An Overview of Alnico
Alnico magnets exhibit excellent temperature stability, high residual induction, and relatively high energies. Alnico is composed primarily of alloys of Aluminum, Nickel, and Cobalt. They are manufactured through either a casting or sintering process. Cast magnets may be manufactured in complex shapes, such as horseshoes, not possible with other magnet materials. Sintered Alnico magnets offer slightly lower magnetic properties but better mechanical characteristics than cast Alnico magnets.
The most commonly used cast Alnico magnet is Cast Alnico 5. This alnico material is used extensively in rotating machinery, meters, instruments, sensing devices, and holding applications, to name a few.
Alnico is hard and brittle. Machining or drilling cannot therefore be accomplished by regular methods. Holes are usually cored in at the foundry, and magnets are cast close to final size and then finish machined to closer tolerances.
Alnico has a low coercive force, and is easily demagnetized if not handled with care. For optimum performance of Alnico 5, the magnetic length should be approximately 5 times the pole diameter or equivalent diameter. For example, a 0.250” diameter magnet should be about 1.250” long.
Because of its higher coercivity, Alnico 8 may be used in shorter lengths and in disc shapes.
Alnico magnets are manufactured through either casting or sintering processes. Cast Alnico magnets are manufactured by pouring a molten metal alloy into a mold and then further processing it through various heat-treat cycles. The resulting magnet has a dark gray exterior appearance, and may have a rough surface. Machined surfaces have a shiny appearance similar to steel.
Sintered magnets are manufactured by compacting fine Alnico powder in a press, and then sintering the compacted powder into a solid magnet.
Alnico Magnet Assemblies
BuyMagnets.com is able to manufacture metal and other components of finished sub assemblies using our machining capabilities. Alnico assemblies can be fabricated by adhering magnets with adhesives to suit a range of environments, by mechanically fastening magnets, or by a combination of these methods. Due to the relatively brittle nature of these magnet materials, press fits are not recommended.
Alnico Surface Treatments
The corrosion resistance of Alnico is considered excellent and no surface treatments are required. However, Alnico magnets are easily plated for cosmetic reasons if required.
Alnico is hard and brittle, and prone to chipping and cracking. Special machining techniques must be used to machine this material. Holes must be made by specified machining methods. BuyMagnets.com is capable of getting our Alnico material to your specifications.
Magnetizing and Handling
Alnico magnets require magnetizing fields of about 3 kOe. Because of their relatively low coercivities, special care should be taken to assure that these magnets are not subjected to adverse repelling fields, since these could partially demagnetize the magnets. Magnetized magnets should be stored with ìkeepersî to reduce the possibility of partial demagnetization. If Alnicos are partially demagnetized, they may be easily remagnetized.
Up to about 1000° F, changes in magnetization are largely reversible and re-magnetizable, while changes above this are largely structural and not fully reversible or re-magnetizable. Approximately 90% of room temperature magnetization is retained at temperatures of up to 1000° F.
Alnico Material Characteristics
Cast Alnico materials commonly contain casting voids and hairline cracks within the material.
Any pull values shown, are approximate and offered only for comparison. They have been measured when pole surfaces are in contact with a 1/2” thick, ground, mild steel plate. Due to the nature of magnetism, it is very difficult to establish a definite holding force to fit all applications. It is suggested that each customer make their own pull tables on an actual model.
More About Alnico
Alnico is an acronym referring to iron alloys which in addition to iron are composed primarily of aluminium (Al), nickel (Ni) and cobalt (Co), hence al-ni-co, with the addition of copper, and sometimes titanium. Alnico alloys are ferromagnetic, with a high coercivity (resistance to loss of magnetism) and are used to make permanent magnets. Before the development of rare earth magnets in the 1970s, they were the strongest type of magnet. Other trade names for alloys in this family are: Alni, Alcomax, Hycomax, Columax, and Ticonal.
The composition of alnico alloys is typically 8–12% Al, 15–26% Ni, 5–24% Co, up to 6% Cu, up to 1% Ti, and the balance is Fe. The development of alnico began in 1931, when T. Mishima in Japan discovered that an alloy of iron, nickel, and aluminum had a coercivity of 400 oersted (Oe), double that of the best magnet steels of the time.
Alnico alloys make strong permanent magnets, and can be magnetized to produce strong magnetic fields. Of the more commonly available magnets, only rare-earth magnets such as neodymium and samarium-cobalt are stronger. Alnico magnets produce magnetic field strength at their poles as high as 1500 gauss (0.15 tesla), or about 3000 times the strength of Earth’s magnetic field. Some brands of alnico are isotropic and can be efficiently magnetized in any direction. Other types, such as alnico 5 and alnico 8, are anisotropic, with each having a preferred direction of magnetization, or orientation. Anisotropic alloys generally have greater magnetic capacity in a preferred orientation than isotropic types. Alnico’s remanence (Br) may exceed 12,000 G (1.2 T), its coercivity (Hc) can be up to 1000 oersted (80 kA/m), its energy product ((BH)max) can be up to 5.5 MG·Oe (44 T·A/m). This means alnico can produce a strong magnetic flux in closed magnetic circuit, but has relatively small resistance against demagnetization.
Alnico is produced by casting or sintering processes. Anisotropic alnico magnets are oriented by heating above a critical temperature, and cooling in the presence of a magnetic field. Both isotropic and anisotropic alnico require proper heat treatment to develop optimum magnetic properties — without it alnico’s coercivity is about 10 Oe, comparable to technical iron, which is a soft magnetic material. After the heat treatment alnico becomes a composite material, named “precipitation material“—it consists of iron and cobalt rich precipitates in rich-NiAl matrix.
Alnico’s anisotropy is oriented along the desired magnetic axis by applying an external magnetic field to it during the precipitate particle nucleation, which occurs when cooling from 900° C (1650° F) to 800° C (1470° F), near the Curie point. Without an external field there are local anisotropies of different orientations, due to spontaneous magnetization. The precipitate structure is a “barrier” against magnetization changes, as it prefers few magnetization states requiring much energy to get the material into any intermediate state. Also, a weak magnetic field shifts the magnetization of the matrix phase only, and is reversible.
Alnico alloys have some of the highest Curie points of any magnetic material, around 800° C (1470° F), although the maximum working temperature is normally limited to around 538° C (1000° F). They are the only magnets that have useful magnetism even when heated red-hot. This property, as well as its brittleness and high melting point, is the result of the strong tendency toward order due to inter-metallic bonding between aluminum and its other constituents. They are also one of the most stable magnets if they are handled properly.
Alnico magnets are widely used in industrial and consumer applications where strong permanent magnets are needed; examples are electric motors, electric guitar pickups, microphones, sensors, loudspeakers, traveling-wave tubes, and cow magnets. In many applications they are being superseded by rare earth magnets, whose stronger fields (Br) and larger energy products (BHmax) allow smaller size magnets to be used for a given application.
The Right Choice for Superior Temperature Stability for Complex Shapes
Alnico Magnets derive their magnetic properties and their name from their main constituents – aluminum, nickel, and cobalt. They have the widest range of temperature stability of any standard magnetic material. Other characteristics include high induction as well as relatively high energy. Manufacture is by sintering or casting.
We stock Alnico magnets in grades 5, 8, and 2. Alnico magnets are your best choice for applications exposed to operating temperatures above 400° F. Up to 1000° F, they maintain about 85 percent of their room-temperature magnetic properties, and changes in magnetization are reversible. See the Stock Magnet Specifications table on the Learn About Magnets page for detailed information.
