It was recently noted that the dynamic domain structure in a ferromagnetic material may differ markedly from that observed under static conditions. In this investigation the dynamic size of magnetic domains in a rectangular single-crystal specimen of an iron-3% silicon alloy (2.5 cm×3.3 cm×0.22 mm with a (001) [100] orientation) was measured using the Kerr magneto-optic effect together with high-speed cinematography. Under dynamic conditions of a 100 Hz sinusoidal induction of saturation amplitude, the average domain width was reduced from the average static width of 2½ times the sheet thickness to about ½ this value. The finer dynamic domain structure could be ``frozen in'' by turning off the applied field during the middle of the magnetization cycle, but the original coarser structure always returned following an ac demagnetization. Measurements in the frequency range 20–100 Hz showed that above a threshold frequency domain width decreased with frequency as (f)−1/2. With a varying amplitude of induction the domain width decreased as (Bmax)−1, above a threshold induction.
