Ferrites
The term “ferrites”—from the Latin word for iron—means different things to different scientists. To metallurgists, ferrite means pure iron. To geologists, ferrites are a group of minerals based on iron oxide. To an electrical engineer, ferrites are also a group of materials based on iron oxide, but ones that have particular useful properties: magnetic properties and dielectric properties.
Having magnetic properties means that a piece of ferrite will attract iron-based materials and will attract magnets of opposite polarity and repel magnets of like polarity. Magnetite, or lodestone, is a naturally occurring iron oxide that is considered a ferrite by both geologists and engineers. Over 2,000 years ago the ancient Greeks recognized the strange properties of lodestone, and almost 1,000 years ago the Chinese used it to invent the magnetic compass.
Having dielectric properties means that even though electromagnetic waves can pass through ferrites, they do not readily conduct electricity. This gives them an advantage over iron, nickel, and other transition metals that have magnetic properties (“ferromagnetic”) in many applications because these metals also conduct electricity. Materials can become magnetic because each of the molecules that make up the material function have a “magnetic moment” —that is they function like a very tiny magnet. When they all line up the overall material can produce a magnetic field. In the “ferrimagnetic” ferrites—as opposed to the “ferromagnetic” metals—there is not one alignment but a distinctive arrangement of parallel and perpendicular magnetic moments. This arrangement gives them their interesting properties. This effect can be achieved through several different crystal structures. Different ferrites lend themselves to different applications, as we will see. Because their magnetism depends on an orderly crystal structure, both ferromagnetic and ferrimagnetic materials can lose their magnetism if they’re heated too high or subjected to mechanical stresses
Although theoretically an engineer’s ferrite could be a single crystal or a collection of crystals grown together—like the geologist’s ferrite—in practice ferrites are made from pressing together iron oxide powders under high heat. Because of this, ferrites can be put into a ceramic or rubber matrix and molded into an endless variety of sizes and shapes. Because there are different types of ferrites, different mixtures of iron oxides—with other materials added as well—can be produced with the exact desired combination electrical and magnetic properties.
Ferrites, therefore, have many very important uses. Whenever a fixed magnet, as opposed to an electromagnet, is needed, ferrites are there. Certain types of electric generators and electric motors use fixed magnets, and ferrites are ideal for these applications. They are used as cores for inductors and transformers. Cassette and video tapes use ferrites coated onto the plastic base to record the signal. And many computers up to the 1970s used magnetic core memories where the cores were made of ferrite (in fact, because of their reliability, ferrite core memories were used in the Space Shuttle until 1990.
Perhaps the most important use of ferrites in recent times is as a medium for transmitting microwaves. This is because some ferrites at very high frequencies (beginning above about 500 MHz, and very strongly in the microwave range of 1 to 30 GHz) exhibit a nonreciprocal effect. That means that electromagnetic waves passing through them behave differently traveling in different directions. This phenomenon allows the construction of one way transmission lines, junctions that can control the “traffic” of microwaves, and other microwave control devices. Our modern telecommunications system would not be possible without ferrites.
