Neodymium Magnets

Types of Neodymium Magnets

NdFeB Neodymium-Iron-Boron Magnets

Neodymium magnets are one of the most influential discoveries of the Twentieth Century and are a central component in the advancement of technology over the last fifty years. They are the most powerful magnets in the world and are used in the production of endless commercial products. For their size, they have incredible power capable of lifting materials far greater than their size. Neodymium magnets are a member of the rare earth group of magnets produced from alloys of the Lanthanide, or lanthanoid, group of elements on the Periodic Table. The Lanthanide group is used to manufacture the rare earth magnets Neodymium and Samarium. Neodymium magnets are produced by combining Neodymium-Iron-Boron while Samarium magnets are made from Samarium and Cobalt.

Neodymium magnets, developed in 1982, are more commonly used in producing products than Samarium (SmCo) magnets. Though Samarium magnets have high magnetic energy, have reliable coercivity, and good temperature characteristics, they are very expensive to produce. The cost of Samarium magnets is the main reason researchers and manufacturers use Neodymium magnets in products.

Sintered and Bonded Neodymium Magnets

Sintered Neodymium Magnets NdFeB magnets are manufactured using a powder metallurgy. It is a stringent process that is controlled to avoid oxidation or explosions when in the fine powder form prior to sintering. After sintering, the material is ground or sliced to size before coating & magnetization. Sintered NIB magnets offer the highest energy per unit volume of any permanent magnetic material. They can lift up to 1000 times their weight and are manufactured in numerous energy and temperature grades. Current energy levels start at 33 MGO with 50 MGO versions having been perfected in laboratory experiments.

The intrinsic (jHc) coercive force determines the operating temperature of an element. Even with a SH (high coercivity) 150°C degree, materials may still have a poor temperature coefficient, which means an open circuit magnet can lose about 0.10% of Remanence per °C. These losses are called Reversible Losses. The Remanence will recover as the temperature fails. Generally, the higher coercivity is achieved by doping the alloy with Dysprosium or Cobalt at the expense of the Iron content. This reduces Remanence and therefore MGO.

Apart from temperature difficulties, NIB also suffers from corrosion because of its high iron content. Although the magnets are normally coated, care must be taken to avoid Hydrogen or saltwater environments. Although Neodymium offers 30-40% higher energy levels than SmCo alloys and is considerably lower in cost, SmCo magnets can still be considered as a first choice when high temperature or hostile environments are encountered.

Preferred surface coatings for Neodymium magnets are:

1. Nickel plating
2. Zinc plating
3. Epoxy coating
4. Parlene coating
5. Clear cellulose

Applications for Neodymium magnets are diverse and wide ranging. Magicians to Aerospace Engineers have found them to be invaluable. Their power to weight ratio has made applications thought to be impossible achievable, which has changed the design concepts for the modern Engineer.

Warning: Because of the brittle nature and high attractive forces generated by NdFeB magnets, care should be taken when handling them. It is important to always study the Safety Instructions.

Bonded Neodymium Magnets

Bonded magnets are made by binding neodymium powder with a resin or polymer that is pressed or extruded into a die cavity. During production, the magnetic field is isotropic, does not have a specific direction, which makes them flexible to be formed into complex shapes or inserted directly into other components.

Compression bonded neodymium magnets offer higher magnetic properties, up to 12 MGOe, than injection-molded magnets that are up to 6 MGOe but are limited to simple patterns. Though bonded magnets are corrosion resistant, they are usually epoxy coated for durability, which increases their resistance to corrosive materials. They are an excellent choice for applications requiring high magnetic strength and tight tolerance.

In the compression process, bonded neodymium magnets are isotropic and can be magnetized in any direction.

Applications and Uses of Neodymium Magnets

Since their introduction by General Motors and Sumitomo Special Metals in the mid-1980’s, Neodymium magnets have become an essential part of several industrial processes and the production of consumer goods. Industries such as electrical motor manufacturers, medical suppliers, renewable energy producers, and technological companies use super strength Neodymium magnets. They have been instrumental in several of the technological advances over the last several years. The major attraction of Neodymium magnets is their ability to be transformed into multiple shapes and sizes. Listed below are a few of the uses of Neodymium magnets:

Hard Disk Drives

A hard disk drive records information by magnetizing and demagnetizing ferromagnetic material on a disk. The disk is separated into several parts with each part having its own miniature magnet. Data is recorded using the head of the drive that reads and writes the information. When the head is writing, a magnetic field is formed, and the recording surface is magnetized. Other magnets are used to move the head into position.

