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The study of the ordering mechanism (1, 2) reveals that it begins by the formation of order@ regions which nucleate and grow within the dieordered matrix. The tetragonal axis of each ordered lfcrystalliteql always remains parallel to one of the three fourfold axes of the disordered lattice. In general all the three types of ordered nucleus (corresponding to the three pqssible orientations of their axes) are present.Thus, a fully ordered crystal is made up of a set of crystallites whose relative volumes are p, q, r, along the axes (loo), (OlO), and (001) of the lattice. (3) show that in an initially disordered Pt-Co single crystal, annealing under a magnetic field in the (100) direction favours the nucleation of Previous experiments