Sintered Alnico has marginally lower magnetic properties, but better mechanical properties, than cast Alnico. Both are hard and brittle materials. They require skillful machining that is best performed on specialized equipment. The MMPA Standards for Alnico magnets state, in part: “These are materials used primarily for their magnetic capabilities…without regard to their mechanical properties. Therefore, it is generally not recommended that these materials be used for structural or decorative purposes.” Alnico magnets can be pressed directly into nonmagnetic materials. For steel pressings, they should be enclosed in a nonferrous bushing. When specifications call for extremely tight tolerances or complex, non-standard shapes that will require considerable machining, magnetic materials that are easier to work with than Alnico should be considered.
Cast Alnico 2 & 5 have maximum energy products of 5.4 and 5.5, respectively, and are popular choices for applications such as holding assemblies, electronic instrumentation, sensing devices, and communications equipment. For best results with Alnico 5 magnets, the length should be no less than 5 times the crosssection diameter – or 5 times the diameter of a circle equal in area to the cross section.
Cast Alnico 8 HE has the highest temperature stability of any commercially available magnetic material. So it is especially wellsuited to high-temperature applications. Improved crystal structure and alloying techniques achieve a 6.0 energy product and high resistance to demagnetization. Typical uses include computer keyboards, drives, printers, microphones, meters, motors, generators, relays, reed-switch relays, transducers, and Hall-Effect devices.
Sintered Alnico 8H has a 5.25 energy product and high temperature stability, coercivity, and demagnetization resistance similar to Cast Alnico 8. But it can be manufactured to closer tolerances. Its fine grain structure results in highly uniform flux distribution and mechanical strength. So it is ideally suited to applications requiring short magnetic length or involving high-speed motion. Some applications include core meters, traveling wave tube stacks, polarized relays, reed switches, torque transmitting devices, and sandwich-type holding assemblies.
Sintered Alnico 2 has an energy product of 1.5. Magnets of this material are unoriented and can be magnetized in any direction.
- To prevent demagnetizing in Alnico 5 Rods, the length / diameter ratio should be 5 or above
- Did you know that Alnico magnets offer the highest maximum operating temperature of any magnet material (1000° F)?
Rare Earth Magnets
Neodymium magnets are a member of the Rare Earth magnet family and are the most powerful permanent magnets in the world. They are referred to as NdFeB magnets, or NIB, because they are composed mainly of Neodymium (Nd), Iron (Fe) and Boron (B).
Neodymium magnets are all graded by the material they are made of. The higher the grade, which is the number following the letter ‘N’, the stronger the magnet. Any letter following the grade refers to the temperature rating of the magnet. If there are no letters following the grade, then the magnet is standard temperature neodymium.
Neodymium magnets are a composition of mostly Neodymium, Iron and Boron. If left exposed to the elements, the iron in the magnet will rust. To protect the magnet from corrosion and to strengthen the brittle magnet material, it is usually preferable for the magnet to be coated.
There are a variety of options for coatings, but nickel is the most common and usually preferred. Some other options for coating are zinc, tin, copper, epoxy, silver and gold.
Caution of Machining
Neodymium material is brittle and prone to chipping and cracking, so it does not machine well by conventional methods. Machining the magnets will generate heat, which if not carefully controlled, can demagnetize the magnet or even ignite the material which is toxic when burned. It is recommended that magnets not be machined.
Rare Earth magnets have a high resistance to demagnetization, unlike most other types of magnets. Neodymium magnets will not lose their magnetization around other magnets or if dropped. They will begin to lose strength if they are heated above their maximum operating temperature, which is 180° F (80° C) for standard N grades. They will completely lose their magnetization if heated above their Curie temperature, which is 590° F (310° C) for standard N grades. BuyMagnets.com offers a selection of magnets that are of high temperature material, which can withstand higher temperatures without losing strength.
The strength of neodymium magnets is very impressive. Neodymium magnets are ten (10) times stronger than the strongest ceramic magnets. It is possible to use a smaller sized neodymium magnet and get a greater holding force than much larger ceramic magnets.
Licensed to Sell
Due to the fact that neodymium magnets are an invention, they are covered under United States patent laws. All neodymium magnets sold in the United States are required by law to be licensed by the patent holders.
It is important to research your supplier regarding their supply of neodymium magnets. There are many cheap variations of neodymium magnets available that are not licensed and not legally allowed to be sold in the U.S.
All BuyMagnets.com magnets are licensed and legal for sale in the U.S. and throughout the world.
The overall strength of the BuyMagnets.com neodymium magnets is underestimated frequently. These neodymium magnets must be handled with care to avoid injury and damage to the magnets. Fingers and other body parts can get severely pinched between two attracting magnets. Neodymium magnets are brittle, and can peel, crack or shatter if allowed to slam together. Eye protection should be worn when handling these magnets, because shattering magnets can launch pieces at great speeds.
The strong magnetic fields of neodymium magnets can also damage magnetic media such as credit cards, magnetic identification cards, video tapes or other such devices. Also, neodymium magnets can damage televisions, computer monitors and other electronic displays. As a general rule, you should never place neodymium magnets near electronic appliances.
It is also important to keep neodymium magnets out of the reach of small children. Children should not be allowed to handle neodymium magnets as they can be dangerous. Small neodymium magnets pose a choking hazard and should never be swallowed or inserted into any part of the body.
It is crucial to never allow neodymium magnets near a person with a pacemaker or similar medical aid. The strong magnetic fields of the magnet can affect the operation of such devices.
Finally, neodymium magnets are brittle and prone to chipping and cracking. They do not take kindly to machining. Neodymium magnets will lose their magnetic properties if heated above 175° F (80° C). Neodymium magnets should never be burned, as burning them will create toxic fumes.
Samarium Cobalt Magnets
Samarium Cobalt Magnets produce energy rivaling that of Neodymium. Typical uses include many hi-tech applications, such as computers, electronics, switches, and automotive “under-the-hood” applications, where elevated temperatures apply.
Samarium Cobalt magnets are extremely hard and brittle and should be protected from shock and mechanical forces in their application when handling. They resist corrosion and retain most of their energy up to 575° F, making them ideal replacements for Alnico when high temperature use or miniaturization is required.
Samarium Cobalt 18 and 22 were the first Samarium Cobalt grades manufactured and are derived from nearly pure Samarium and Cobalt. Samarium Cobalt 26 has an even higher temperature stability than the 18 and 22 grades and is especially suited for applications demanding high energy in high-temperature environments.
Samarium Cobalt magnets are another rare earth magnet material that offer the best value when comparing performance and size in high temperature or adverse environments. Samarium Cobalt magnets are higher in cost, but magnetically very strong and typically allow for dimensional reductions. Samarium Cobalt magnets offer excellent corrosion resistance and typically do not require a surface treatment.
No magnet material should be employed as a structural element in a design. Samarium Cobalt magnets are especially prone to fracturing and is very weak under tensile or compressive loads. Samarium Cobalt magnets have good resistance to external demagnetizing fields because of its high Intrinsic Coercive Force (Hci). This resistance makes Samarium Cobalt rare earth magnets an excellent choice for electromechanical applications.
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. They are expensive, brittle, and prone to cracking and chipping. Samarium-cobalt magnets have maximum energy products (BHmax) that range from 16 megagauss-oersteds (MGOe) to 32 MGOe; their theoretical limit is 34 MGOe. They are available in two “series”, namely Series 1:5 and Series 2:17.
These samarium-cobalt magnet alloys (generally written as SmCo5, or SmCo Series 1:5) have one atom of rare earth samarium and five atoms of cobalt. By weight this magnet alloy will typically contain 36% samarium with the balance cobalt. The energy products of these samarium cobalt alloys range from 16 MGOe to 25 MGOe. These Samarium Cobalt magnets generally have a reversible temperature coefficient of -0.05%/°C. Saturation magnetization can be achieved with a moderate magnetizing field. This series of magnet is easier to calibrate to a specific magnetic field than the SmCo 2:17 series magnets.
