def D_theo(D0, T, Tc, mu):
    """
    Calculates the theoretical value for spinwave stiffness D as a function of temperature
    
    Parameters
    ----------
    D0   :   float
        spin-wave stiffness at 0 K meV/angstroem^2
    T    :   float
        temperature of the system in K (needs to be <= Tc)
    Tc   :   float
        Curie temperature of the material in K
    beta :   float, optional
        critical exponent from dynamical scaling theory beta = 1/3 is exact within the theory

    Return
    ------

    Note
    ----
    * D0_Fe = 280 meV / angstroem^2
    * D0_Ni = 430 meV / angstroem^2
    """
    
    return D0*(1-T/Tc)**mu