Rare-earth magnet - AbsoluteAstronomy.com

Rare-earth magnets are strong permanent magnets made from alloy

Alloy

An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...

s of rare earth element

Rare earth element

As defined by IUPAC, rare earth elements or rare earth metals are a set of seventeen chemical elements in the periodic table, specifically the fifteen lanthanides plus scandium and yttrium...

s. Developed in the 1970s and 80s, rare-earth magnets are the strongest type of permanent magnets made and have significant performance advantages over ferrite

Ferrite (magnet)

Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...

or alnico

Alnico

Alnico is an acronym referring to iron alloys which in addition to iron are composed primarily of aluminium , nickel and cobalt , hence al-ni-co, with the addition of copper, and sometimes titanium. Alnico alloys are ferromagnetic, with a high coercivity and are used to make permanent magnets...

magnets. The magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

typically produced by rare-earth magnets can be in excess of 1.4 teslas

Tesla (unit)

The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...

, whereas ferrite or ceramic magnets typically exhibit fields of 0.5 to 1 tesla. There are two types: neodymium magnet

Neodymium magnet

A neodymium magnet , the most widely-used type of rare-earth magnet, is a permanent magnet made from an alloy of neodymium, iron, and boron to form the Nd2Fe14B tetragonal crystalline structure. Developed in 1982 by General Motors and Sumitomo Special Metals, neodymium magnets are the strongest...

s and samarium-cobalt magnet

Samarium-cobalt magnet

A samarium–cobalt magnet, a type of rare earth magnet, is a strong permanent magnet made of an alloy of samarium and cobalt. They were developed in the early 1970s. They are generally the second-strongest type of magnet made, less strong than neodymium magnets, but have higher temperature ratings...

s. Rare earth magnets are extremely brittle and also vulnerable to corrosion, so they are usually plated

Plating

Plating is a surface covering in which a metal is deposited on a conductive surface. Plating has been done for hundreds of years, but it is also critical for modern technology...

or coated to protect them from breaking and chipping.

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. Interest in rare earth compounds as permanent magnets began in 1966, when K. J. Strnat and G. Hoffer of the US Air Force Materials Laboratory discovered that an alloy of yttrium

Yttrium

Yttrium is a chemical element with symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and it has often been classified as a "rare earth element". Yttrium is almost always found combined with the lanthanides in rare earth minerals and is...

and cobalt

Cobalt

Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

, YCo₅, had by far the largest magnetic anisotropy

Magnetic anisotropy

Magnetic anisotropy is the direction dependence of a material's magnetic properties. In the absence of an applied magnetic field, a magnetically isotropic material has no preferential direction for its magnetic moment while a magnetically anisotropic material will align its moment with one of the...

constant of any material then known.

Explanation of strength

The rare earth (lanthanide

Lanthanide

The lanthanide or lanthanoid series comprises the fifteen metallic chemical elements with atomic numbers 57 through 71, from lanthanum through lutetium...

) elements are metals that are ferromagnetic, meaning that like iron

Iron

Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

they can be 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 metal

Transition metal

The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...

s 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

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 orbital

Atomic orbital

An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...

s are strongly localized and therefore easily retain their magnetic moments and function as paramagnetic centers

Paramagnetism

Paramagnetism is a form of magnetism whereby the paramagnetic material is only attracted when in the presence of an externally applied magnetic field. In contrast with this, diamagnetic materials are repulsive when placed in a magnetic field...

. Magnetic moments in other orbitals are often lost due to strong overlap with the neighbors; 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 (high anisotropy) results in high strength.

Magnetic properties

Some important properties used to compare permanent magnets are: remanence

Remanence

Remanence or remanent magnetization is the magnetization left behind in a ferromagnetic material after an external magnetic field is removed. It is also the measure of that magnetization. Colloquially, when a magnet is "magnetized" it has remanence...

(B_r), which measures the strength of the magnetic field; coercivity

Coercivity

In materials science, the coercivity, also called the coercive field or coercive force, of a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation...

(H_ci), the material's resistance to becoming demagnetized; energy product (BH_max), the density of magnetic energy; and Curie temperature (T_c), 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 magnet, neodymium (Nd₂Fe₁₄B) and samarium-cobalt (SmCo₅), with other types of permanent magnets.

Magnet	B_r (T)	H_ci (kA/m)	(BH)_max (kJ/m³)	T_c (°C)
Nd₂Fe₁₄B (sintered)	1.0–1.4	750–2000	200–440	310–400
Nd₂Fe₁₄B (bonded)	0.6–0.7	600–1200	60–100	310–400
SmCo₅ (sintered)	0.8–1.1	600–2000	120–200	720
Sm(Co,Fe,Cu,Zr)₇ (sintered)	0.9–1.15	450–1300	150–240	800
Alnico (sintered)	0.6–1.4	275	10–88	700–860
Sr-ferrite (sintered)	0.2–0.4	100–300	10–40	450

Samarium-cobalt

Samarium-cobalt magnets (chemical formula: Sm

Samarium

Samarium is a chemical element with the symbol Sm, atomic number 62 and atomic weight 150.36. It is a moderately hard silvery metal which readily oxidizes in air. Being a typical member of the lanthanide series, samarium usually assumes the oxidation state +3...

Cobalt

₅), 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 temperature

Operating temperature

An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...

s. 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

Thermal shock

Thermal shock is the name given to cracking as a result of rapid temperature change. Glass and ceramic objects are particularly vulnerable to this form of failure, due to their low toughness, low thermal conductivity, and high thermal expansion coefficients...

Neodymium

Neodymium is a chemical element with the symbol Nd and atomic number 60. It is a soft silvery metal that tarnishes in air. Neodymium was discovered in 1885 by the Austrian chemist Carl Auer von Welsbach. It is present in significant quantities in the ore minerals monazite and bastnäsite...

magnets, invented in the 1980s, are the strongest and most affordable type of rare-earth magnet

Magnet

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...