In the presence of a moderately strong magnetic field, unmagnetized magnets of this series will try to align its orientation axis to the magnetic field. Unmagnetized magnets of this series when exposed to moderately strong fields will become slightly magnetized. This can be an issue if postprocessing requires that the magnet be plated or coated. The slight field that the magnet picks up can attract debris during the plating or coating process causing for a potential plating or coating failure or a mechanically out-tolerance condition.
Reversible temperature coefficient
Br drifts with temperature and it is one of the important characteristics of magnet performance. Some applications, such as inertial gyroscopes and travelling wave tubes (TWTs), need to have constant field over a wide temperature range. The reversible temperature coefficient (RTC) of Br is defined as(∆Br/Br) x (1/∆ T) × 100%
To address these requirements, temperature compensated magnets were developed in the late 1970s1. For conventional SmCo magnets, Br decreases as temperature increases. Conversely, for GdCo magnets, Br increases as temperature increases within certain temperature ranges. By combining samarium and gadolinium in the alloy, the temperature coefficient can be reduced to nearly zero.
In SmCo5 magnets are fabricated by packing wide-grain lone-domain magnetic powders. All of the motes are aligned with the easy axis direction. In this case, all of the domain walls are 180 degrees. When there are no impurities, the reversal process of the bulk magnet is equivalent to lone-domain motes, where coherent rotation is the dominant mechanism. However, due to the imperfection of fabricating, impurities may be introduced in the magnets, which form nuclei. In this case, because the impurities may have lower anisotropy or misaligned easy axis, their directions of magnetization are easier to spin, which breaks the 180° domain wall configuration. In such materials, the coercivity is controlled by nucleation. To obtain much coercivity, impurity control is critical in the fabrication process.
These alloys (written as Sm2Co17, or SmCo Series 2:17) are age-hardened with a composition of two atoms of rare-earth samarium and 13–17 atoms of transition metals ™. The TM content is rich in cobalt, but contains other elements such as iron and copper. Other elements like zirconium, hafnium, and such may be added in small quantities to achieve better heat treatment response. By weight the alloy will generally contain 25% of samarium. The maximum energy products of these alloys range from 20 to 32 MGOe. These alloys have the best reversible temperature coefficient of all rare earth alloys, typically being -0.03%/° C. The “second generation” materials can also be used at higher temperatures .
In Sm2Co17 magnets, the coercivity mechanism is based on domain wall pinning. Impurities inside the magnets impede the domain wall motion and thereby resist the magnetization reversal process. To increase the coercivity, impurities are intentionally added during the fabrication process.
Machining samarium cobalt
The alloys are typically machined in the unmagnetized state. Samarium-cobalt should be ground using a wet grinding process (water based coolants) and a diamond grinding wheel. The same type of process is required if drilling holes or other features that are confined. The grinding waste produced must not be allowed to completely dry as samarium-cobalt has a low ignition point. A small spark, such as that produced with static electricity, can easily commence combustion. The fire produced will be extremely hot and difficult to control.
Samarium Cobalt magnet rare earth material is very brittle and conventional machine tools and cutters are not appropriate. The brittle nature combined with the powder metal grain/crystal structure hampers the use of carbide tools. Electrostatic discharge machines (EDM), diamond tooling and some abrasives are the usual means of fabrication for this rare earth magnet alloy. Most magnet materials are machined in the un-magnetized state. Once the fabrication and cleaning operation are complete the Samarium Cobalt magnet is then magnetized to saturation .
BuyMagnets.com is capable of fabricating simple or complex shapes from Samarium Cobalt magnet alloy. We stock a variety of standard and exotic grades for prototype or production fabrication.
A BuyMagnets.com team member can help determine if custom machining is required or if “pressed to size” option is possible. The determining factors are usually required lead-time, cost, and the alloy required.
- Samarium-cobalt magnets can easily chip; eye protection must be worn when handling them.
- Keep them away from children.
- Allowing magnets to snap together can cause the magnets to shatter, which can cause a potential hazard.
- Samarium-cobalt is manufactured by a process called sintering, and as with all sintered materials, inherent cracks are very possible. Design engineers must not expect the magnet to provide mechanical integrity; instead the magnet must be utilized for its magnetic functions and other mechanical systems must be designed to provide the mechanical reliability of the system.
Attributes and Corrosion Characteristics
- Resistant to demagnetization
- Good temperature stability (maximum use temperatures from 250° to 550 °C; Curie temperatures from 700° to 800 °C)
- Expensive and subject to price fluctuations (cobalt is market price sensitive)
- Rare earth magnets are very resistant to corrosion and do not normally require any surface treatment
- Density: 8.4 g/cm³
- Electrical resistivity 0.8×10−4 Ω·cm
- Coefficient of thermal expansion (perpendicular to axis): 12.5 µm/(m·K)
- Fender is using one of legendary designer Bill Lawrence’s latest designs named the Samarium Cobalt Noiseless series of pickups (SCN) in Fender’s American Deluxe Series Guitars and Basses.3
- High-end electric motors used in the more competitive classes in Slotcar racing.
- Traveling-wave tube.
- Applications that will require the system to function at cryogenic temperatures or very hot temperatures (over 180° C).
- Applications where performance is required to be consistent with temperature change. The flux density of a samarium cobalt magnet will vary under 5% per 100° C change in temperature (in the range of 25° to 250° C).
Fully dense rare earth magnets “Samarium Cobalt” are usually manufactured by a powdered metallurgical process. Micron size Samarium Cobalt powder is produced and then compacted in a rigid steel mold. The steel molds will produce shapes similar to the final product, but the mechanical properties of the alloy usually inhibit complex features at this stage of the manufacturing process.
The Samarium Cobalt magnetic performance is optimized by applying a magnetic field during the pressing operation. This applied field imparts a preferred direction of magnetization, or orientation to the Samarium Cobalt magnet alloy. The alignment of particles results in an anisotropic alloy and vastly improves the residual induction (Br) and other magnetic characteristics of the finished magnet.
After pressing, the Samarium Cobalt magnets are sintered and heat treated until they reach their fully dense condition. The rare earth magnet alloy is then machined to the final dimensional requirements and cleaned.
Samarium Cobalt Temperature Characteristics
Sintered Samarium Cobalt rare earth magnets are extremely resistant to demagnetization and they can operate at temperatures up to 500° F (260° C). There are many Samarium Cobalt grades which can withstand higher temperatures, but several factors will dictate the overall performance of the Samarium Cobalt rare earth magnet. One of the most pertinent variables is the geometry of the magnet or magnetic circuit. Samarium Cobalt magnets which are relatively thin compared to their pole cross-section (Magnetic Length / Pole Area) will demagnetize easier than Samarium Cobalt magnets which are thick. Magnetic geometries utilizing backing plates, yokes, or return path structures will respond better to increased temperatures. The maximum recommended operating temperatures listed on the Samarium Cobalt magnetic characteristics page do not take into account all geometry conditions. Please contact a BuyMagnets.com team member for Samarium Cobalt rare earth magnet design assistance when elevated temperatures are involved in your application.
Samarium Cobalt rare earth magnets are extremely strong and they require a huge magnetizing field. Huge magnetizing fields require special equipment and aren’t generally magnetized by customers. The anisotropic nature of sintered Samarium Cobalt magnets results in a single direction of magnetization. This direction must be observed when magnetizing and when integrating the magnet into the final assembly. Often times an indicator is used to identify a specific magnetic pole for the customer’s assembly process. This indicator can be a laser engraved mark or simple paint dot.
The high field required for magnetizing Samarium Cobalt will often times restrict the design of the magnet or magnetic assembly. Many variables must be taken into account and a BuyMagnets.com team member can assist with the design process.
