Rare-earth magnets are strong permanent magnets made from alloys of rare earth elements. Developed
in the 1970s and '80s, rare-earth magnets are the strongest type of permanent magnets
made, producing significantly stronger magnetic fields than other types such as ferrite or
alnico magnets. The magnetic field typically produced by rare-earth magnets can be in excess
of 1.4 teslas, whereas ferrite or ceramic magnets typically exhibit fields of 0.5 to
1 tesla. There are two types: neodymium magnets and samarium-cobalt magnets. Rare earth magnets
are extremely brittle and also vulnerable to corrosion, so they are usually plated or
coated to protect them from breaking, chipping, or crumbling into powder.
The term "rare earth" can be misleading as these metals are not particularly rare or
precious; they are about as abundant as tin or lead. The development of rare earth magnets
began around 1966, when K. J. Strnat and G. Hoffer of the US Air Force Materials Laboratory
discovered that an alloy of yttrium and cobalt, YCo5, had by far the largest magnetic anisotropy
constant of any material then known.
Explanation of strength The rare earth elements are metals that are
ferromagnetic, meaning that like iron they can be permanently magnetized, but their Curie
temperatures 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 have Curie temperatures well above room temperature. Rare earth magnets
are made from these compounds. The advantage of the rare earth compounds
over other magnets is that their crystalline structures have very high magnetic anisotropy.
This means that a crystal of the material is easy to magnetize in one particular direction,
but resists being magnetized in any other direction.
Atoms of rare earth elements can retain high magnetic moments in the solid state. This
is a consequence of incomplete filling of the f-shell, which can contain up to 7 unpaired
electrons with aligned spins. Electrons in such orbitals are strongly localized and therefore
easily retain their magnetic moments and function as paramagnetic centers. Magnetic moments
in other orbitals are often lost due to the strong overlap with their neighboring electrons;
for example, electrons participating in covalent bonds form pairs with zero net spin.
High magnetic moments at the atomic level in combination with a stable alignment of
those atoms results in a high magnetic field strength.
Magnetic properties Some important properties used to compare
permanent magnets are: remanence, which measures the strength of the magnetic field; coercivity,
the material's resistance to becoming demagnetized; energy product, the density of magnetic energy;
and Curie temperature, the temperature at which the material loses its magnetism. Rare
earth magnets have higher remanence, much higher coercivity and energy product, but
lower Curie temperature than other types. The table below compares the magnetic performance
of the two types of rare earth magnet, neodymium and samarium-cobalt, with other types of permanent
magnets. Types
Samarium-cobalt
Samarium-cobalt magnets, the first family of rare earth magnets invented, are less used
than neodymium magnets because of their higher cost and weaker magnetic field strength. However,
samarium-cobalt has a higher Curie temperature, creating a niche for these magnets in applications
where high field strength is needed at high operating temperatures. 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
Neodymium magnets, invented in the 1980s, are the strongest and most affordable type
of rare-earth magnet. They are made of an alloy of neodymium, iron and boron, sometimes
abbreviated as NIB. Neodymium magnets are used in numerous applications requiring strong,
compact permanent magnets, such as electric motors for cordless tools, hard drives, magnetic
holddowns and jewelry clasps. They have the highest magnetic field strength and have a
higher coercivity, but they have a lower Curie temperature and are more vulnerable to oxidation
than samarium-cobalt magnets. Corrosion can cause unprotected magnets to spall off a surface
layer, or to crumble into a powder. Use of protective surface treatments such as gold,
nickel, zinc and tin plating and epoxy resin coating can provide corrosion protection.
Originally, the high cost of these magnets limited their use to applications requiring
compactness together with high field strength. Both the raw materials and the patent licenses
were expensive. Beginning in the 1990s, NIB magnets have become steadily less expensive,
and the low cost has inspired new uses such as magnetic construction toys.
Hazards The greater force exerted by rare earth magnets
creates hazards that are not seen with other types of magnet. Magnets larger than a few
centimeters are strong enough to cause injuries to body parts pinched between two magnets,
or a magnet and a metal surface, even causing broken bones. Magnets allowed to get too near
each other can strike each other with enough force to chip and shatter the brittle material,
and the flying chips can cause injuries. There have even been cases where young children
who have swallowed several magnets have had a fold of the digestive tract pinched between
the magnets, causing injury and in one case intestinal perforations, sepsis and death.
Applications Since their prices became competitive in the
1990s, neodymium magnets have been replacing Alnico and ferrite magnets in the many applications
in modern technology requiring powerful magnets. Their greater strength allows smaller and
lighter magnets to be used for a given application. Common applications
Common applications of rare-earth magnets include:
computer hard drives wind turbine generators
audio speakers / headphones bicycle dynamos
fishing reel brakes permanent magnet motors in cordless tools
high-performance AC servo motors traction motors and integrated starter-generators
in hybrid and electric vehicles mechanically powered flashlights, employing
rare earth magnets for generating electricity in a shaking motion
industrial uses such as maintaining product purity, equipment protection, and quality
control Other applications
Other applications of rare-earth magnets include: Linear motors
Stop motion animation as tie-downs when the use of traditional screw and nut tie-downs
is impractical Diamagnetic levitation experimentation, the
study of magnetic field dynamics and superconductor levitation
Electrodynamic bearings Launched roller coaster technology found on
roller coaster and other thrill rides LED throwies, small LEDs attached to a coin
battery and a rare earth magnet Neodymium magnet toys
Electric guitar pickups Miniature figures, for which rare-earth magnets
have gained popularity in the miniatures gaming community for their small size and relative
strength assisting in basing and swapping weapons between models
Rare earth free permanent magnets The United States Department of Energy has
identified a need to find substitutes for rare earth metals in permanent magnet technology,
and has begun funding such research. The Advanced Research Projects Agency has sponsored a Rare
Earth Alternatives in Critical Technologies program, to develop alternative materials.
In 2011, ARPA-E awarded 31.6 million dollars to fund Rare-Earth Substitute projects.
References
Further reading Edward P. Furlani, "Permanent Magnet and Electromechanical
Devices: Materials, Analysis and Applications", Academic Press Series in Electromagnetism.
ISBN 0-12-269951-3. Peter Campbell, "Permanent Magnet Materials
and their Application"(1996). ISBN 978-0-521-56688-9. Brown, D.N.; B. Smith, B.M. Ma, P. Campbell.
"The Dependence of Magnetic Properties and Hot Workability of Rare Earth-Iron-Boride
Magnets Upon Composition". IEEE Transactions on Magnetics 40: 2895–2897. Bibcode:2004ITM....40.2895B.
doi:10.1109/TMAG.2004.832240. ISSN 0018-9464. External links
MMPA 0100-00, Standard Specifications for Permanent Magnet Materials
Edwards, Lin. "Iron-nitrogen compound forms strongest magnet known". PhysOrg.