Interdiffusion in Co/C soft X-ray multilayer mirrors

The interdiffusion behaviours in the amorphous Co/C multilayers prepared by dual-facing-target sputtering techniques are investigated quantitatively by monitoring the enhancement of the first-order modulation peak on annealing in the temperature range of similar to 473-523 K. Effective interdiffusion coefficients as low as 10(-25) m(2) s(-1) were measured. The negative true macroscopic interdiffusion coefficients were obtained by measuring the effective interdiffusion coefficient as a function of modulation wavelength, indicating that then is a tendency to phase separation in the Co-C system. This result is also predicted by the positive enthalpy of mixing, calculated based on Miedema's macroscopic atom model. The temperature dependence of the true macroscopic interdiffusion coefficients can be described as D = - 1.66 x 10(10) exp[-(128 +/- 10) KJ mol(-1)/RT] m(2) s(-1). The existence of a critical wavelength below which the effective interdiffusion coefficicnt changes its sign is proposed, and it was calculated to be similar to 19.8-20.5 Angstrom at temperatures ranging from 473 K to 523 K. The spread values of the critical wavelength are believed to be attributed to the composition dependence of the gradient energy coefficient k and the second derivative of the Helmholtz energy f(0)(n).