Samarium Cobalt magnets are very strong and brittle, and appropriate handling and packing is required. Most receiving departments are not familiar with the strength of rare earth magnets and this can result in injury or broken parts. All personnel that may come in contact with this alloy should be made aware of the dangers of handling these magnets. The brittle nature of the alloy can lead to flying chips if the magnets are allowed to impact each other or a solid surface. Larger magnets can become a pinching hazard if caution is not exercised.
The packaging methods of magnetized alloys are dependent upon the magnet size and the customer requirement. A brief overview of our standard packaging is listed below. Please alert your BuyMagnets.com contact if you require a different shipping method.
Quarter-sized or smaller magnets are usually put attracting in rows. They may or may not have plastic spacers between them in order to reduce the attracting force between the magnets. These rows may be wrapped in corrosion inhibiting paper (VCI) and the wrapped rows are arranged attracting in a brick. The bricks may be skin packaged on cardboard or wrapped in foam.
Magnets up to 2” square will be arranged attracting in rows with sizable spacers between each magnet. The rows can be arranged attracting with spacers running the length of the rows or individually wrapped in foam. Smaller quantities of these large magnets can go into an appropriate cardboard box, but larger volumes must be crated.
Large magnets, arrays, or assemblies will be packaged in wooden crates. Many times these products must be shipped via a LTL carrier.
A majority of our products are shipped ground by UPS, Fed Ex Ground, or LTL carriers. FAA guidelines consider magnetized material as hazardous goods when the field density emanating from the package sides exceed a specified value. Many air carriers will not accept magnetized material for air shipments. BuyMagnets.com is able to ship via air, but a packing/handling charge may be applied for larger volumes. These charges are only applied in extreme cases to cover the cost of extra shielding and packaging material, labor, and necessary paperwork.
Although fairly inert, Samarium Cobalt magnets should be stored in a low humidity and mild temperature environment. The magnetized alloy is very strong and it will attract ferrous particle from the air and surrounding surfaces. These particles will accumulate and appear as small “hairs” on the surface of the magnet or packaging. To combat the accumulated debris, the magnets should be kept in closed, clean containers and left in their original packaging. The magnets should remain in the attracting condition with all spacers intact. Metal shelving with poor clearance may cause the magnets to jump or shift as they are accessed. Do not store any magnetic material near sensitive electronics, equipment with cathode ray tubes (CRT), or magnetic storage media. Magnets which are not of the same alloy may need to be buffered from each other because of demagnetizing effects.
Samarium Cobalt magnets are used in a variety of applications such as motors, compact high force magnetic assemblies, dipole assemblies, microphones, speakers, sputtering arrays for vacuum deposition, triggering hall sensors, particle accelerators and many other applications.
The Magnetic Glossary
- Air Gap – Basically the “external” distance from one pole of the magnet to the other though a non-magnetic material (usually air).
- Anisotropic – Materials that have a “preferred” magnetization direction. These materials are typically manufactured in the influence of strong magnetic fields, and can only be magnetized through the preferred axis. Neodymium (Iron Boron) and Samarium Cobalt magnets are anisotropic.
- B/H Curve – The result of plotting the value of the magnetic field (H) that is applied against the resultant flux density (B) achieved. This curve describes the qualities of any magnetic material.
- BHmax (Maximum Energy Product) – The magnetic field strength at the point of maximum energy product of a magnetic material. The field strength of fully saturated magnetic material measured in Mega Gauss Oersteds, MGOe.
- Brmax (Residual Induction) – Also called “Residual Flux Density”. It is the magnetic induction remaining in a saturated magnetic material after the magnetizing field has been removed. This is the point at which the hysteresis loop crosses the B axis at zero magnetizing force, and represents the maximum flux output from the given magnet material. By definition, this point occurs at zero air gap, and therefore cannot be seen in practical use of magnet materials.
- C.G.S. – Abbreviation for the “Centimeter, Grams, Second” system of measurement.
- Coercive Force – The demagnetizing force, measured in Oersteds, necessary to reduce observed induction, B, to zero after the magnet has previously been brought to saturation.
- Curie Temperature (Tc) – The temperature at which a magnet loses all of its magnetic properties. Read more.
- Demagnetization Curve – The second quadrant of the hysteresis loop, generally describing the behavior of magnetic characteristics in actual use. Also known as the B-H Curve.
- Demagnetization Force – A magnetizing force, typically in the direction opposite to the force used to magnetize it in the first place. Shock, vibration and temperature can also be demagnetizing forces.
- Dimensions – The physical size of a magnet including any plating or coating.
- Dimensional Tolerance – An allowance, given as a permissible range, in the nominal dimensions of a finished magnet. The purpose of a tolerance is to specify the allowed leeway for imperfections in manufacturing.
- Electromagnet – A magnet consisting of a solenoid with an iron core, which has a magnetic field only during the time of current flow through the solenoid. Read more.
- Ferromagnetic Material – A material that either is a source of magnetic flux or a conductor of magnetic flux. Any ferromagnetic material must have some component of iron, nickel, or cobalt.
- Gauss – Unit of magnetic induction, B. Lines of magnetic flux per square centimeter in the C.G.S. system of measurement. Equivalent to lines per square inch in the English system, and Webers per square meter or Tesla in the S.I. system. Read more.
- Gauss meter – An instrument used to measure the instantaneous value of magnetic induction, B, usually measured in Gauss (C.G.S.). Read more.
- Gilbert – The unit of magnetomotive force, F, in the C.G.S. system.
- Hysteresis Loop – A plot of magnetizing force versus resultant magnetization (also called a B/H curve) of the material as it is successively magnetized to saturation, demagnetized, magnetized in the opposite direction and finally remagnetized. With continued recycles, this plot will be a closed loop which completely describes the characteristics of the magnetic material. The size and shape of this “loop” is important for both hard and soft materials. With soft materials, which are generally used in alternating circuits, the area inside this “loop” should be as thin as possible (it is a measure of energy loss). But with hard materials the “fatter” the loop, the stronger the magnet will be. The first quadrant of the loop (that is +X and +Y) is called the magnetization curve. It is of interest because it shows how much magnetizing force must be applied to saturate a magnet. The second quadrant (+X and -Y) is called the Demagnetization Curve.
- Induction, (B) – The magnetic flux per unit area of a section normal to the direction of flux. Measured in Gauss, in the C.G.S. system of units.
- Intrinsic Coercive Force (Hci) – Indicates a materials’ resistance to demagnetization. It is equal to the demagnetizing force which reduces the intrinsic induction, Bi, in the material to zero after magnetizing to saturation; measured in oersteds.
- Irreversible Losses – Partial demagnetization of the magnet, caused by exposure to high or low temperatures, external fields, shock, vibration, or other factors. These losses are only recoverable by remagnetization. Magnets can be stabilized against irreversible losses by partial demagnetization induced by temperature cycles or by external magnetic fields.
- Isotropic Material – A material that can be magnetized along any axis or direction (a magnetically unoriented material). The opposite of Anisotropic Magnet.
- Keeper – A soft iron piece temporarily added between the poles of a magnetic circuit to protect it from demagnetizing influences. Also called a shunt. Not needed for Neodymium and other modern magnets.
- Kilogauss – One Kilogauss = 1,000 Gauss = Maxwells per square centimeter.
- Magnet – A magnet is an object made of certain materials which create a magnetic field. Every magnet has at least one north pole and one south pole. By convention, we say that the magnetic field lines leave the North end of a magnet and enter the South end of a magnet. This is an example of a magnetic dipole (“di” means two, thus two poles).
If you take a bar magnet and break it into two pieces, each piece will again have a North pole and a South pole. If you take one of those pieces and break it into two, each of the smaller pieces will have a North pole and a South pole. No matter how small the pieces of the magnet become, each piece will have a North pole and a South pole. It has not been shown to be possible to end up with a single North pole or a single South pole which is a monopole (“mono” means one or single, thus one pole).
