With the object of improving the properties of MnAl compound suitable for the application to permanent magnet, the effect of pulverizing and swaging on the magnetic properties of MnAl, especially on its coercive force, has been studied.
When MnAl is pulverized in a ball mill,
IHC increases at the expense of
Br. Pulverizing in a hammer mill produces a striking increase in
IHC and after being pulverized 130 times in the mill,
IHC reaches the highest value of 6500 Oe. The relation between
IHC and grain size of the hammer milled MnAl powder shows close resemblance to those of Ba ferrite, MnBi and Fe
3O
4, but an increase in
IHC takes place at far more coarse grains than in the case of these typical single domain magnets. Saturation magnetization
Is, lowerd by hammer milling is recovered by tempering, and the increased
IHC is reduced. The permanent magnet properties obtained are not sufficent to practical applications. Magnetic press was not recognized to be effective.
MnAl bars swaged at room temperature by the reduction of 86% and 71% improved
IHC to be 4000 Oe and 3000 Oe respectively. Decrease in saturation magnetization by swaging was not observed. Saturation magnetization and maximum energy product of the swaged MnAl is raised by tempering. Swaging at 400°C gives the result inferior to room temperature swaging.
MnAl containing Ti, especially, the specimen having a high
Is×
IHC value (68.93%Mn, 28.16%Al, 1.94%Ti and 0.97%Cu) shows a remarkable increase in
IHC and decrease in
Is by swaging at room temperature. When tempered, this specimen acquires excellent permanent magnet properties. For example, the specimen swaged by 84% and tempered at 500°C for 1 hr has the following properties;
Br=4840 gauss,
IHC=3100 Oe,
BHC=2400 Oe and (
BH)
max=4.5×10
6 gauss·Oe. This (
BH)
max value is higher than that obtained by Koch et al. with MnAl containing no additional element.
On the MnAl specimens containing Ti, swaged and tempered, the magnetic anisotropy energy derived from approach to saturation seems to be proportional to
Is×
IHC. This fact may give a suggestion on the mechanism of coercive force of swaged MnAl.
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