Interdiffusion in Fe–Pt multilayers

Fe ∕ Pt multilayers with modulation periods Λ=24.1±0.2 and 37.2±0.1Å and [110]‖[111] bcc-fcc texture were fabricated by magnetron sputtering on thermally oxidized Si wafers. The structural evolution of the multilayers with annealing temperature in the range of 300–600K was studied by in situ x-ray diffraction (XRD) and x-ray reflectivity. Two temperature regimes were found from the XRD data. Below 534±4K slow, short-range diffusion is observed without significant broadening of the satellite peaks or changes in the texture. Above 534K fast, long-range diffusion is observed accompanied by significant broadening of the satellites and rapid increase of the misorientations of the grains. The multilayers crystallize at about 583K into the tetragonal FePt phase with a small degree of ordering and strong [111] texture. The transition resembles a first-order phase transition with a critical exponent β=0.48±0.01 which practically does not depend on Λ. The bulk interdiffusion coefficient, determined from the decay of the −1 satellite of the (001) Bragg peak of the multilayers, can be expressed in Arrhenius form as D(T)=(1.37±0.26)×10−6exp(−1.7±0.6∕kBT)m2∕s. The gradient-energy coefficient k, entering the Cahn-Hilliard diffusion equation [Acta Metallurg. 9, 795 (1961), 10, 179 (1962); J. Chem. Phys. 28, 258 (1959)], was estimated from the Λ dependence of the diffusion coefficient to be (−6.8±0.2)×107eV∕cm.