Magnetic properties of MnAl system alloys

Abstract

The ferromagnetic phase of MnAl alloy exists at a position slightly different from that of Mn:Al = 1:1 toward the Mn rich side. The single crystal of MnAl alloy was prepared by the horizontal type zone melting process. Domain patterns of that single crystal were observed microscopically with or without applying the magnetizing field, using a colloidal solution of fine iron oxide powders. Domain patterns on the ferromagnetic phase are similar to those of cobalt and MnBi alloy. From magnetizing curves measured at room temperature, the saturation magnetization ( 4\piIs ) was found to be 6900 gauss and then crystalline anisotropy constant K was calculated to be 9.3 \times 10^{6} ergs/cm3. When cooled from the room temperature to -140° C, 4\piIs and K were hardly varied, but the both parameters were slightly decreased when heated up to about 60°C. MnAl alloy properties are magnetically excellent in themselves, but it is very interesting to note that they are remarkably improved by adding up to two percent boron. The improved magnetic characteristics are as follows: 1) Ferromagnetic phase is present in the sample of water-quenched state after a solid solution treatment of 1100°C; 2) Needle-shaped crystals are precipitated into the matrix as the secondary ferromagnetic phase; 3) The more the ternary MnAl-B alloys are swaged, the better magnetic properties are gained, for example, Br 5900 G, B H c 1780 Oe, and (BH)max 3.5 M.G.Oe at 70 percent reduction of 71.0 percent Mn, 26.65 percent Al, 1.76 percent B alloy.