During an investigation of the Mn-Al system from 40–100% Mn, a new metastable phase was found,which has a tetragonal structure with lattice constants a = 2.77 A and c = 3.57 A and lattice positions 000 and 121212 with a preference of the Mn atoms for one of these positions. This τ phase, which has the approximate composition Mn1.11 Al0.89, can only be obtained by special heat treatment. It possesses remarkable magnetic properites. From the approach to saturation an anisotropy constant of about 107 ergs/cm3 can be calculated, while the extrapolated value σ∞ amounts to 96 erg/oe g; the latter corresponds to a 4πIs of 6200 G. Particles obtained by grinding the bulk material possess an HCI value of up to 6000 oe. No fixed correlation exists between particle size and the value of HCI; the latter is strongly dependent on the method of pulverization, and the authors are inclined to think that HCI is largely determined by the extent of deformation. The increase of HCI caused by grinding is accompanied by a decrease of the magnetic moment. This decrease can be partially reversed by aging. The observed decrease in the magnetic moment on pulverizing agrees with the value calculated from X-ray and neutron diffraction data. The current fine-particle theory is unable to give an adequate explanation of the HCI. An anisotropic permanent magnet was obtained by swaging cast bars, with the following properties in the preferred direction: Br = 4280 G, HCI = 4600 oe, HCB = 2750 oe, BHmax = 3.5×106 G oe.

Koch, A. J. J., Hokkeling, P., v. d. Steeg, M. G., & de Vos, K. J. (1960). New Material for Permanent Magnets on a Base of Mn and Al. Journal of Applied Physics, 31(5), S75–S77.