- Magnetic Circuit – Consists of all elements, including air gaps and non-magnetic materials that the magnetic flux from a magnet travels on, starting from the north pole of the magnet to the south pole.
- Magnetic Field (B) – When specified on our site, the surface field or magnetic field refers to the strength in Gauss. For axially magnetized discs and cylinders, it is specified on the surface of the magnet, along the center axis of magnetization. For blocks, it is specified on the surface of the magnet, also along the center axis of magnetization. For rings, you may see two values. By,center specifies the vertical component of the magnetic field in the air at the center of the ring. By,ring specifies the vertical component of the magnetic field on the surface of the magnet, mid-way between the inner and outer diameters. Some depictions of magnet fields can be found here.
- Magnetic Field Strength (H) – Magnetizing or demagnetizing force, is the measure of the vector magnetic quantity that determines the ability of an electric current, or a magnetic body, to induce a magnetic field at a given point; measured in Oersteds.
- Magnetic Flux – Is a contrived but measurable concept that has evolved in an attempt to describe the “flow” of a magnetic field. When the magnetic induction, B, is uniformly distributed and is normal to the area, A, the flux, Ø = BA.
- Magnetic Flux Density – Lines of flux per unit area, usually measured in Gauss (C.G.S.). One line of flux per square centimeter is one Maxwell.
- Magnetic Induction (B) – The magnetic field induced by a field strength, H, at a given point. It is the vector sum, at each point within the substance, of the magnetic field strength and the resultant intrinsic induction. Magnetic induction is the flux per unit area normal to the direction of the magnetic path.
- Magnetic Line of Force – An imaginary line in a magnetic field, which, at every point, has the direction of the magnetic flux at that point.
- Magnetic Pole – An area where the lines of flux are concentrated.
- Magnetomotive Force (F or mmf) – The magnetic potential difference between any two points. Analogous to voltage in electrical circuits. That which tends to produce a magnetic field. Commonly produced by a current flowing through a coil of wire. Measured in Gilberts (C.G.S.) or Ampere Turns (S.I.).
- Material Grade – Neodymium (NdFeB) magnets are graded by the magnetic material from which they are manufactured. Generally speaking, the higher the grade of material, the stronger the magnet. Neodymium magnets currently range in grade from N27 to N52. The theoretical limit for Neodymium magnets is grade N64. The grade of most of our stock magnets is N42 because we feel that N42 provides the optimal balance between strength and cost. We also stock a wide range of sizes in grade N52 for customers who need the strongest permanent magnets available.
- Maximum Energy Product (BHmax) – The magnetic field strength at the point of maximum energy product of a magnetic material. The field strength of fully saturated magnetic material measured in Mega Gauss Oersteds, MGOe.
- Maximum Operating Temperature (Tmax) – Also known as maximum service temperature, is the temperature at which the magnet may be exposed to continuously with no significant long-range instability or structural changes.
- Maxwell – Unit of magnetic flux in the C.G.S. electromagnetic system. One maxwell is one line of magnetic flux.
- Magnetization Curve – The first quadrant portion of the hysteresis loop (B/H) Curve for a magnetic material.
- Magnetizing Force (H) – The magnetomotive force per unit of magnet length, measured in Oersteds (C.G.S.) or ampere-turns per meter (S.I). Maxwell – The C.G.S. unit for total magnetic flux, measured in flux lines per square centimeter.
- MGOe – Mega (million) Gauss Oersteds. Unit of measure typically used in stating the maximum energy product for a given material. See Maximum Energy Product.
- North Pole – The north pole of a magnet is the one attracted to the magnetic north pole of the earth. This north-seeking pole is identified by the letter N. By accepted convention, the lines of flux travel from the north pole to the south pole.
- Oersted (Oe) – The C.G.S. unit for magnetizing force. The English system equivalent is Ampere Turns per Inch (1 Oersted equals 79.58 A/m). The S.I. unit is Ampere Turns per Meter.
- Orientation – Used to describe the direction of magnetization of a material.
- Orientation Direction – The direction in which an anisotropic magnet should be magnetized in order to achieve optimum magnetic properties.
- Paramagnetic Materials – Materials that are not attracted to magnetic fields (wood, plastic, aluminum, etc.). A material having a permeability slightly greater than 1.
- Permanent Magnet – A magnet that retains its magnetism after it is removed from a magnetic field. A permanent magnet is “always on”. Neodymium magnets are permanent magnets.
- Permeance (P) – A measure of relative ease with which flux passes through a given material or space. It is calculated by dividing magnetic flux by magnetomotive force. Permeance is the reciprocal of reluctance.
- Permeance Coefficient (Pc) – Also called the load-line, B/H or “operating slope” of a magnet, this is the line on the Demagnetization Curve where a given magnet operates. The value depends on both the shape of the magnet, and it’s surrounding environment (some would say, how it’s used in a circuit). In practical terms, it’s a number that define how hard it is for the field lines to go from the north pole to the south pole of a magnet. A tall cylindrical magnet will have a high Pc, while a short, thin disc will have a low Pc.
- Permeability (µ) – The ratio of the magnetic induction of a material to the magnetizing force producing it (B/H). The magnetic permeability of a vacuum (µo) is 4π×10-7 N/Amp2.
- Pole – An area where the lines of magnetic flux are concentrated.
- Plating/Coating – Most neodymium magnets are plated or coated in order to protect the magnet material from corrosion. Neodymium magnets are mostly composed of neodymium, iron, and boron. The iron in the magnet will rust if it is not sealed from the environment by some sort of plating or coating. Most of the neodymium magnets that we stock are triple plated in nickel-copper-nickel, but some are plated in gold, silver, or black nickel, while others are coated in epoxy, plastic or rubber.
- Polarity – The characteristic of a particular pole at a particular location of a permanent magnet. Differentiates the North from the South Pole.
- Pull Force – The force required to pull a magnet free from a flat steel plate using force perpendicular to the surface. The limit of the holding power of a magnet. Read more.
- Rare Earth – Commonly used to describe high energy magnet material such as NdFeB (Neodymium-Iron-Boron) and SmCo (Samarium-Cobalt). Read more.
- Relative Permeability – The ratio of permeability of a medium to that of a vacuum. In the C.G.S. system, the permeability is equal to 1 in a vacuum by definition. The permeability of air is also for all practical purposes equal to 1 in the C.G.S. system.
- Reluctance ®- A measure of the relative resistance of a material to the passage of flux. It is calculated by dividing magnetomotive force by magnetic flux. Reluctance is the reciprocal of permeance.
- Remanence, (Bd) – The magnetic induction that remains in a magnetic circuit after the removal of an applied magnetizing force.
- Residual Flux Density (Brmax) – Also called “Residual Induction”. It is the magnetic induction remaining in a saturated magnetic material after the magnetizing field has been removed. This is the point at which the hysteresis loop crosses the B axis at zero magnetizing force, and represents the maximum flux output from the given magnet material. By definition, this point occurs at zero air gap, and therefore cannot be seen in practical use of magnet materials.
- Residual Induction (Brmax) – Also called “Residual Flux Density”. It is the magnetic induction remaining in a saturated magnetic material after the magnetizing field has been removed. This is the point at which the hysteresis loop crosses the B axis at zero magnetizing force, and represents the maximum flux output from the given magnet material. By definition, this point occurs at zero air gap, and therefore cannot be seen in practical use of magnet materials.
- Return Path – Conduction elements in a magnetic circuit which provide a low reluctance path for the magnetic flux. Reversible Temperature Coefficient: A measure of the reversible changes in flux caused by temperature variations.
