Rare Earth Magnets

What are Rare Earth Magnets?

Rare-earth magnets fall under the category of permanent magnets having a composition of alloys manufactured from rare earth element (mostly elements from lanthanide series, and  alloys).

They were developed in the years 1970s and 1980s and are the strongest types of permanent magnets. They produce magnetic fields that are quite stronger than other types of magnets like alnico and ferrite magnets.  The rare earth magnets have a field strength of more than 1.4 tesla, whereas ceramic and ferrite magnets have fields of 0.5 to 1.0 Tesla.  Some other types of magnetostrictive rare earth magnets such as the Terfenol-D find applications in the audio systems.

  • Rare earth magnets tend to be highly fragile and also would be exposed to corrosion, so they would be typically coated or plated to protect them from breaking or crumbling into powder.
  • The term “rare earth” could be confusing as these metals are not particularly rare or precious; they are usually abundant in plenty like tin or lead.

Types of Rare Earth Magnets

There are two types of rare earth magnets:

  • Samarium-cobalt
  • Neodymium

Samarium-cobalt Magnets: 

  • These are the first group of rare earth magnets discovered, which are less used unlike neodymium magnets owing to their cost being higher and magnetic field strength being weaker.
  • The samarium-cobalt has a Curie temperature being higher and has a higher resistance to oxidation, but sintered samarium-cobalt magnets are fragile and vulnerable to chipping and cracking, and might crumble when exposed to thermal shock.

Neodymium Magnets:

  • These types of magnets were discovered during the 1980s and are the most affordable and strongest type of rare-earth magnet. They are manufactured out of an alloy of iron, neodymium and boron, termed by the abbreviation NIB.
  • Neodymium magnets have a higher coercivity. They are of use in various applications such as electric motors for jewellery clasps, hard disk drives and magnetic hold-downs.

How are Rare Earth Magnets made?

  • Rare earth magnets are made from the rare earth elements whose atomic numbers are between 57 and 71.
    The most common and strongest rare earth magnet is obtained from the alloys of neodymium, boron, and iron.
  • To make rare earth magnets, solid ingots of neodymium, boron, and iron are ground into powder. This process of making rare earth magnet consists of three phases.
  • In the first phase, the ingots are crushed into coarse particles mechanically, in the second phase they are ground into finer particles. In the final phase, these particles are jet milled into spherical particles with few micron diameters.
  • In jet milling, high pressure gas is used to make particles smaller in size as well as to maintain specific size.
    This powder is transferred to steel molds to get the shape of the magnet.
  • Rubber molds are used for neodymium alloy and this process is known as isostatic pressing.
  • The magnetic properties are provided by applying magnetic field of the range 4 Tesla.
  • Liquid phase sintering is used after applying magnetic field and the temperature used is 1000°C.
  • The final procedure is to coat the rare earth magnets based on their applications. There are many choices such as gold, nickel, zinc, and tin. But the most common is epoxy resin.

Uses of Rare Earth Magnets

Common Applications:  Rare-Earth Magnets have some common applications like:

  • Bicycle dynamos
  • Permanent magnet motors present in cordless tools
  • Magnetic resonance imaging devices
  • Audio headphones
  • High-performance AC servo motors
  • Computer hard disk drives
  • Industrial uses such as maintaining product purity, equipment protection, and quality control
  • Wind turbine generators
  • Integrated starter-generators and traction motors generators in electric vehicles
  • Fishing reel brakes

Other Applications: Rare-earth magnets have other applications like:

  • LED Throwies
  • Launched roller coaster
  • Electrodynamic bearings
  • Linear motors (used in magnetic levitation trains)
  • Electric guitar pickups
  • Stop motion animation
  • Diamagnetic levitation experimentation – It is the study of the superconductor dynamics and magnetic field levitation
  • Neodymium magnet toys

Difference between Rare Earth Magnet and Ceramic Magnet

Parameter Rare earth magnet Ceramic magnet
Chemical composition These are obtained from the rare earth elements These are obtained from strontium and barium ferrite
Magnetic field strength 26 and 40 are the magnetic field strength of SmCo and NdFeB respectively 3.5 is the magnetic field strength
Resistance to thermal stress 150°C 300°C
Thermal stress beyond Tmax 310° 460°

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