On the Internal Friction of Ferromagnetic Substances

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In ferromagnetic substance, the origin of internal friction under low stress amplitude has been considered to be due to two kinds of eddy currents, the first named macroscopic eddy current was studied by Brown and its theoretically predicted contribution to internal friction was known to be consistent with the experimental data. But the second named microscopic eddy current has not yet been investigated except in demagnetized state. In this paper, the latter was calculated theoretically as a function of the magnetization and the results were compared with the experimental data obtained with the substances which have neglisibly small anisotropic energy. Two different theories were respectively developed under two assumptions on the generating mechanism of microscopic eddy current. The first theory which assumes that the microscopic eddy current is induced by the rotation of the spontaneous magnetization concludes that the relation between internal friction and the magnetization is given by a curve with a maximum starting from a finite value at demagnetized state, while the second assuming the displacement of the 180° magnetic domain boundary concludes that internal friction is proportional to the square of the magnetization. To confirm these theoretical results, following three series of measurements of internal friction were made using transversal vibration of about 500～1,000 cps. (1) The effect of superlattice formation on the internal friction of Ni_3Fe was observed and the results were well explained by considering both macro- and micro-scopic eddy currents. In this case, microscopic eddy current is considered to be induced by the rotation of the spontaneous magnetization. (2) Internal friction of the field-cooled Ni_3Fe was also explained by the microscopic eddy current due to the rotation of the spontaneous magnetization. (3) The behaviour of internal friction of 65-Permalloy showed remarkable difference from that of Ni_3Fe. It could be explained neither by considering the rotation of the spontaneous magnetization nor by considering the displacement of 180° domain boundaries, so that the other process must be introduced to explain the experimental facts. The internal friction of Perminvar shows intermediate character between Ni_3Fe and 65-Permalloy. It will be explained by considering both the rotation of the spontaneous magnetization and the other process which is responsible for the internal friction of 65-Permalloy.

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