- Saturation – The state where an increase in magnetizing force produces no further increase in magnetic induction in a magnetic material.
- Shunt – A soft iron piece temporarily added between the pole of a magnetic circuit to protect it from demagnetizing influences. Also called a keeper. Not needed for Neodymium and other modern magnets.
- S.I. – Abbreviation for “Système International”. Refers to the International Standard System of units. It is also known as the MKS system.
- South Pole – The south pole of a magnet is the one attracted to the south pole of the earth. This south-seeking pole is identified by the letter S. By accepted convention, the lines of flux travel from the north pole to the south pole.
- Stabilization – The process of exposing a magnet or a magnetic assembly to elevated temperatures or external magnetic fields to demagnetize it to a predetermined level. Once done the magnet will suffer no future degradation when exposed to that level of demagnetizing influence.
- Surface Field (Surface Gauss) – The magnetic field strength at the surface of the magnet as measured by a Gauss meter.
- Temperature Coefficient – A factor that is used to calculate the decrease in magnetic flux corresponding to an increase in operating temperature. The loss in magnetic flux is recovered when the operating temperature is decreased.
- Tesla – The S.I. unit for magnetic induction (flux density). One Tesla equals 10,000 Gauss.
- Weber – The S.I. unit for total magnetic flux. The practical unit of magnetic flux. It is the amount of magnetic flux which, when linked at a uniform rate with a single-turn electric circuit during an interval of 1 second, will induce in this circuit an electromotive force of 1 volt.
- Weight – The weight of a single magnet.
An electromagnet is a type of magnet whose magnetic field is produced by the flow of electric current. The magnetic field disappears when the current ceases. Electromagnets are very widely used as components of other electrical devices, such as motors, generators, relays, loud speakers, hard disks, MRI machines, scientific instruments, and magnetic separation equipment, as well as being employed as industrial lifting electromagnets for picking up and moving heavy iron objects like scrap iron.
Electromagnets attract paper clips when current is applied creating a magnetic field. The electromagnet loses them when current and magnetic field are removed.
A simple electromagnet consisting of a coil of insulated wire wrapped around an iron core. The strength of magnetic field generated is proportional to the amount of current.
Current (I) through a wire produces a magnetic field (B). The field is oriented according to the right-hand rule.
An electric current flowing in a wire creates a magnetic field around the wire (see drawing below). To concentrate the magnetic field of a wire, in an electromagnet the wire is wound into a coil, with many turns of wire lying side by side. The magnetic field of all the turns of wire passes through the center of the coil, creating a strong magnetic field there. A coil forming the shape of a straight tube (a helix) is called a solenoid; a solenoid that is bent into a donut shape so that the ends meet is called a toroid.
Much stronger magnetic fields can be produced if a “core” of ferromagnetic material, such as soft iron, is placed inside the coil. The ferromagnetic core magnifies the magnetic field to thousands of times the strength of the field of the coil alone, due to the high magnetic permeability μ of the ferromagnetic material. This is called a ferromagnetic-core or iron-core electromagnet.
The direction of the magnetic field through a coil of wire can be found from a form of the right-hand rule. If the fingers of the right hand are curled around the coil in the direction of current flow (conventional current, flow of positive charge) through the windings, the thumb points in the direction of the field inside the coil. The side of the magnet that the field lines emerge from is defined to be the north pole.
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 amount of electric current. However, a continuous supply of electrical energy is required to maintain the field.
How the iron core works
The material of the core of the magnet (usually iron) is composed of small regions called magnetic domains that act like tiny magnets (see ferromagnetism). Before the current in the electromagnet is turned on, the domains in the iron core point in random directions, so their tiny magnetic fields cancel each other out, and the iron has no large scale magnetic field. When a current is passed through the wire wrapped around the iron, its magnetic field penetrates the iron, and causes the domains to turn, aligning parallel to the magnetic field, so their tiny magnetic fields add to the wire’s field, creating a large magnetic field that extends into the space around the magnet. The larger the current passed through the wire coil, the more the domains align, and the stronger the magnetic field is. Finally all the domains are lined up, and further increases in current only cause slight increases in the magnetic field: this phenomenon is called saturation.
When the current in the coil is turned off, most of the domains lose alignment and return to a random state and the field disappears. However some of the alignment persists, because the domains have difficulty turning their direction of magnetization, leaving the core a weak permanent magnet. This phenomenon is called hysteresis and the remaining magnetic field is called remanent magnetism. The residual magnetization of the core can be removed by degaussing.
Danish scientist Hans Christian Ørsted discovered in 1820 that electric currents create magnetic fields. British scientist William Sturgeon invented the electromagnet in 1824. His first electromagnet was a horseshoe-shaped piece of iron that was wrapped with about 18 turns of bare copper wire (insulated wire didn’t exist yet). The iron was varnished to insulate it from the windings. When a current was passed through the coil, the iron became magnetized and attracted other pieces of iron; when the current was stopped, it lost magnetization. Sturgeon displayed its power by showing that although it only weighed seven ounces (roughly 200 grams), it could lift nine pounds (roughly 4 kilos) when the current of a single-cell battery was applied. However, Sturgeon’s magnets were weak because the uninsulated wire he used could only be wrapped in a single spaced out layer around the core, limiting the number of turns. Beginning in 1827, US scientist Joseph Henry systematically improved and popularized the electromagnet. By using wire insulated by silk thread he was able to wind multiple layers of wire on cores, creating powerful magnets with thousands of turns of wire, including one that could support 2063 pounds. The first major use for electromagnets was in telegraph sounders.
The magnetic domain theory of how ferromagnetic cores work was first proposed in 1906 by French physicist Pierre-Ernest Weiss, and the detailed modern quantum mechanical theory of ferromagnetism was worked out in the 1920s by Werner Heisenberg, Lev Landau, Felix Bloch and others.
Uses of electromagnets
- Electromagnets are very widely used in electric and electromechanical devices, including:
- Motors and generators
- Relays, including reed relays originally used in telephone exchanges
- Electric bells
- Magnetic recording and data storage equipment: tape recorders, VCRs, hard disks
- Scientific instruments such as MRI machines and mass spectrometers
- Particle accelerators
- Magnetic locks
- Magnetic separation of material
- Industrial lifting magnets
- Electromagnetic suspension used for MAGLEV trains
- Analysis of ferromagnetic electromagnets
For definitions of the variables below, see box at end of article.
The magnetic field of electromagnets in the general case is given by Ampere’s Law:
Which says that the integral of the magnetizing field H around any closed loop of the field is equal to the sum of the current flowing through the loop. Another equation used, that gives the magnetic field due to each small segment of current, is the Biot-Savart law. Computing the magnetic field and force exerted by ferromagnetic materials is difficult for two reasons. First, because the strength of the field varies from point to point in a complicated way, particularly outside the core and in air gaps, where fringing fields and leakage flux must be considered. Second, because the magnetic field B and force are nonlinear functions of the current, depending on the nonlinear relation between B and H for the particular core material used. For precise calculations, computer programs that can produce a model of the magnetic field using the finite element method are employed.
Magnetic circuit – the constant B field approximation
Magnetic field (green) of a typical electromagnet, with the iron core C forming a closed loop with two air gaps G in it. Most of the magnetic field B is concentrated in the core. However some of the field lines BL, called the “leakage flux”, do not follow the full core circuit and so do not contribute to the force exerted by the electromagnet. In the gaps G the field lines spread out beyond the boundaries of the core in “fringing fields” BF. This increases the “resistance” (reluctance) of the magnetic circuit, decreasing the total magnetic flux in the core. Both the leakage flux and the fringing fields get larger as the gaps are increased, reducing the force exerted by the magnet. Line L shows the average length of the magnetic circuit, used in equation (1) below. It is the sum of the length Lcore in the iron core and the length Lgap in the air gaps.
