1156 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER, 1953 



Although we are concerned more with magnetic phenomena than with 

 the properties of particular materials, we will relate our discussion to 

 ferrites, since these are the materials in which high frequency phenomena 

 have been explored. We begin, therefore, with a brief description of these 

 materials. 



II. DESCRIPTION OF FERRITES 



The term "ferrite" as used here refers to a class of ferromagnetic 

 oxides that are structurally the same as magnetite (the naturally occur- 

 ring magnetic mineral commonly known as lodestone) and as the min- 

 eral spinel from which the structure derives its name.^' ' These com- 

 pounds form extensive solid solutions of both the substitional and the 

 subtractional type. Nickel zinc and manganese zinc ferrites are important 

 examples of the substitutional type. In these, the zinc and nickel or 

 manganese are thought to be in solid solution in magnetite (Fe304) 

 where they have directly replaced equivalent amounts of iron in the 

 lattice. An example of the subtractional type of solution is T-FeoOs- 

 Here, oxygen is considered to be in solution in magnetite, not, however, 

 having replaced iron, but having eliminated it, thus leaving vacant sites 

 in the lattice. Magnetically, the ferrites are thought of as consisting of 

 two interpenetrating lattices of metal ions whose magnetic moments 

 point in opposite directions. Since, however, these moments are in 

 general not equal, the material has a net magnetic moment. 



Ferrites are manufactured by carefully mixing oxides of the constituent 

 materials. The resulting powder is then pressed into desired shapes. These 

 formed parts are fired at a temperature of 1000°C or more to produce the 

 finished materials. The finished product is technically classed as a 

 ceramic, and among its properties is extraordinarily high resistivity 

 compared with magnetic alloys. Mechanically, ferrites have some of the 

 characteristics of ceramics. They are extremely hard and brittle and 

 cannot be machined by ordinary methods. They may be ground and 

 lapped by use of abrasive cutting tools. 



Experience has sho^vn that the properties of the finished product 

 depend upon composition (both what elements are present and in what 

 proportions) and upon heat treating conditions (atmosphere, maximum 

 firing temperature, and time of firing). It is apparent that, since there 

 are so many possible variables in manufacturing procedure, one may 

 expect a wide variety of electrical and magnetic characteristics. 



Ferrites have been commercially available for several years. NiZn 

 ferrite is being used extensively in deflection coils and flyback trans- 

 formers in television sets. MnZn ferrite has found specialized but im- 