Microphones, Acoustic Pick-ups, Headphones, and Loudspeakers

Magnets are used in speakers with a coil that converts electricity into mechanical energy to move the cone of the speaker to create sound. With a microphone, a diaphragm is connected to the coil that has a magnet. When sound moves the diaphragm, the coil moves. As the coil moves, in the magnetic field, an electrical signal is converted to sound.

Dentures

Miniature Neodymium magnets are used to hold dentures in place where several teeth are missing. These extremely tiny magnets are coated for protection from corrosion and wear.

Door Catches

Neodymium magnets are used in public and commercial buildings in the creation of magnetic door catches. They are placed in the door to attract another magnet or steel plate. Due to their superior strength, they can easily hold the weight of a door in place but can be just as easily separated.

Motors and Generators

Electrical motors use a combination of electromagnetic and permanent magnet energy to convert electrical energy into mechanical energy. At the heart of most electrical motors are Neodymium magnets. Generators work in reverse. They convert mechanical energy into electrical energy by moving a conductor through a magnetic field.

Magnetic Bearings

Magnetic bearings support moving parts without making physical contact. They produce relative motion with very low friction or wear at high speeds. There are two types of magnetic bearings: passive and active. Neodymium magnets are used to make passive bearings while active bearings use electromagnets.

MRI Scanners

Magnetic resonance imaging is a medical technique used to form pictures of the body’s anatomy and its physiological processes. MRI scanners use strong Neodymium magnetic fields, magnetic field gradients, and radio images to produce images of the body’s internal organs. An MRI is a preferred method of observing internal bodily functions over X-rays since there isn’t any exposure to radiation.

Magnetic Therapy

Transcranial magnetic stimulation (TMS) uses magnetic fields to stimulate nerve cells in the brain to improve symptoms associated with depression. It is a treatment that is typically used when other traditional forms of therapy or treatment have failed. The magnet is placed on the scalp near the forehead. The process is designed to activate parts of the brain that have had decreased activity.

Magnetic Separators

Production facilities use Neodymium magnet systems to remove contaminating ferrous or paramagnetic materials from production and processing lines. The materials to be checked are moved along a conveyor that passes near the magnet.

Magnetically Powered Pumps

A magnetic pump has a motor driven shaft with a ring of magnets that is connected to a smaller ring of magnets attached to another shaft attached to an impeller that sits in an even larger ring of magnets. As the motor turns the drive shaft and magnets, a magnetic field turns the other set of magnets that powers the impeller. There is no mechanical contact between the motor and impeller. If any part fails to function, there others remain in motion and will not burnout.

The Properties of Neodymium Magnets

The term Neodymium comes from two Greek words: Neo for new and didymos for twin. Neodymium does not occur as a natural metal but is a chemical element present in lanthanide ores like monazite and bastnasite. All varieties of lanthanide contain Neodymium, which makes it very common and easily available for general use. Lanthanide is mined in Australia, Brazil, China, India, Sri Lanka, and the United States. When lanthanide is exposed to moisture and air, the oxidation process produces a yellow, pink, and bluish purple compound, which is Neodymium. In the early part of the Twentieth Century, Neodymium oxide was used as a dye for drinking glasses and glazes of red and purple.

Neodymium magnets, known as NdFeB, NIB, or Neo magnets, are an essential part of multiple worldwide manufacturing processes. They are a permanent magnet that is produced from an alloy of neodymium, iron and boron to form a tetragonal crystalline structure. The method used to manufacture them determines whether they will be sintered or bonded with Neodymium magnets.

Neodymium is ferromagnetic. Its magnetism only happens at extremely low temperatures such as a Curie temperature of - 425.5°F. When it is combined with other metals, such as iron, its Curie temperature rises to room temperature. It is this combination used to make Neodymium magnets.

There are several factors contributing to the reason Neodymium magnets are so strong. One important aspect is its tendency to magnetize along a single crystal axis. Also, Neodymium magnets are composed of microcrystalline grains that are aligned to create a very strong and powerful magnetic field. Since they are magnetized along a single axis, Neodymium magnets have a very high coercivity, or resistance to being demagnetized.

