Ferrite Magnet

Ferrite magnets are a composite of a barium or strontium carbonate ceramic and powdered iron (III) oxide, also known as Fe2O3, or its more popular name, rust. Developed in Japan in the 1930s, ferrite magnets were once the strongest magnets in existence until rare earth magnets were developed in the 1970s, and in many instances their more powerful cousins have replaced them. However, this ferrite magnet is less brittle and is considerably cheaper to make than rare earth magnets, since its raw materials rust (90%) and strontium (or barium) carbonate (10%) are economically available and abundant. 

 

Judging by its markings, this magnet is likely to have come from a loudspeaker. Whilst ferrite magnets aren’t as powerful as neodymium magnets, they are a popular choice for everyday applications like loudspeakers and microphones. In a loudspeaker, a ferrite magnet is positioned close to a metal coil. This coil produces a magnetic field when electric current flows through it, and the direction of the magnetic field can flip when the direction of current is reversed. The ferrite magnet is fixed in position, and the nearby coil is attached to a flexible membrane made from paper or plastic. Pulses of electrical current set the coil and membrane vibrating back and forth as it is attracted to and repelled by the ferrite magnet, translating electricity into sound. Microphones use the same mechanism but in reverse; the diaphragm vibrates from your voice and the interaction between ferrite magnet and metal coil converts sound into an electrical signal. 

 

To make a ferrite magnet, the constituent powders are milled to a very small particle size, pressed into a shape, dried and sintered (heated). To enhance the pull of the final magnet, the process can be done in an electromagnetic field to orientate the magnetic powder particles.

 

Magnetic iron oxides like ferrite have a long history of use by humans, with naturally occurring magnetised ‘lodestones’ (magnetite) used in navigation as primitive compasses since antiquity. Compasses work by the principle that the Earth itself has a magnetic field, and the magnetised free-moving needle or pointer aligns itself with that field, with one end pointing towards the south magnetic pole, and the other to the north magnetic pole.