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RARE-EARTH MAGNET

Rare-earth magnets are strong permanent magnets made from alloys of rare-earth elements.
There are two types: neodymium magnets and samarium–cobalt magnets. The magnetic field typically produced by rare-earth magnets can exceed 1.4 teslas, whereas ferrite or ceramic magnets typically exhibit fields of 0.5 to 1 tesla.

Explanation of strength

The rare-earth (lanthanide) elements are metals that are ferromagnetic, meaning that like iron they can be magnetized to become permanent magnets, but their Curie temperatures (the temperature above which their ferromagnetism disappears) are below room temperature, so in pure form their magnetism only appears at low temperatures. However, they form compounds with the transition metals such as iron, nickel, and cobalt, and some of these compounds have Curie temperatures well above room temperature. Rare-earth magnets are made from these compounds.

Magnetic properties

Some important properties used to compare permanent magnets are: remanence (Br), which measures the strength of the magnetic field; coercivity (Hci), the material's resistance to becoming demagnetized; energy product (BHmax), the density of magnetic energy; and Curie temperature (TC), the temperature at which the material loses its magnetism. Rare-earth magnets have higher remanence, much higher coercivity and energy product, but (for neodymium) lower Curie temperature than other types. The table below compares the magnetic performance of the two types of rare-earth magnets, neodymium (Nd2Fe14B) and samarium-cobalt (SmCo5,Sm2Co17), with other types of permanent magnets.

Type of Magnet    

 Br (T)

Hci (kA/m) 

 (BH)max (kJ/m3)

 TC (°C)

Nd2Fe14B (sintered)  

 1.0–1.5

750–2000 

 200–440 

  310–350

Nd2Fe14B (bonded)  

 0.6–0.7 

 600–1200

 60–100 

310–400 

SmCo5 (sintered)   

 0.8–1.10 

 600–2000

 120–200

 720

Sm2Co17 (sintered)   

 0.9–1.15

 450–1300 

 150–240

 800

Alnico (sintered)

 0.6–1.4 

275 

10–88 

 700–860

Alnico (casting)

 

 

 

 

Sr-ferrite (sintered) 

0.2–0.4 

100–300 

10–40

 450

MnZn-ferrite(sintered)