Attributes of Neodymium

• Very high resistance to demagnetization
• High energy for size
• Good in ambient temperature
• Moderately priced
• Material is corrosive and should be coated for long term maximum energy output
• Low working temperature for heat applications, but higher levels of heat resistance materials are being introduced periodically.

Neodymium atoms have four unpaired electrons in its structure, which is different from the average number of three. The unpaired electrons is what creates the magnetic field and is the reason Neodymium magnets are so strong. They have a magnetic energy that is 18 times stronger than a ferrite magnet as well as being more powerful than samarium cobalt magnets, which Neodymium magnets replaced.

Tolerances

For a pressed material, tolerance on the thickness (direction of magnetization) is ± .005. Other dimensions are ± 2.5% or ± .010, whichever is greater. According to IMA standards, visual imperfections such as hairline cracks, porosity and minor chips are commonly found in sintered metallic magnets. A chipped edge is considered acceptable if no more than 10% of the surface is missing. Cracks are acceptable as long as they do not extend across more than 50% of pole surface.

Neodymium Compounds

Neodymium Oxide

Neodymium Oxide, or Neodymia, is mainly used for glass and capacitors. It can color glass with delicate shades ranging from pure violet through wine-red and warm gray. Light transmitted through such glass shows unusually sharp absorption bands. The glass is used in astronomical work to produce sharp bands by which spectral lines may be calibrated. Glass containing neodymium is a laser material in place of ruby to produce coherent light.

Neodymium oxide is an attractive mauve color, which can be seen by looking carefully at the original surface of cut pieces. They can be protected from further oxidation by an argon atmosphere inside an ampoule.

• Formula: Nd2O3
• CAS No.: 1313-97-9
• Molecular Weight: 336.48
• Appearance: Light purple
• Solubility: Insoluble in water, moderately soluble in strong mineral acids
• Stability: Slightly hygroscopic

Neodymium Metal

Neodymium Metal is mainly used in manufacturing very powerful permanent magnets-Neodymium-Iron-Boron magnets. A few Neodymium Metals are applied in making superalloy.

• Formula: Nd
• CAS No.: 7440-00-8
• Molecular Weight: 144.24
• Appearance: Silvery
• Stability: Moderately reactive in air

Neodymium Acetate

Neodymium Acetate is mainly used for glass, crystal and capacitors. It colors glass with delicate shades ranging from pure violet through wine-red and warm gray. Light transmitted through such glass shows unusually sharp absorption bands.

• Formula: Nd(O2C2H3)3.xH2O
• CAS No.: 6192-13-8
• Molecular Weight: 339.38
• Appearance: Light purple
• Solubility: Soluble in water, moderately soluble in strong mineral acids
• Stability: Slightly hygroscopic

Neodymium Chloride

Neodymium Chloride is mainly used for glass, crystal and capacitors. As with neodymium acetate, it colors glass with delicate shades ranging from pure violet through wine-red and warm gray. Light transmitted through such glass shows unusually sharp absorption bands.

• Formula: NdCl3.xH2O
• CAS No.: 10024-93-8
• Molecular Weight: 250.60
• Appearance: Light purple
• Solubility: Soluble in water, moderately soluble in strong mineral acids
• Stability: Slightly hygroscopic

Neodymium Fluoride

Neodymium Fluoride is mainly used for glass, crystal and capacitors, and is the main raw material for making Neodymium Metal and alloys.

• Formula: NdF3
• CAS No.: 13709-42-7
• Molecular Weight: 201.24
• Appearance: Light purple
• Solubility: Soluble in water, moderately soluble in strong mineral acids
• Stability: Slightly hygroscopic

Neodymium Hydroxide

Neodymium Hydroxide, mainly used as a catalyst for glass, crystal and capacitors, has shades ranging from pure violet through wine-red and warm gray. Light transmitted through such glass shows unusually sharp absorption bands.

• Formula: Nd(OH)3.xH2O
• CAS No.: 16469-17-3
• Molecular Weight: 195.26
• Appearance: Light purple
• Solubility: Soluble in water, moderately soluble in strong mineral acids
• Stability: Slightly hygroscopic

Neodymium Nitrate

Neodymium Nitrate has shades ranging from pure violet through wine-red and warm gray. Light transmitted through such glass shows unusually sharp absorption bands.

