Rotational hysteresis in polycrystalline alloys

Rotational hysteresis has been studied at very low rotational speeds in nearly isotropic polycrystalline materials. Measurements were made on disc shaped samples of pure iron, 1.2% C steel and mild steel. The magnitude and direction of the moment were measured as the disc was rotated starting from an initial magnetization parallel to the external field. For large angles of rotation the measurements give the usual hysteresis loss measured in continually rotating specimens. However it was also possible to measure the approach to the steady state and the response of the moment to small changes in the direction of the external field. The results are explained in terms of two models, one appropriate to low fields, below the maximum in the rotational loss curve, the other to the high field regime. At low fields the losses are due to domain wall motion, and the model predicts that the ratio of alternating to rotational loss is
\pi\sqrt{2}
. At high fields the loss is thought to be due to a mechanism not present in alternating fields. This is the unstable rotation of the magnetization in each grain past its direction of difficult magnetization. To test the model Barkhausen noise measurements were made under alternating and rotating conditions. The results are compared with numerical calculations on the behaviour of 25 interacting dipoles.