Measurements of Rotational Magnetic Losses in Ferrites at Very Low Frequencies

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Abstract
For some time this laboratory has been interested in the rotational magnetic losses in ferrites at low frequencies. Apparatus has been developed and successfully operated in which these losses are measured at frequencies of the order of 0.1 cps in fields up to about 5000 gauss and over a reasonably wide temperature range. It consists of a quartz fiber torsion pendulum (the fiber perpendicular to the field) operating in a vacuum and in a reasonably constant temperature enclosure. The principle involved is that of observing the decay of free oscillations of the system, and determining the torques due to the damped oscillatory system. By a process developed in our laboratory the samples are ground into spheres to accuracies of the order of 0.1% which can be improved it desired. In the absence of magnetic anisotropy of the sample, the method can, in principle, distinguish between Coulomb or constant torque losses (customarily assumed for rotational hysteresis loss torque) and loss torques proportional to the angular velocity (viscous). However, for all samples so far measured the anisotropy has been sufficiently large to prevent the disentangling of the Coulomb and viscous losses. (Qualitative indications of the anisotropy are obtained from the measurements and it is interesting that the losses and anisotropy seem to be proportional.) The procedure has been to assume the losses are of the constant torque type and determine them by making the best engineering fit with the decay curves.

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