There are three main types of magnets:
Permanent magnets
Temporary magnets
Electromagnets
Permanent Magnets
Permanent magnets are those we are most familiar with, such as the magnets hanging onto our refrigerator doors. They are permanent in the sense that once they are magnetized, they retain a level of magnetism. As we will see, different types of permanent magnets have different characteristics or properties concerning how easily they can be demagnetized, how strong they can be, how their strength varies with temperature, and so on.
Temporary Magnets
Temporary magnets are those which act like a permanent magnet when they are within a strong magnetic field, but lose their magnetism when the magnetic field disappears. Examples would be paperclips and nails and other soft iron items.
Electromagnets
An electromagnet is a tightly wound helical coil of wire, usually with an iron core, which acts like a permanent magnet when current is flowing in the wire. The strength and polarity of the magnetic field created by the electromagnet are adjustable by changing the magnitude of the current flowing through the wire and by changing the direction of the current flow.
Materials used for permanent magnets
There are four classes of permanent magnets:
Neodymium Iron Boron (NdFeB or NIB)
Samarium Cobalt (SmCo)
Alnico
Ceramic or Ferrite
This table gives us some of the special characteristics of the four classes of magnets.
Br is the measure of its residual magnetic flux density in Gauss, which is the maximum flux the magnet is able to produce. ( 1Gauss is like 6.45 lines/sq in)
Hc is the measure of the coercive magnetic field strength in Oersted, or the point at which the magnet becomes demagnetized by an external field. ( 1Oersted is like 2.02 ampere-turns/inch)
BHmax is a term of overall energy density. The higher the number, the more powerful the magnet.
Tcoef of Br is the temperature coefficient of Br in terms of % per degree Centigrade. This tells you how the magnetic flux changes with respect to temperature. -0.20 means that if the temperature increases by 100 degrees Centigrade, its magnetic flux will decrease by 20%!
Tmax is the maximum temperature the magnet should be operated at. After the temperature drops below this value, it will still behave as it did before it reached that temperature (it is recoverable). (degrees Centigrade)
Tcurie is the Curie temperature at which the magnet will become demagnetized. After the temperature drops below this value, it will not behave as it did before it reached that temperature. If the magnet is heated between Tmax and Tcurie, it will recover somewhat, but not fully (it is not recoverable). (degrees Centigrade)
(please note that this data is from www.hs-magnets.com)
Material Br Hc BHmax Tcoef of Br Tmax Tcurie
NdFeB 12,800 12,300 40 -0.12 150 310
SmCo 10,500 9,200 26 -0.04 300 750
Alnico 12,500 640 5.5 -0.02 540 860
Ceramic or Ferrite 3,900 3,200 3.5 -0.20 300 460
Both the Neodymium Iron Boron and the Samarium Cobalt magnets are generally known as rare earth magnets since their compounds come from the rare earth or Lanthanide series of the periodic table of the elements. They were developed in the 1970's and 1980's. As can be seen in the table, these are the strongest of the permanent magnets, and are difficult to demagnetize. However, the Tmax for NdFeB is the lowest.
Alnico is made of a compound of aluminum, nickel and cobalt. Alnico magnets were first developed in the 1940's. As can be seen in the table, this magnet is least affected by temperature, but is easily demagnetized. This is the reason why bar magnets and horseshoe magnets made of alnico will easily become demagnetized by other magnets, by dropping it, and by not storing it with a keeper. Its Tmax, though, is the highest.
Ceramic or Ferrite magnets are the most popular types of magnets available today. The flexible magnets we use are a type of ceramic magnet, with the magnetic powders fixed in a flexible binder. These were first developed in the 1960's. This is a fairly strong magnet, not as easy to demagnetize as alnico, but its magnetic strength will vary the most as its temperature changes.