Magnetics: miniaturization of permanent, high-performance magnets

• Applications
• Magnetic Properties of Rare-Earths

• Applications

Permanent magnets are materials that retain their magnetic properties after having been exposed to a magnetic field. They are found in a great variety of materials used in a significant and increasing number of industrial and commercial applications:

• micro-motors and capacitors used in the fields of: · computers: miniaturization of high performance magnets · audio-visuals (speakers, VCR's, etc.) · automobiles (directional aids, power windows, ABS, dashboard computers, etc.) · household electronics (dishwashers, washing machines, air conditioners, etc.)

• timing motors used in:
- industrial robots
- military and space technology
- clocks and watches

New powders with magnetic properties have been developed for new applications:
- re-recordable CDs and DVDs (Digital Video Disc) (magneto-optics)
- Sonar using alloys that change form under the effect of a magnetic field.

This change in the properties of magnets is illustrative of progress that can be directly related to the use of Rare Earths:
- miniaturizing systems
- increased magnetic performance
- smaller sizes and more complex forms
- multiple high tech applications

• Magnetic Properties of Rare-Earths

Rare-earths have exceptional magnetic properties. Their anisotropic magnetocrystalline constancy is 10 to 100 times greater than that normally encountered, magnetization under absolute saturation is vastly superior to iron; however, the magnetic order exists only in temperatures that are below ambient temperature, the internal position of the 4¦ orbital leading to weak annealing both at the level of direct interactions between nearby atoms and magnetic annealing with elongated exchanges via conduction electrons. At ambient temperature, Rare Earths, then, become paramagnetic or diamagnetic, the highest order temperature (or Curie temperature, Tc) being around 293 K, below which gadolinium ferromagnetism appears.In an effort to increase Curie temperatures, an attempt was made to associate Rare Earths with elements that contained both good magnetic properties and high Curie temperatures (greater than 400-500°C), such as the transition elements of iron, cobalt or nickel.

The enormous amount of work done since the 1960's on the preparation and categorization of rare-earth / metal alloys initially resulted in industrializing samarium-cobalt magnets (SmCo₅ or Sm₂Co₁₇) which had a Tc greater than 700°C and per unit volume energy product greater than 0.16 MJ/m³ (20MG.Oe), for coercivity of approximately 800 kA/m (10 kOe) contrasted with values that did not surpass 32 kJ/m³
(4MG.Oe) and 320 kA/m (4 kOe), respectively, for traditional ferrites or Al-Ni-Co (Alnico) alloys. This performance made it possible to obtain intense magnetic energy in low volume, and subsequently, miniaturization utilized in timing motors, or, even more spectacularly, in audiovisuals where the use of the Sm-Co magnet, for example, made it possible to refine the miniature earphones used with Walkman®s.

Even more high-performing neodymium-iron-boron magnets appeared at the start of the 1980's. The Nd₂Fe₁₄B structure, with doping to increase the Curie temperature or coercivity, delivers the highest performance ever achieved industrially to date: per unit volume energy product (B.H)max greater than 320kJ/m³ (40 MG. Oe) [calculated theoretical value being 512 kJ/m³ (64 MG.Oe)] with coercivity Hc around 960 Ka/m (12kOe). These magnets, being developed rapidly by industry, are being examined thoroughly by the automobile industry (miniaturized electromagnets) and they have a very promising future. In addition to other industrial or medical uses (NMR imaging, for example), they are projected to become essential elements in a variety of home applications such as stereos, telephones, or home appliances.

A new area of application for the magnetic properties of rare earth transition element alloys is under development. It is magneto-optical recording where the magnetic (high Hc, low Tc) and optical (high Kerr rotation angle) properties of amorphous alloys (Gd, Tb)/(Co, Fe) are utilized to obtain very high density recording (20 Mbit/cm²) in erasable and re-recordable systems for future use in laser technology. A mass production illustration of this is perfection of the minidisk.

• Contacts

If you would like more informations, please contact:
ec-electronics@eu.rhodia.com