Ferrite magnets are a large class of magnets that include chemical compounds such as hematite (Fe23) and magnetite (Fe3O4), which is the most naturally occurring magnetic mineral on Earth. They are ceramics and magnets, meaning they attract other ferromagnetic materials and attract or repel other magnets. A ferrite magnet is the simplest type of magnet, in contrast to more sophisticated magnets like Alnico magnets or neodymium magnets (rare-earth magnets). They are also permanent magnets, unlike electromagnets, which are only magnetic when supplied with an electric current.
The first magnets that people are exposed to are usually ferrite magnets, as they are the cheapest and most common. Children are often given toys made up of iron filings that can be moved around with this type of magnet — usually magnetite. If the magnet is placed on the filings, the resulting pattern they are pulled into reveals the shape of the magnetic field. The Earth itself produces a similar magnetic field, though trillions of times larger and about 10,000 times weaker.
Ferrite magnets, also known as ferromagnetic materials, are generally classified into two categories based on their magnetic coercivity, or persistence of internal magnetism: soft ferrites and hard ferrites. These categorizations do not refer to the actual hardness of the magnets — both types are brittle ceramics — but rather their coercive force. Depending on whether a magnet is soft or hard, it may have different applications. For instance, a hard magnet might be used in a radio or a hard disk, while a soft one could be used as a transformer core or an electromagnet core.
The strength of a ferrite magnet, like any other magnet, can be measured in teslas. One tesla represents one weber per square meter, with a weber being a unit of magnetic flux such that it would produce one volt of electricity if reduced to zero in one second. Here are some examples, starting with the weaker magnetic fields and moving to stronger:
- Paramagnetic materials like aluminum or oxygen have a magnetic field of about 300 nanoteslas.
- The Earth's magnetic field has a strength of about 31 microteslas.
- The strength of a typical refrigerator magnet is about 5 milliteslas.
- A bar magnet is about 35 milliteslas.
- A loudspeaker magnet has a strength of about 1 T to 2.4 teslas.
- The strength of the most powerful continuous magnetic field yet produced in a laboratory is 45 teslas.