  3D numerical simulations of ferromagnetic materials can be compared with
experimental results via microwave susceptibility. In this paper, an optimised
computation of this microwave susceptibility for large meshes is proposed. The
microwave susceptibility is obtained by linearisation of the Landau and
Lifchitz equations near equilibrium states and the linear systems to be solved
are very ill-conditionned. Solutions are computed using the Conjugate Gradient
method for the Normal equation (CGN Method). An efficient preconditioner is
developed consisting of a projection and an approximation of an ``exact''
preconditioner in the set of circulant matrices. Control of the condition
number due to the preconditioning and evolution of the singular value
decomposition are shown in the results.
