The method previously used to calculate the magnetization curve of an infinite cylinder is applied to the new Meiklejohn and Bean material which is made of Co particles in a CoO shell. It is assumed that the interface of the ferromagnetic and antiferromagnetic materials is held parallel to the cylindrical axis and does not change its direction for any value of the applied field. The crystal anisotropy is neglected.It is shown that the easiest nucleation mode for small radii is buckling and for large radii is curling. The transition from buckling to curling is at aboutRIsA−½=2, whereRis the radius of the cylinder,Isis the saturation magnetization andAis the exchange constant.Numerical solution of the nonlinear equations involved, yields two branches in the magnetization curve, which are both stable with respect to curling perturbations. The general stability is not tested.A similar calculation is carried out for an infinite slab with the spins on the surface held at a fixed direction and with the crystal anisotropy neglected.