Rare Earth Magnetism | Magnetic properties and Types
Rare Earth Magnetism
Rare earth Magnetism are powerful permanent magnets composed of alloys of rare earth elements. Developed in the 1970s and 1980s, rare earth magnets are the most powerful type of permanent magnets, as they produce a significantly stronger magnetic field than other types such as ferrite or alnico magnets. The magnetic field typically produced by rare earth magnets can exceed 1.4 tesla, while ferrite or ceramic magnets typically exhibit fields from 0.5 to 1 tesla.
Ferrofluid on glass, with a rare earth magnet in the lower part There are two types: neodymium magnets and samarium-cobalt magnets. Rare earth Magnetismre extremely brittle and also vulnerable to corrosion, so they are usually plated or coated to prevent cracking, chipping, or breaking into dust.
The development of rare earth magnets began around 1966 when K.J. Strnat and G. Hoffer of the US Air Force Materials Laboratory found that an alloy of yttrium and cobalt, YCo 5, was by far the largest magnetic unipolar state of any material then known to be constant.
The term “rare earth” can be misleading, as some of these metals may be as abundant in the earth’s crust as tin or lead, but rare earth ore seams (such as coal or copper), hence any cubic kilometer of crust. They are “rare”. The major source is currently China.
Some countries classify rare earth metals as strategically important, and recent Chinese export restrictions on these materials have prompted some to begin research programs to develop stronger magnets that can be used to identify rare earth metals. is not required.
Neodymium magnets (small cylinders) lifting steel balls. As shown here, rare earth Magnetism can easily lift thousands of times their own weight.
Some of the important properties used to compare permanent magnets are: residual (br), which measures the strength of the magnetic field; The resistance of the material to coercion (HCI), becoming paramagnetic; energy product ( B H max ), the density of the magnetic energy; and the Curie temperature (Tc), the temperature at which the material loses its magnetism.
Rare earth Magnetism have a high residue, very high coercivity and energy product, but (for neodymium) the Curie temperature is lower than for other types. The table below compares the magnetic performance of two types of rare-earth magnets, neodymium (Nd 2 Fe 14 B) and samarium-cobalt (SmCo 5 ) with other types of permanent magnets.
Samarium-cobalt magnets (chemical formula: SmCo5), the first family of rare-earth Magnetism envented, are less used than neodymium magnets due to their higher cost and lower magnetic field strength.
However, samarium-cobalt has a high Curie temperature, which creates a niche for these magnets in applications where high field strengths at high operating temperatures are required. They are highly resistant to oxidation, but sintered samarium-cobalt magnets are brittle and prone to chipping and cracking and may fracture when subjected to thermal shock.
Neodymium magnets, invented in the 1980s, are the strongest and most economical type of rare-earth magnet. They are composed of an alloy of neodymium, iron and boron (Nd 2 Fe 14b), sometimes abbreviated as NIB.
Neodymium magnets are used in many applications that require strong, compact permanent magnets, such as electric motors for cordless tools, hard disk drives, magnetic hold downs, and jewelry clasps.
They have the highest magnetic field strength and a high coercivity (which makes them magnetically stable), but they have a lower Curie temperature and are more vulnerable to oxidation than samarium-cobalt magnets.
Vulnerable to corrosion, magnets can cause a surface layer to spell or crumble off into a powder. The use of protective surface treatments such as gold, nickel, zinc, and tin plating and epoxy-resin coating can provide corrosion protection; Most neodymium magnets use nickel plating to provide a strong protection.
Basically, the high cost of these magnets limited their use to applications requiring compactness with high field strength. Both raw materials and patent licenses were expensive. However, since the 1990s, NIB magnets have become increasingly less expensive, and their low cost has inspired new uses such as magnetic construction toys.