• Formula: Nd(NO3)3.6H2O
• CAS No.: 16454-60-7
• Molecular Weight: 330.25
• Appearance: Light purple
• Solubility: Soluble in water, moderately soluble in strong mineral acids
• Stability: Slightly hygroscopic

Neodymium Oxalate

Neodymium Oxalate has colors ranging from pure violet through wine-red and warm gray. Light transmitted through such glass shows unusually sharp absorption bands.

• Formula: Nd2(C2O4)3.10H2O
• CAS No.: 1186-50-1
• Molecular Weight: 552.53
• Appearance: Light purple
• Solubility: Soluble in water, moderately soluble in strong mineral acids
• Stability: Slightly hygroscopic

Neodymium Sulfate

Neodymium Sulfate, also Neodymium Sulphate, mainly used for catalyst, glass, crystal and capacitors, with delicate shades ranging from pure violet through wine-red and warm gray. Light transmitted through such glass shows unusually sharp absorption bands.

• Formula: Nd2(SO4)3.8H2O
• CAS No.: 13477-91-3
• Molecular Weight: 576.66
• Appearance: Light purple
• Solubility: Soluble in water, moderately soluble in strong mineral acids
• Stability: Slightly hygroscopic

Neodymium Metal

Neodymium metal is available commercially and does not need to be produced in a laboratory, which is just as well, since it is difficult to separate from pure metal. It is very prevalent in nature since lanthanoids are found in a number of minerals such as xenotime, monazite, and bastnaesite. Xenotime and monazite are orthophosphate minerals, LnPO4, while bastnaesite is a fluoride carbonate, LnCO3F. The Ln denotes a mixture of all the lanthanoids except promethium, which is very rare Lanthanoids, with even atomic numbers, are more common. The most common lanthanoids are found in cerium, lanthanum, neodymium, and praseodymium. Monazite contains thorium and yttrium making it difficult to handle since thorium and its decomposition products are radioactive.

Separation of the metals is very complex. The metals are extracted as salts from the ores using Sulphuric acid (H2SO4), hydrochloric acid (HCl), and sodium hydroxide (NaOH). Modern purification techniques of the salt mixtures involve selective complexation techniques, solvent extractions, and ion exchange chromatography.

Pure neodymium is available through the reduction of NdF3 with calcium metal.

2NdF3 + 3Ca 2Nd + 3CaF2

This would work for the other calcium halides as well but the product CaF2 is easier to handle under the reaction conditions, which is heat to 50°C above the melting point of the element in an argon atmosphere. Excess calcium is removed from the reaction mixture under a vacuum.

Other Magnetic Materials

Ferrite (Ceramic)

Ferrite magnets, also known as hard ceramic magnets, are made from Strontium or Barium Ferrite. They were developed in the 1960s as a low-cost and more powerful alternative to AlNiCo and steel magnets. They are less expensive than NdFeB magnets but are powerful and resistant to demagnetization. Ferrite magnets are lower in power compared to other formulations and very brittle. However, they have very high Hc and good Tc as well as being corrosion-resistant.

AlNiCo (Aluminum-Nickel-Cobalt)

AlNiCo (Aluminum-Nickel-Cobalt) have medium strength but excellent machinability. Developed in the 1940s, they perform much better than plain steel but are weaker in strength and must be carefully stored since they are prone to demagnetization. Contact with a NdFeB magnet can easily reverse or destroy the field of an AlNiCo magnet.

SmCo (Samarium Cobalt)

SmCo (Samarium Cobalt) have high power and are resistant to high temperatures and corrosion. Developed in the 1970s, they were the first of the 'rare earth' magnets. They are almost as powerful as NdFeB magnets but far more powerful than all the others. They are the most expensive magnet formulation and are only used where resistance to high temperatures and corrosion are needed. Samarium cobalt magnets are very brittle and hard to machine.

Permanent Magnets

Permanent magnets are used in the following major groups: acoustic transducers, motors and generators, magneto mechanical devices, and magnetic field and imaging systems as well as televisions, telephones, computers, audio systems and automobiles.

The permanent magnet family consists of non-rare earth permanent magnets and rare earth magnets. The non-rare earth magnets include Alnico (Aluminum-Nickel-Cobalt) magnets and Ceramic (Strontium and Barium Ferrite) magnets. Rare earth magnets include Sm-Co (Samarium-Cobalt) magnets and Nd-Fe-B (Neodymium-Iron-Boron) magnets.

