FERRITES IN MICROWAVE APPLICATIONS 1315 



haps a few which are higher still. Therefore, we have as typical fre- 

 quencies 



/i = T^^anis ^ 300 to 3,000 mc (15) 



and 



/2 = yM, ^ 3,000 to 9,000 mc. (16) 



It is evident that at 9,000 mc only that loss associated with M, will 

 contribute to the "Low Field Loss" and since the mechanism depends 

 upon the existence of domain walls it will disappear when the material 

 is saturated in agreement with our observations. 



Curve C, Fig. 5 



Some ferrites exhibit a low-field loss which disappears at saturation 

 for only the negative component and increases for the positive com- 

 ponent. This is thought to be due to an effective anisotropy field in the 

 material. In order for such behavior to be present at 9,000 mc, however, 

 the internal field must be of the order of 240,000 ampere turns per meter. 

 While such a value of crystalline anisotropy might be found in cobalt or 

 other high anisotropy ferrites it appears to be somewhat too high for a 

 nickel-zinc ferrite such as that in which this characteristic was first ob- 

 served. However, high internal fields could result from demagnetizing 

 effects similar to those discussed under item B but differing in that the 

 poles are set up on nonmagnetic grain boundaries instead of domain 

 walls. These, of course, would persist when the body is saturated, but 

 there would be loss for only one circular component inasmuch as all of 

 the crystallites are then magnetized in the same direction. Such a loss 

 characteristic can be quite useful where one wishes to absorb one circular 

 component selectively without the necessity for applying a large dc field. 



Curve D, Fig. 5 



Dielectric losses are present in all of the ferrites which have been made 

 to date, although in some materials this loss is very low. Low dc con- 

 ductivity in itself is not a sufficient criterion of the dielectric properties 

 of a material as some ferrites appear to consist of conducting regions 

 surrounded by an insulating matrix, and these have fairly high loss 

 tangents at microwave frequencies. 



The major mechanism of dielectric loss involves the exchange of 

 electrons between ions in the crystal lattice. It has been found that the 

 presence of ions of the same metal in different valence states on the same 



