Direct Observation of Domain Rotation in Ferrites

Abstract

A direct experimental observation of domain rotation in the initial permeability region of sintered nickel ferrite has been made using a technique similar to that originated by F. Bloch to observe nuclear precession. A 5-mm diameter sphere of ferrite is surrounded by crossed transmitter and received loops adjusted for minimum rf signal when the sample is demagnetized. Application of an external steady field (as low as 5 oersteds, perpendicular to both loops) then causes a measurable received loop output which cannot be eliminated by readjusting the planes of the loops. The motion of the net magnetic moment is thus rotational. In the present case, the external field is needed only to polarize the sphere, the field for Larmor precession being supplied by the equivalent anisotropy fields seen by each individual domain. Since the applied field is much smaller than these anisotropy fields, a plot of the detector loop output (per unit applied field) vs frequency delineates the rotational resonance spectrum of the polycrystalline sample. Measurements have been made from 20 Mc/sec to 800 Mc/sec and can be described qualitatively using the theory of domain rotation. In order to understand the relatively low dispersion frequency indicated by the measurements, it is hypothesized that intermediate conditions exist in some polycrystalline ferrites between the situation in which the initial permeability is purely a result of domain rotation and that in which motion of domain walls is the dominant mechanism. The distinction between the models is thus probably not very sharp.