In many practical applications of electromagnets, such as motors, generators, transformers, lifting magnets, and loudspeakers, the iron core is in the form of a loop or magnetic circuit, possibly broken by a few narrow air gaps. This is because iron presents much less “resistance” (reluctance) to the magnetic field than air, so a stronger field can be obtained if most of the magnetic field’s path is within the core.
Since most of the magnetic field is confined within the outlines of the core loop, this allows a simplification of the mathematical analysis. See the drawing at right. A common simplifying assumption satisfied by many electromagnets, which will be used in this section, is that the magnetic field strength B is constant around the magnetic circuit and zero outside it. Most of the magnetic field will be concentrated in the core material ©. Within the core the magnetic field (B) will be approximately uniform across any cross section, so if in addition the core has roughly constant area throughout its length, the field in the core will be constant. This just leaves the air gaps (G), if any, between core sections. In the gaps the magnetic field lines are no longer confined by the core, so they ‘bulge’ out beyond the outlines of the core before curving back to enter the next piece of core material, reducing the field strength in the gap. The bulges (BF) are called fringing fields. However, as long as the length of the gap is smaller than the cross section dimensions of the core, the field in the gap will be approximately the same as in the core. In addition, some of the magnetic field lines (BL) will take ‘short cuts’ and not pass through the entire core circuit, and thus will not contribute to the force exerted by the magnet. This also includes field lines that encircle the wire windings but do not enter the core. This is called leakage flux. Therefore the equations in this section are valid for electromagnets for which:
- 1. The magnetic circuit is a single loop.
- 2. The core has roughly the same cross sectional area throughout its length.
- 3. Any air gaps between sections of core material are not large compared with the cross sectional dimensions of the core.
- 4. There is negligible leakage flux.
The main nonlinear feature of ferromagnetic materials is that the B field saturates at a certain value, which is around 1.6 teslas (T) for most high permeability core steels. The B field increases quickly with increasing current up to that value, but above that value the field levels off and becomes almost constant, regardless of how much current is sent through the windings. So the strength of the magnetic field possible from an iron core electromagnet is limited to 1.6–2 T.
Magnetic field created by a current
The magnetic field created by an electromagnet is proportional to both the number of turns in the winding, N, and the current in the wire, I, hence this product, NI, in ampere-turns, is given the name magnetomotive force. For an electromagnet with a single magnetic circuit, of which length Lcore is in the core material and length Lgap is in air gaps, Ampere’s Law reduces to:
is the permeability of free space (or air).
This is a nonlinear equation, because the permeability of the core, μ, varies with the magnetic field B. For an exact solution, the value of μ at the B value used must be obtained from the core material hysteresis curve. If B is unknown, the equation must be solved by numerical methods. However, if the magnetomotive force is well above saturation, so the core material is in saturation, the magnetic field won’t vary much with changes in NI anyway. For a closed magnetic circuit (no air gap) most core materials saturate at a magnetomotive force of roughly 800 ampere-turns per meter of flux path.
So in equation (1) above, the second term dominates. Therefore, in magnetic circuits with an air gap, the behavior of the magnet depends strongly on the length of the air gap, and the length of the flux path in the core doesn’t matter much.
Force exerted by magnetic field
When none of the magnetic field bypasses any sections of the core (no flux leakage), the force exerted by an electromagnet on a section of core material is:
The 1.6 T limit on the field mentioned above sets a limit on the maximum force per unit core area, or pressure, an iron-core electromagnet can exert; roughly:
Given a core geometry, the B field needed for a given force can be calculated from (2); if it comes out to much more than 1.6 T, a larger core must be used.
Closed magnetic circuit
Cross section of lifting electromagnet like that in above photo, showing cylindrical construction. The windings © are flat copper strips to withstand the Lorentz force of the magnetic field. The core is formed by the thick iron housing (D) that wraps around the windings.
For a closed magnetic circuit (no air gap), such as would be found in an electromagnet lifting a piece of iron bridged across its poles, equation (1) becomes:
Substituting into (2), the force is:
It can be seen that to maximize the force, a core with a short flux path L and a wide cross sectional area A is preferred. To achieve this, in applications like lifting magnets (see photo above) and loudspeakers a flat cylindrical design is often used. The winding is wrapped around a short wide cylindrical core that forms one pole, and a thick metal housing that wraps around the outside of the windings forms the other part of the magnetic circuit, bringing the magnetic field to the front to form the other pole.
Force between electromagnets
The above methods are inapplicable when most of the magnetic field path is outside the core. For electromagnets (or permanent magnets) with well defined ‘poles’ where the field lines emerge from the core, the force between two electromagnets can be found using the ‘Gilbert model’ which assumes the magnetic field is produced by fictitious ‘magnetic charges’ on the surface of the poles, with pole strength m and units of Ampere-turn meter. Magnetic pole strength of electromagnets can be found from:
The force between two poles is:
This model doesn’t give the correct magnetic field inside the core, and thus gives incorrect results if the pole of one magnet gets too close to another magnet.
Side effects in large electromagnets
There are several side effects which become important in large electromagnets and must be provided for in their design.
The only power consumed in a DC electromagnet is due to the resistance of the windings, and is dissipated as heat. Some large electromagnets require cooling water circulating through pipes in the windings to carry off the waste heat.
Since the magnetic field is proportional to the product NI, the number of turns in the windings N and the current I can be chosen to minimize heat losses, as long as their product is constant. Since the power dissipation, P = I2R, increases with the square of the current, the power lost in the windings can be minimized by reducing I and increasing the number of turns N proportionally. This is one reason most electromagnets have windings with many turns of wire.
However, the limit to increasing N is that the larger number of windings takes up more room between the magnet’s core pieces. If the area available for the windings is filled up, more turns require going to a smaller diameter of wire, which has higher resistance, which cancels the advantage of using more turns. So in large magnets there is a minimum amount of heat loss that can’t be reduced. This increases with the square of the magnetic flux B2.
Inductive voltage spikes
An electromagnet is a large inductor, and resists changes in the current through its windings. Any sudden changes in the winding current cause large voltage spikes across the windings. This is because when the current through the magnet is increased, such as when it is turned on, energy from the circuit must be stored in the magnetic field. When it is turned off the energy in the field is returned to the circuit.
If an ordinary switch is used to control the winding current, this can cause sparks at the terminals of the switch. This doesn’t occur when the magnet is switched on, because the voltage is limited to the power supply voltage. But when it is switched off, the energy in the magnetic field is suddenly returned to the circuit, causing a large voltage spike and an arc across the switch contacts, which can damage them. With small electromagnets a capacitor is often used across the contacts, which reduces arcing by temporarily storing the current. More often a diode is used to prevent voltage spikes by providing a path for the current to recirculate through the winding until the energy is dissipated as heat. The diode is connected across the winding, oriented so it is reverse-biased during steady state operation and doesn’t conduct. When the supply voltage is removed, the voltage spike forward-biases the diode and the reactive current continues to flow through the winding, through the diode and back into the winding.
Large electromagnets are usually powered by variable current electronic power supplies, controlled by a microprocessor, which prevent voltage spikes by accomplishing current changes in gentle ramps. It may take several minutes to energize or deenergize a large magnet.
In powerful electromagnets, the magnetic field exerts a force on each turn of the windings, due to the Lorentz force acting on the moving charges within the wire. The Lorentz force is perpendicular to both the axis of the wire and the magnetic field. It can be visualized as a pressure between the magnetic field lines, pushing them apart. It has two effects on an electromagnet’s windings:
- The field lines within the axis of the coil exert a radial force on each turn of the windings, tending to push them outward in all directions. This causes a tensile stress in the wire.
- The leakage field lines between each turn of the coil exert a repulsive force between adjacent turns, tending to push them apart.
