Full Text
From Current Literature
PERMANENT MAGNET MADE FROM MnBi POWDER
The idea of creating permanent magnets by emulsifying or pressing oriented powder of ferromagnetic substances is not new. As early as 1930, F. J. Frenkel and J. Dorfman drew attention to the fact that small ferromagnetic particles whose dimensions are close to domain dimensions possess a distinctly pronounced magnetization. On the other hand, the search for new materials for making permanent magnets is stimulated by the circumstance that the alloys usually used for these purposes—for example, alnico—contain scarce cobalt and nickel. The authors of the paper under review developed methods for producing permanent magnets from powder of the intermetallic compound MnBi. This material was chosen on the basis of the following considerations. Its permanent crystalline anisotropy \((K)\) has an extremely high value, of the order of \(11.6 \cdot 10^6\) erg/cm\(^3\); the saturation magnetization corresponds to 600 gauss, and the Curie temperature is \(340 \div 360^\circ\). Further, the critical particle dimensions (corresponding to the dimensions of domains) reach \(0.8\,\mu\), while the coercive force inherent in them, according to calculations, approaches 35,000 oersteds.
The greatest technological difficulty is the preparation of an alloy of Bi and Mn in sufficiently pure ferromagnetic MnBi phase. The technology developed by the authors is as follows. Fine-grained powders of Mn (16.65%) and Bi (83.35%) are mixed and melted at a temperature of \(700^\circ\)C in a slowly rotating alumina crucible (in a helium atmosphere). After a five-hour hold in the above regime, the melt temperature is slowly lowered to \(440^\circ\)C. After six hours the temperature is again slowly lowered to \(320^\circ\)C, and after another 8 hours cooling to room temperature is carried out. The alloy is then ground into powder (in a mechanical mill) to dimensions of the order of several microns, i.e., close to domain dimensions. In some cases subsequent annealing of the powder is applied for several hours.
Even in the most favorable cases, the powder obtained in this way contains, in addition to MnBi, a considerable amount of impurity in the form of unreacted Mn and Bi. Therefore, before using it to make a magnet, it is necessary to carry out an enrichment process. For this purpose a thin stream of powder is slowly poured through a glass tube passing between the poles of a magnet; the ferromagnetic part of the powder (MnBi) settles on the walls of the tube, while the nonmagnetic part passes through the tube without being retained. As a result of this operation an enriched fraction is obtained, containing about 90% MnBi, 4% Mn, and 6% Bi.
After enrichment the powder is pressed under low pressure at a temperature of \(300^\circ\)C (to prevent oxidation, pressing
is carried out in an atmosphere of an inert gas). At the same time, the particles are oriented by the action upon them of a pulsating magnetic field (up to 20,000 oersteds), parallel to the direction of pressing. Since the powder contains a certain amount of pure bismuth, heating to 300° leads to the appearance of a liquid phase, which facilitates the orientation of the particles, ensures their effective fixation after cooling, and also helps to obtain specimens with higher density.
The specimens prepared in this way were tested in order to determine the influence of various factors on their magnetic properties. The authors established that a decrease in particle size leads to a sharp increase in the coercive force of the magnet. Likewise, the degree of orientation of the particles in the process of pressing the specimen in a strong field affects both the magnitude of the coercive force and the residual magnetization. The density of the specimen has no lesser influence. However, the use of high pressures during pressing, although it leads to an increase in density, is usually associated with a decrease in the degree of orientation of the particles, and also with deformation of the particles themselves; as a result, the qualities of magnets obtained by pressing under high pressure prove worse than when pressing under low pressure.
In conclusion, the authors give the following table, in which the characteristics of some of the powder magnets they prepared are compared with the characteristics of magnets made from solid metal:
| Magnet | Density | Residual induction | Coercive force | $2K/I_{\mathrm{sat}}$ | $(BH)_{\max}\times 10^{-6}$ |
|---|---|---|---|---|---|
| MnBi—4 . . . | 7.0 | 3,160 | 1,110 | 1,240 | 1.3 |
| MnBi—7 . . . | 7.5 | 3,400 | 1,995 | 4,650 | 1.9 |
| MnBi—11 . . . | 6.6 | 3,375 | 31,100 | 7,740 | 2.9 |
| Bismanol *) . . | 8.1 | 4,300 | 3,400 | 7,000 | 4.3 |
| Alnico II . . . | 7.1 | 7,200 | 540 | 540 | 1.6 |
| Alnico V . . . | 7.3 | 12,000 | 580 | 580 | 4.5 |
| Pt—Fe . . . | 10.0 | 5,800 | 1,570 | 1,700 | 3.0 |
| Pt—Co . . . | 11.0 | 4,500 | 2,600 | 3,600 | 3.8 |
| Silmanal . . . | 9.0 | 550 | 550 | 6,000 | 0.08 |
As is evident from the table, magnets made from pressed MnBi powder possess a coercive force considerably exceeding the coercive force of magnets of all other types. At the same time, the residual magnetization obtained with magnets of this type is substantially lower than, for example, that of magnets made from alnico. Apparently, magnets produced by the method described above will find application in apparatus where the principal requirement is not the production of the strongest possible magnetic fields, but rather high stability of permanent magnets with respect to demagnetizing factors (for example, in electrical measuring instruments).
R. G.
*) A typical specimen of a powder magnet made from MnBi, obtained by the method described above, but with an increased degree of enrichment of the powder with the ferromagnetic phase.
References
- J. Frenkel and J. Dorfman, Nature 126, 274 (1930).
- E. Adams, W. M. Hubbard and A. M. Syles, J. Appl. Phys. 23, No. 11, 1207 (1952).