Determination of diffusion coefficient D and activation energy Q_a of nickel into titanium in Ni–Ti multilayers by grazing-angle neutron reflectometry

Diffusion in Ni–Ti multilayers with periods of 120 Å is studied in the temperature range 293–743 K by grazing-angle unpolarized neutron reflectometry. The effective diffusion coefficient, D_eff, of nickel into titanium and its corresponding activation energy, Q_a, are determined by measurement of the decay of the reflectivity of the first Bragg peak, which arises from the nuclear scattering-length density modulation, as a function of annealing temperature at constant time. The direction of diffusion is determined by simulation of the Kiessig fringes located between the total-reflection plateau and the first Bragg peak. Two diffusion regimes with a transition temperature T_c ≃ 543 K are observed in this Ni–Ti multilayer; the corresponding activation energy values are 0.21 and 0.43 eV, respectively.