A REVIEW OF NEW MAGNETIC PHENOMENA 



1161 



the sample is in the form of a slab, cut to fit snugly against the walls 

 of the waveguide. 



Fig. 3 shows in a qualitative way the behavior of a typical ferrite. 

 In this figure we have plotted the real and imaginary parts of permea- 

 bility as functions of frequency. Examination of the results obtained by 

 various investigators for a number of different ferrites leads to the con- 

 clusion that they all behave in a fashion similar to that shown in Fig. 3. 

 Frequency /i usually lies in the region between 1 and 100 mc and ji 

 frequently lies in the neighborhood of 3000-4000 mc. Available data are 

 not sufficient to provide similar curves for dielectric constant, but what 

 there are indicate a decrease from the very high apparent value at low 

 frequencies to a constant value of the order of 10. This decrease generally 

 occurs somewhere around 10 mc, although it depends upon the material 

 and also probably upon sample dimensions as will be discussed below. 



The consensus is that the high experimental values of dielectric con- 

 stant observed at low frequencies result from the peculiar structure of 

 ferrites. They are considered to consist of grains of conduction material 

 (of moderately high conductivity) separated by thin layers of dielectric 

 material having a dielectric constant of the order of 20. Measurements 

 on such a structure would give very high apparent dielectric constant 

 and low Q. Such behavior was observed several years ago in samples of 

 powdered permalloy, and has also been found in samples of plastic in 

 which finely divided particles of copper have been dispersed. 



f. ^2 



Fig. 3 — Frequency characteristics of a typical ferrite. The components of 

 complex permeability (/x = m' — i^") as functions of frequency. The behavior in 

 the neighborhood of /i is attributed to domain wall motion or to dimensional 

 effects. /2 represents the frequency at which ferromagnetic resonance occurs. 