Although non-rare earth magnets are used in the majority of these applications due to their cost, rare earth permanent magnets have many distinguishing characteristics such as a large Maximum Energy Product, the performance index for permanent magnets. Dozens of magnetic materials, which contain rare earth, have been developed recently. Two major families of rare earth permanent magnets, Sm-Co magnets and Nd-Fe-B, have been widely used in a variety of applications. Each family has its own advantages and disadvantages.

Alnico

Alnico magnets are composed of iron, cobalt, nickel, aluminum, and copper. They are magnetized to produce a strong magnetic field. Of the more commonly available magnets, only rare-earth magnets, such as samarium-cobalt and neodymium-iron, are stronger. Alnico magnets produce magnetic field strength at their poles as high as 1500 gauss or about 3000 times the strength of the earth's magnetic field. They have one of the highest Curie points of any magnetic material, around 800 degrees Celsius.

Alnico magnets are used in electric motors, sensors, and loudspeakers and are produced by casting or sintering processes. Some varieties are isotropic so they can be magnetized in any direction. Other types, such as Alnico 5 and Alnico 8, are anisotropic and have a preferred direction of magnetization or orientation. Anisotropic types generally have greater magnetic capacity than isotropic types. Anisotropic Alnico magnets are oriented by heating them above a critical temperature and cooling them in the presence of a magnetic field.

Electromagnet

An electromagnet has its magnetic field induced by a flow of electric current. A current, flowing through a wire, produces a magnetic field (M) around the wire. The field is oriented according to the right-hand rule. The simplest type of electromagnet is a coiled piece of wire. A coil, forming the shape of a straight tube, similar to a corkscrew, is called a solenoid, which is bent so that the ends meet in a toroid. Much stronger magnetic fields can be produced if a "core" of paramagnetic or ferromagnetic material, commonly iron, is placed inside the coil. The field causes the iron to magnetize and generate a field of its own. This field can be hundreds or thousands of times stronger than that of the coil itself.

Magnetic fields caused by coils of wire follow a form of the right-hand rule. If the fingers of the right hand are curled in the direction of current flow through the coil, the thumb points in the direction of the field inside the coil. The side of the magnet, where the field lines emerge, is defined as the north pole.

Electromagnets vs. Permanent Magnets

The main advantage of an electromagnet over a permanent magnet is that the magnetic field can be rapidly manipulated over a wide range by controlling the electric current. A disadvantage is that if an electromagnet, with a ferromagnetic core, is turned on and off again, the core retains some residual magnetization due to hysteresis. This magnetic field can persist indefinitely.

Applications that do not require a variable magnetic field are perfect for a permanent magnet. Since an electromagnet requires a constant flow of electricity, it consumes electrical power. Additionally, permanent magnets can be manufactured to produce stronger fields than any electromagnet of similar size.

Rare Earth Magnet – Samarium Cobalt

Samarium Cobalt magnets are the second most common type of magnetic material found in nature after neodymium. They are called rare earth, because their composition elements are found in the Lanthanides portion of the Periodic Table. Samarium cobalt magnets are composed of samarium, cobalt and iron. They are extremely strong for their small size, metallic in appearance, and can be found in simple shapes such as rings, blocks and discs.

Samarium-cobalt magnets have been available since the early 1970s and are very powerful. However, they are also very brittle and prone to cracking and chipping. Although more expensive, and not as powerful as neodymium magnets, samarium-cobalt magnets have a higher Curie temperature and can be used for higher-temperature applications.

Attributes

• High resistance to demagnetization
• High energy (magnetic strength is strong for its size)
• Good temperature stability (maximum use temperatures between 250 and 350°C; Curie temperatures from 700 to 800°C)
• Expensive material (cobalt is market price sensitive)

Applications

• Computer disc drives
• Sensors
• Traveling wave tubes
• Linear actuators
• Satellite systems
• Motors where temporary stability is vital

Material properties

• Density (g/cc): 8.4
• Electrical Resistivity (Ohm/cm) 0.8×10-4
• Coefficient of Thermal Expansion (perpendicular to axis) (10-6/k) 12.5

The material used to produce Samarium-cobalt magnets is a fire hazard if ground when dry. The ground dust has a low ignition temperature.

Caution:

Any type of strong permanent magnet is not a toy. Even small samarium-cobalt magnets can be hazardous, able to pinch skin or fingers when suddenly attracted to another magnetic object. Magnets should be stored away from electrical appliances, magnetic bank cards and computer monitors.