Differentiating the free energy will then give the electric polarization P i = − − ∂ ∂ F ∂ ∂ E i and the magnetization M i = − − 1 μ μ 0 ∂ ∂ F ∂ ∂ H i . Here, P s and M s are the static polarization, resp. magnetization of the material, whereas χ χ e and χ χ v are the electric, resp. magnetic susceptibilities. The tensor α α describes the linear magnetoelectric effect, which corresponds to an electric polarization induced linearly by a magnetic field, and vice versa. The higher terms with coefficients β β and γ γ describe quadratic effects. For instance, the tensor γ γ describes a linear magnetoelectric effect which is, in turn, induced by an electric field.