Polycrystalline bulk and epitaxially deposited multilayer (200A Pt/[20A 57FePt, 30A 56FePt] x10/MgO) samples of stoichiometric L10-ordered FePt intermetallic compound were examined for “order-order” kinetics (resistometry of bulk at 650 K < T < 1050 K and of the multilayer at 650 K < T < 800 K) and Fe diffusion (grazing incidence nuclear resonant scattering of X-rays (NRS) from multilayer at 773 K < T < 873 K.
Two time scales (fast and slow relaxation components) were observed in “order-order” relaxations. Strong and discontinuous change of chemical ordering dynamics was detected in bulk at T0 = 800 K, definitely away from the Curie point (735 K). The change affected, however, only the slow relaxation component, whose activation energy dropped from about (2,7 ± 0,1) eV above T0 (the value close to the activation energy for Fe* tracer diffusion in FePt measured traditionally at high temperatures) to (1,5 ± 0,3) eV below T0. The rate of the fast relaxation component showed very weak temperature dependence yielding the activation energy of about 0,3 eV. The analogous relaxations measured in the multilayer showed the activation energy of about 1,6 eV.
Dynamics of Fe in the multilayer was investigated with nuclear resonant scattering (NRS) of synchrotron radiation in grazing incindence geometry. A new method for investigating diffusion in very slow diffusing systems was used. The FePt multilayer samples were successively annealed at four temperatures between 773 K and 873 K followed by the reflectivity measurement at room temperature. The vanishing of the superstructure Bragg peaks was observed. From the intensity loss the activation energy for Fe diffusion in a FePt thin film was evaluated as equal to 1,7 eV.
It is concluded that diffusion mechanism in FePt changes at about 800 K – as directly evidenced by resistometry carried out in bulk over sufficiently wide temperature range.

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