INTERDIFFUSION IN CO C SOFT-X-RAY MULTILAYER MIRRORS/

The interdiffusion behaviours in the amorphous Co/C multilayers prepar ed by dual-facing-target sputtering techniques are investigated quanti tatively by monitoring the enhancement of the first-order modulation p eak on annealing in the temperature range of similar to 473-523 K. Eff ective interdiffusion coefficients as low as 10(-25) m(2) s(-1) were m easured. The negative true macroscopic interdiffusion coefficients wer e 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 macr oscopic atom model. The temperature dependence of the true macroscopic interdiffusion coefficients can be described as D = - 1.66 x 10(10) e xp[-(128 +/- 10) KJ mol(-1)/RT] m(2) s(-1). The existence of a critica l wavelength below which the effective interdiffusion coefficicnt chan ges 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 s econd derivative of the Helmholtz energy f(0)(n).