Using Magnetic Fields and Storing Data

Abstract

If you were asked to give an example of a magnetic material instinctively you would probably say iron. It is a good example, but in its pure form iron is not a very useful magnet. Ceramics can be magnetic too and they were the first magnets known to humans. About 600,000 t of ceramic magnets are produced each year making them, in terms of volume, commercially more important than metallic magnets. The largest market segment is hard ferrites (permanent magnets) that are used in a range of applications including motors for electric toothbrushes and windshield wipers in automobiles, refrigerator door seals, speakers, and stripes on the back of the ubiquitous credit and ATM cards. Soft ferrites can be magnetized and demagnetized easily and are used in cell telephones, transformer cores, and, now to a somewhat lesser extent, magnetic recording.

Ferrite is a term used for ceramics that contain Fe₂O₃ as a principal component.

Magnetism has probably fascinated more people, including Socrates and Mozart (listen to Così fan tutte), over the years than any other materials property. For over four thousand years the strange power of magnets has captured our imagination. Yet it remains the least well understood of all properties. In this chapter we will start by describing some of the basic characteristics of magnetic materials, which often contain one of the first row transition metals, Fe, Co, or Ni. The electron arrangement in the 3d level of these atoms is the key. The manganates are a very interesting class of magnetic ceramic. Although they are not new, the recent discovery that they exhibit colossal magnetoresistance (just like the giant magnetoresistance observed in metal multilayers only much bigger) has renewed interest in these materials. Structurally the manganates are very similar to the high-temperature superconductors (HTSCs). The similarity may be more than coincidental.