1346 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



lattice site gives rise to high conductivity and hence high dielectric loss. 

 Conversely, when a ferrite is carefully prepared so that all of the con- 

 stituents are present in exactly stoichiometric porportion, and when the 

 possibility of multiple valence states is eliminated the conductivity is 

 very low. To illustrate this point a series of measurements is reported in 

 which the iron content of theferriteswas carefully varied about stoichiom- 

 etry in a nickel-zinc ferrite. These measurments are discussed at a 

 later point in this paper. 



A ferrite is made by reacting a mixture of metallic oxides at a tempera- 

 tures below the melting point of these oxides. As the oxides react a new 

 crystal structure is evolved in which the metallic ions occupy positions 

 in the interstices of a close-packed oxygen lattice. There is a very well 

 authenticated theory due to Neel^ explaining the way in which the spin 

 orientations of the ions are distributed in the two types of lattice site 

 which exist in the Spinel oxygen lattice. Whenever metal ions in more 

 than one valence state occupy the same type of site, e.g., the octahedral 

 position, there is a possibility for the easy transfer of an electron from 

 one to the other since the crystal structure is unchanged by the trans- 

 fer.* In the case of nickel ferrite which has the composition NiOFe203 an 

 excess of iron will tend to replace some nickel atoms by entering the 

 lattice in the divalent state. Since the remainder of the iron is trivalent, 

 comparatively high conductivity is observed. The problem of producing 

 ferrites with extremely low dielectric losses appears to be fairly well 

 understood and is progressing satisfactorily. By choosing the proper set 

 of metal ions to insure the absence of multiple valence states and by 

 maintaining the proper oxygen stoichiometry one may be able to achieve 

 loss tangents as low as 0.001. The subject of dielectric losses is well 

 covered in the literature.^' ^° 



Curves E, F and G, Fig. 6 



The loss mechanisms indicated in Fig. 5 by Curves E and F all arise 

 from the particular behavior of ferrites in waveguides as differentiated 

 from the plane wave theory. 



For example, the erratic behavior indicated by Curve E has been 

 shown to be due to the presence of higher order modes in the ferrite 

 region in a waveguide, and the subsidiary hump on the absorption Curve 

 F has been shown to be a "cavity resonance" which is strongly dependent 



• L. Neel, Physica, 16, pp. 350-53, 1950, and Zeit. Anorg. Chem., 262, pp. 175- 

 184, 1950. 



» E. J. W. Vcrwey and J. II. DeBoer, Rec. des Travaus Chemiques des Pays- 

 Bas, 56, pp. 531-54, 1936. 



10 E. J. W. Vcrwey et al., Phillips Res. Reps., 5, pp. 173-187, 1950. 



