Manganese-aluminum alloys in the vicinity of the equiatomic composition exhibit an attractive combination of magnetic properties for technological applications, including bulk permanent magnets and thin-film devices. The technical magnetic properties derive from the formation of a metastable L1 0 intermetallic phase ( -MnAl) characterized by a high, uniaxial magnetocrystalline anisotropy with an "easy" c-axis. Carbon is generally added to stabilize the tetragonal phase with respect to the stable phases in the system. The magnetic hysteresis behavior of the Mn-Al-C genre of permanent magnet alloys is extremely sensitive to the microstructure and defect structure produced during the formation of the phase (L1 0 ) within the high-temperature phase (hcp). In this study, modern metallographic techniques, including high-resolution electron microscopy (HREM), have been applied to elucidate the nature of the phase transformation and the evolution of the unique microstructure and defect structure characterizing the structural state of the ferromagnetic phase. It is concluded that the metastable phase is the product of a compositionally invariant, diffusional nucleation and growth process or massive transformation. The massive product nucleates preferentially at the grain boundaries of the parent phase and is propagated by the migration of incoherent interphase interfaces. The interphase interfaces are revealed to be faceted on various length scales. It is concluded that this faceting is not a feature of the bicrystallography of the parent and product phases. The high density of lattice defects within the phase, generated by the phase transformation, is attributed to growth faults produced during atomic attachment at the migrating interfaces. Classical nucleation theory has been applied quantitatively to the grain-boundary nucleation process and was found to be consistent with the observed time-temperature-transformation (TTT) behavior. Analysis of the growth kinetics gives an ⌬H D value of 154 kJ mol Ϫ1 for the activation energy of the transboundary diffusional process controlling boundary migration.
Transmission electron microscopy techniques are used to characterize the development of the polytwinned microstructures during Al → Ll0 ordering in FePd and FePt alloys. Also, Lorentz microscopy methods are used to image the magnetic domain structures which are associated with these magnetically modulated twin assemblies. A modified APB pinning model including thermally activated wall motion is formulated to account for the levels of coercivity and the temperature dependence of the coercivity in polytwinned ferromagnets.
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