Effects of disorder on the spin wave spectra of ferromagnetic ultrathin films: A critique of naive effective medium approaches

We show that the neglect of vertex corrections in the calculation of the dynamic transverse spin susceptibility for disordered systems can lead to qualitatively incorrect predictions regarding spin wave energies and even the stability of the ferromagnetic ground state. We show numerical evidence indicating that the most critical approximation is the neglect of vertex correction in the RPA equation.