. Neodymium alloy (Nd

Neodymium

₂Fe

Iron

₁₄B

Boron

Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

) is made of neodymium

Neodymium

, iron

Iron

and boron

Boron

. Neodymium magnets are typically used in most computer hard drives and a variety of audio speakers

Loudspeaker

A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...

. They have the highest magnetic field strength, but are inferior to samarium-cobalt in resistance to oxidation and Curie temperature. Use of protective surface treatments such as gold

Gold

Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

, nickel

Nickel

Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

, zinc

Zinc

Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...

and tin

Tin

Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...

plating and epoxy resin coating can provide corrosion

Corrosion

Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

protection where required.

Originally, the high cost of these magnets limited their use to applications requiring compactness together with high field strength. Both raw materials and patent licenses were expensive. Beginning in the 1990s, NIB

Neodymium magnet

magnets have become steadily less expensive, and the low cost has inspired new uses such as magnetic building 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 and fractures. 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

Gastrointestinal tract

The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....

pinched between the magnets, causing injury or death. The stronger magnetic fields can be hazardous also, and can erase magnetic media such as hard disk

Hard disk

A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...

s and credit cards, and magnetize the shadow mask

Shadow mask

The shadow mask is one of two major technologies used to manufacture cathode ray tube televisions and computer displays that produce color images. The other approach is aperture grille, better known by its trade name, Trinitron. All early color televisions and the majority of CRT computer monitors...

s of CRT

Cathode ray tube

The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...

type monitors at a significant distance.

Applications

Since their prices became competitive in the 1990s, neodymium magnets have been replacing Alnico

Alnico

and ferrite

Ferrite (magnet)

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
Wind turbine
A wind turbine is a device that converts kinetic energy from the wind into mechanical energy. If the mechanical energy is used to produce electricity, the device may be called a wind generator or wind charger. If the mechanical energy is used to drive machinery, such as for grinding grain or...

generators
audio speakers
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...

/ headphones
Headphones
Headphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...
bicycle dynamos
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
fishing reel brake
Brake
A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....

s
permanent magnet motors in cordless tools
self-powered flashlights, employing rare earth magnets for generating electricity in a shaking motion

Other applications

Other applications of rare-earth magnets include:

Linear motors
Linear motor
A linear motor is an electric motor that has had its stator and rotor "unrolled" so that instead of producing a torque it produces a linear force along its length...

(used in Mag-lev trains, etc.)
Stop motion
Stop motion
Stop motion is an animation technique to make a physically manipulated object appear to move on its own. The object is moved in small increments between individually photographed frames, creating the illusion of movement when the series of frames is played as a continuous sequence...

animation as tie-downs when the use of traditional screw and nut tie-downs is impractical
Diamagnetic
Diamagnetism
Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the...

levitation experimentation, the study of magnetic field dynamics and superconductor
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...

levitation
Meissner effect
The Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state. The German physicists Walther Meissner and Robert Ochsenfeld discovered the phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin...

.
Electrodynamic bearings.
Launched roller coaster
Launched roller coaster
The launched roller coaster is a modern form of roller coaster which has increased in use in the last decade. In place of a traditional chain lift, the launched coaster initiates a ride with high amounts of acceleration via one or series of Linear Induction Motors , Linear Synchronous Motors ,...

technology found on roller coaster and other thrill rides.
LED Throwies An LED throwie is a small LED attached to a coin battery and a rare earth magnet (usually with conductive epoxy or electrical tape), used for the purpose of creating non-destructive graffiti and light displays.
Electric guitar pickups.
Miniature figure (gaming) in particular Warhammer 40,000
Warhammer 40,000
Warhammer 40,000 is a tabletop miniature wargame produced by Games Workshop, set in a dystopian science fantasy universe. Warhammer 40,000 was created by Rick Priestley in 1987 as the futuristic companion to Warhammer Fantasy Battle, sharing many game mechanics...

and Warhammer Fantasy Battle
Warhammer Fantasy Battle
Warhammer: The Game of Fantasy Battles is a tabletop wargame created by Games Workshop. It is the origin of the Warhammer Fantasy setting....

Rare Earth magnets have gained popularity in the miniatures gaming community for their small size and relative strength assisting in swapping weapons between models to adhere to WYSIWYG
WYSIWYG
WYSIWYG is an acronym for What You See Is What You Get. The term is used in computing to describe a system in which content displayed onscreen during editing appears in a form closely corresponding to its appearance when printed or displayed as a finished product...

conversions.
Windbelt
Windbelt
The Windbelt is a device for converting wind power to electricity. A windbelt is essentially an aeolian harp except that it exploits the motion of the string produced by the aeroelastic flutter effect to move a magnet closer and farther from one or more electromagnetic coil and thus induce current...

for electricity generation through electromagnetic induction
Electromagnetic induction
Electromagnetic induction is the production of an electric current across a conductor moving through a magnetic field. It underlies the operation of generators, transformers, induction motors, electric motors, synchronous motors, and solenoids....

and aeroelastic flutter
Aeroelasticity
Aeroelasticity is the science which studies the interactions among inertial, elastic, and aerodynamic forces. It was defined by Arthur Collar in 1947 as "the study of the mutual interaction that takes place within the triangle of the inertial, elastic, and aerodynamic forces acting on structural...

principles.

External links

Magnet Man Cool experiments with magnets
MMPA 0100-00, Standard Specifications for Permanent Magnet Materials

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Explanation of strength

Magnetic properties

Samarium-cobalt

Neodymium

Hazards

Applications

Common applications

Other applications

Further reading

External links