Structure, Definition, and Types of Permanent Magnets

Defining permanent magnets

Magnets are metals that develop an invisible force field around themselves. These invisible fields are called magnetic fields. A magnetic field attracts ferromagnetic materials, including iron, steel, nickel, cobalt, etc. 

The field is concentrated at the ends of the magnet, called poles. A magnet comprises two poles: the north pole and the south pole. Opposite poles attract, and identical poles repel. Magnetic field lines travel from the north pole to the south pole in closed loops, and do not end at the tip of the magnet. 

The total magnetic field which passes through a given area is called the magnetic flux. Metallic objects inside or near the magnetic flux tend to experience a magnetic force. The magnetic flux intensity, denoted by H, is the extent to which a material can be magnetised.

Structure of a permanent magnet

A permanent magnet retains its magnetic properties for an extended period. The term ‘magnet’ originated from the magnetite, or lodestone, which is a naturally occurring magnet. 

Inside cells, the nucleus and the electrons act like magnets and have their magnetic fields. The magnetic fields are generated by electrons orbiting the nucleus. The magnetic field of a magnet is the sum of the nuclear spins. In ferromagnets such as iron, cobalt, and nickel, all the spins of the electrons line up, creating a magnetic field that affects the objects around it. 

Permanent magnets are formed by heating a ferromagnetic material to a specific temperature and cooling it down into a shape we want. They can be magnetised by placing them inside an external magnetic field, and they do not lose their magnetic properties.

The magnetic strength and the polarity of permanent magnets cannot be changed. They have high permeability for easy magnetisation and high coercivity and retentivity. Magnetic bistability is achieved due to constant magnetic fields. As opposed to a temporary magnet, the field can’t be turned off and on when required. 

The structure of permanent magnets can be explained using the B-H curve. This curve tells us about the strength of a magnetic field when a demagnetising field is applied. Permanent magnets are widely used in speakers, headphones, generators, fridge magnets and so on.

Types of permanent magnet

1. Ceramic magnets: Ceramic magnets or ferrite magnets are made from iron oxide and barium. They are manufactured by the process of pressing and sintering and are fragile. They are superior in terms of magnetic strength and resistance to demagnetising. Due to this, they are used widely in many instruments. Ceramic magnets have a high coercive force and are resistant to corrosion.
2. Alnico magnets: Alnico magnets are made of aluminium, nickel, cobalt and other materials that enhance magnetic abilities. They can remain stable at high temperatures and are resistant to corrosion. They are made from casting and sintering. They are stable at high temperatures and can withstand force.
3. Samarium-Cobalt (SmCo) magnets: These are made from rare earth metal with high oxidation state and magnetic strength. They are used in applications that are used at high temperatures. They have a high resistance to corrosion. SmCo magnets are costly and have low magnetic strength. They have high energy products. 
4. Neodymium Iron Boron magnets: These permanent magnets are made from Neodymium, Iron and Boron. They get oxidised easily and are moderately resistant to temperature. They are costly and used only in selective applications. These magnets are highly coercive. They have high energy products. 
5. Injection-molded magnets: These permanent magnets are used in molding different materials and are quite expensive. They have moderate energy products and coercive force. They can’t withstand high temperatures.
6. Flexible magnets: They are similar to injection-molded magnets but are produced in flat strips and sheets. They are cheap. They have a moderate coercive force and high corrosion resistance.

Conclusion

Magnets are metals that develop an invisible field around them. The invisible field is called a magnetic field. The magnetic direction can be represented as magnetic field lines which travel from the north pole to the south. A permanent magnet is a type of magnet that retains its magnetic properties over a long period. Permanent magnets are used for various industrial purposes, and they do not lose magnetisation over a large range of reverse magnetic fields.