The Lorentz forces increase with B2. In large electromagnets the windings must be firmly clamped in place, to prevent motion on power-up and power-down from causing metal fatigue in the windings. In the Bitter design, below, used in very high field research magnets, the windings are constructed as flat disks to resist the radial forces, and clamped in an axial direction to resist the axial ones.
In alternating current (AC) electromagnets, used in transformers, inductors, and AC motors and generators, the magnetic field is constantly changing. This causes energy losses in their magnetic cores that are dissipated as heat in the core. The losses stem from two processes:
- Eddy currents: From Faraday’s law of induction, the changing magnetic field induces circulating electric currents inside nearby conductors, called eddy currents. The energy in these currents is dissipated as heat in the electrical resistance of the conductor, so they are a cause of energy loss. Since the magnet’s iron core is conductive, and most of the magnetic field is concentrated there, eddy currents in the core are the major problem. Eddy currents are closed loops of current that flow in planes perpendicular to the magnetic field. The energy dissipated is proportional to the area enclosed by the loop. To prevent them, the cores of AC electromagnets are made of stacks of thin steel sheets, or laminations, oriented parallel to the magnetic field, with an insulating coating on the surface. The insulation layers prevent eddy current from flowing between the sheets. Any remaining eddy currents must flow within the cross section of each individual lamination, which reduces losses greatly. Another alternative is to use a ferrite core, which is a nonconductor.
- Hysteresis losses: Reversing the direction of magnetization of the magnetic domains in the core material each cycle causes energy loss, because of the coercivity of the material. These losses are called hysteresis. The energy lost per cycle is proportional to the area of the hysteresis loop in the BH graph. To minimize this loss, magnetic cores used in transformers and other AC electromagnets are made of “soft” low coercivity materials, such as silicon steel or soft ferrite.
The energy loss per cycle of the AC current is constant for each of these processes, so the power loss increases linearly with frequency.
High field electromagnets
When a magnetic field higher than the ferromagnetic limit of 1.6 T is needed, superconducting electromagnets can be used. Instead of using ferromagnetic materials, these use superconducting windings cooled with liquid helium, which conduct current without electrical resistance. These allow enormous currents to flow, which generate intense magnetic fields. Superconducting magnets are limited by the field strength at which the winding material ceases to be superconducting. Current designs are limited to 10–20 T, with the current (2009) record of 33.8 T. The necessary refrigeration equipment and cryostat make them much more expensive than ordinary electromagnets. However, in high power applications this can be offset by lower operating costs, since after startup no power is required for the windings, since no energy is lost to ohmic heating. They are used in particle accelerators, MRI machines, and research.
Since both iron-core and superconducting electromagnets have limits to the field they can produce, the highest manmade magnetic fields have been generated by air-core nonsuperconducting electromagnets of a design invented by Francis Bitter in 1933, called Bitter electromagnets. These consist of a solenoid made of a stack of conducting disks, arranged so that the current moves in a helical path through them.
This design has the mechanical strength to withstand the extreme Lorentz forces of the field, which increase with B2. The disks are pierced with holes through which cooling water passes to carry away the heat caused by the high current. The highest continuous field achieved with a resistive magnet is currently (2008) 35 T. The highest continuous magnetic field, 45 T, was achieved with a hybrid device consisting of a Bitter magnet inside a superconducting magnet.
The factor limiting the strength of electromagnets is the inability to dissipate the enormous waste heat, so higher fields, up to 90 T, have been obtained from resistive magnets by pulsing them. The highest magnetic fields of all have been created by detonating explosives around a pulsed electromagnet as it is turned on. The implosion compresses the magnetic field to values of around 1000 T for a few microseconds.
The Basics of an Electromagnet
The basic idea behind an electromagnet is extremely simple: By running electric current through a wire, you can create a magnetic field.
By using this simple principle, you can create all sorts of things, including motors, solenoids, read/write heads for hard disks and tape drives, speakers, and so on.
An electromagnet starts with a battery (or some other source of power) and a wire. What a battery produces is electrons.
If you look at a battery, say at a normal D-cell from a flashlight, you can see that there are two ends, one marked plus (+) and the other marked minus (-). Electrons collect at the negative end of the battery, and, if you let them, they will gladly flow to the positive end. The way you “let them” flow is with a wire. If you attach a wire directly between the positive and negative terminals of a D-cell, three things will happen:
1. Electrons will flow from the negative side of the battery to the positive side as fast as they can.
2. The battery will drain fairly quickly (in a matter of several minutes). For that reason, it is generally not a good idea to connect the two terminals of a battery to one another directly. Normally, you connect some kind of load in the middle of the wire so the electrons can do useful work. The load might be a motor, a light bulb, a radio or whatever.
3. A small magnetic field is generated in the wire. It is this small magnetic field that is the basis of an electromagnet.
An Overview of Ceramic Magnets
Ceramic magnets are comprised of a combination of iron oxide and strontium carbonate. These materials are easily attainable, and are available at a lower cost than other materials used to make permanent magnets. It’s one of the reasons ceramic magnets are more popular among the permanent magnets; they’re less expensive!
Ceramic magnets are made using a sintering process. The wet milling process produces slurry which is fed into a die. This material is pressed into a green product which is later sintered at a high temperature. Once cooled, these magnets can be ground and cut to desired shapes.
Ceramic Magnets are Permanent Magnets
Did you know the Earth itself is a permanent magnet? A permanent magnet is one that is always a magnet, as opposed to something like an electromagnet, which is only magnetized when there is an actual electric current flowing through it. It’s true, permanent magnets aren’t as strong as electromagnets, but they still have plenty of uses. Ceramic magnets in particular are used for things like speaker magnets, MRI machines, motors, magnetic lifters, magnetic separators, and other types of magnetic assemblies and tools.
Properties of Ceramic Magnets
Ceramic magnets are typically hard and brittle. They require diamond wheels if grinding is necessary. They have a good balance of magnetic strength, resistance to demagnetizing, as well as economy. Ceramic magnets are made using pressing and sintering techniques.
Grades of Ceramic Magnets
Ceramic magnets come in different grades: 1, 5 and 8. Grade one is an isotropic grade, which means it possesses equal magnetic properties from every direction. Grades 5 and 8, though, are considered anisotropic grades. This means they’re only magnetized in the direction they’re pressed. The anisotropic grades are more powerful. Grade one is less powerful magnetic strength.
Here’s a table that breaks down the properties of ceramic properties:
Magnet Grade Max. Energy product (BH)max (MGO) Residual Induction Br(gauss) Coercive Force Hc(oersteds) Intrinsic Coercive Force Hci(oersteds) Max Operating Temperature °F / °C C1 1.00 2200 1825 3250 480/249 C5 3.40 3800 2400 2420 480/249 C8 3.50 3850 2950 3250 480/249
Ceramic Magnet Tolerance
The tolerance on the dimensions between its unfinished surfaces is ±2% or ±.015” (whichever is greater). Tolerance on the dimensions between finished surfaces is ±.005”. Tolerance on thickness of C5 and C8 ceramic magnets are ±.005”.
Machining Ceramic Magnets
Ceramic material is very brittle. Because it breaks easily, there are special tools required to handle it.
Handling a Ceramic Magnet
As we’ve mentioned before, ceramic magnets are brittle and break easily. Because of this, you must be aware that it could chip, break, or even shatter if dropped or allowed to jump to something it is attracted to. The weakest type of ceramic magnet is the grade 1, so be extra careful with those.
The Pros and Cons
As we discussed above, the pros to ceramic magnets include their low cost, their high resistance to demagnetization, and high resistance to corrosion. Their negatives are fewer in number, though, which makes them the most widely-used magnet today. They’re considered a low-energy product, and are brittle.