1164 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER, 1953 



Fig. 4 — Typical behavior of the components of complex permeability (jx 

 — ill") in a ferrite in the neighborhood of domain wall resonance. 



cross section area, /i moved to a higher value. Brockman, Dowling and 

 Steneck attributed this to a cavity type resonance effect resulting from 

 a high dielectric constant which gave wavelengths in the material of the 

 same order of magnitude as the dimensions of the sample under test. 

 Another way" of looking at the phenomenon is to recognize that in the 

 ferrites there are displacement eddy currents which correspond to the 

 conduction eddy currents in magnetic alloys. Analysis based on this 

 approach gives resonance frequencies which are the same as those cal- 

 culated by Brockman, Dowling and Steneck. 



In any sample it is possible that both mechanisms — domain wall 

 motion and dimensional resonance — contribute to the behavior of the 

 material in Region A. Futhermore, it is evident that considerable cau- 

 tion is required in interpreting results of experiments designed to meas- 

 ure M and e as functions of frequency. One must always bear in mind 

 that what one obtains is the effective ji and e of the sample under test, 

 and the actual fx and e for the material may be different from the ob- 

 served values, depending upon the effect of sample dimensions. 



It is clear that in a design problem the communications engineer must 

 take into account the dimensions of the ferrite part as related to the 

 permeability and dielectric constant of the material and to the frequency 

 at which it is being used. This may impose a practical limitation on the 

 size of a part for a particular application. 



VIII. FERROMAGNETIC RESONANCE 



The behavior indicated in Region B of Fig. 3 is attributed to ferro- 

 magnetic resonance. This phenomenon was first observed in magnetic 

 metals by Griffiths^® and has been studied intensively in ferrites.^^ 



