalline anisotropy characterized by one, three or four preferred axes and ( c ) simple movement of 90° and 180° boundary walls. It is then shown how the types and relative importance of the domain processes operative in the initial magnetization region may be determined from measurements of χ and Δ E . Examples of this procedure are given and include the case of annealed cobalt for which the Δ E effect is due entirely to rotations against uniaxial magnetocrystalline anisotropy energy, while the susceptibility arises mainly from 180° boundary wall movement. For annealed nickel, experimental values of Δ E and χ obtained by a number of workers vary widely and this is shown to be due to (i) differences in the relative proportions of 90° and 180° boundaries and (ii) the influence of the annealing process on the ease of boundary wall movement.