New Material for Permanent Magnets on a Base of Mn and Al

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 $\text{a\hspace{0.17em}=\hspace{0.17em}2.77\hspace{0.17em}}\mathrm{A}$ and $\text{c\hspace{0.17em}=\hspace{0.17em}3.57\hspace{0.17em}}\mathrm{A}$ and lattice positions 000 and $\frac{1}{2}\frac{1}{2}\frac{1}{2}$ with a preference of the Mn atoms for one of these positions. This $\tau$ phase, which has the approximate composition ${\mathrm{Mn}}_{\text{1.11}}\hspace{0.17em}{\mathrm{Al}}_{\text{0.89}}$ , can only be obtained by special heat treatment. It possesses remarkable magnetic properites. From the approach to saturation an anisotropy constant of about ${10}^7\hspace{0.17em}\mathrm{ergs}/{\mathrm{cm}}^3$ can be calculated, while the extrapolated value ${\sigma }_{\infty }$ amounts to 96 erg/oe g; the latter corresponds to a ${\text{4πI}}_s$ of $\text{6200\hspace{0.17em}G}$ . Particles obtained by grinding the bulk material possess an ${}_I{H}_C$ value of up to 6000 oe. No fixed correlation exists between particle size and the value of ${}_I{H}_C$ ; the latter is strongly dependent on the method of pulverization, and the authors are inclined to think that ${}_I{H}_C$ is largely determined by the extent of deformation. The increase of ${}_I{H}_C$ 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 ${}_I{H}_C$ . An anisotropic permanent magnet was obtained by swaging cast bars, with the following properties in the preferred direction: $B_r\text{\hspace{0.17em}=\hspace{0.17em}4280\hspace{0.17em}G}$ , ${}_I{H}_C\text{\hspace{0.17em}=\hspace{0.17em}4600\hspace{0.17em}}\mathrm{oe}$ , ${}_B{H}_C\text{\hspace{0.17em}=\hspace{0.17em}2750\hspace{0.17em}}\mathrm{oe}$ , $\mathrm{BH}\max {\text{\hspace{0.17em}=\hspace{0.17em}3.5×10}}^6\hspace{0.17em}\mathrm{G}\hspace{0.17em}\mathrm{oe}$ .