Ferromagnetic Phase of Mn-Al

The metastable, ferromagnetic, tetragonal phase in the Mn-Al system has been obtained by isothermal transformation of a high-temperature phase. Transmission electron micrographs of specimens of this phase so obtained (bulk ${}_i{H}_c\text{\hspace{0.17em}=\hspace{0.17em}1130\hspace{0.17em}}\mathrm{oe}$ ) show striations of light (transmitting) and dark (non-transmitting) material. The striations are presumed to arise from stresses induced by the transformation and/or cooling. Transmission electron diffractograms contain individual patterns both with and without superlattice reflections. Respective patterns reveal axial ratios of 0.90 and 0.94, and are correlated with the dark and light areas of the micrographs. From diffraction data it is concluded that the trace of the boundary between the light and dark regions coincides with a projection of the c axis. The possibility of coherency as a mechanism for the high coercive force, analogous to CoPt, is discussed. The type of cooperative atomic interaction as related to the shift in the axial ratio is also discussed.