We report on the development of structural and magnetic order in epitaxially grown L10 FePt thin films. Upon annealing, the easy axis of magnetization changes from the out-of-plain into the in-plain direction. We found that the overall fraction of reoriented domains first increases but after certain time decreases before achieving a saturated state. The results are based on conversion electron Mössbauer spectroscopy studies and confirm Monte Carlo simulations in nano-layered FePt. We present a modified version of the Johnson–Mehl–Avrami (JMA) model adequately describing the experimental findings. Two dynamical processes, the first being a 2D-growth, dominate the initial state of sample annealing and the second being a 3D-growth, dominate the late stage close to saturation. From an Arrhenius plots of JMA coefficients for both processes we extracted the activation energies of the underlying dynamics which are 1.5(1) eV for disordering and 0.8(2) eV for ordering.
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