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Neodymium Magnets Definitive Guide

Magnets History, Measures, Earth Demonstrations, Uses by Industry, Science & by Casual Consumers 

This article is about magnetics  and magnetic products that produce magnetic fields. 

Some portions of this article came from Wikipedia, the free encyclopedia and are being used under the creative commons share & Share alike clause.  Those portions may be used by anyone. Other portions belong to CMS Magnetics & Magnets For Sale & are copyrighted and may not be used without written permission. 

 

MAGNETS 

 

A magnet is a material or item that creates an attractive field. This attractive field is imperceptible yet is liable for the most outstanding property of a magnet: a power that pulls on other ferromagnetic materials, for example, iron, and draws in or repulses different magnets. 

 

A lasting magnet is an item produced using a material that is charged and makes its own determined attractive field. An ordinary model is a fridge magnet used to hold notes on a cooler entryway. Materials that can be polarized, which are likewise the ones that are emphatically pulled in to a magnet, are called ferromagnetic (or ferrimagnetic). These incorporate the components iron, nickel and cobalt and their compounds, some combinations of uncommon earth metals, and some normally happening minerals, for example, lodestone. Albeit ferromagnetic (and ferrimagnetic) materials are the main ones pulled in to a magnet unequivocally enough to be usually viewed as attractive, every single other substance react feebly to an attractive field, by one of a few different kinds of attraction. 

 

Ferromagnetic materials can be partitioned into attractively "delicate" materials like strengthened iron, which can be polarized yet don't will in general remain charged, and attractively "hard" materials, which do. Perpetual magnets are produced using "hard" ferromagnetic materials, for example, alnico and ferrite that are exposed to unique handling in a solid attractive field during production to adjust their inside microcrystalline structure, making them exceptionally difficult to demagnetize. To demagnetize a soaked magnet, a specific attractive field must be applied, and this edge relies upon coercivity of the separate material. "Hard" materials have high coercivity, while "delicate" materials have low coercivity. The general quality of a magnet is estimated by its attractive minute or, on the other hand, the all out attractive transition it produces. The nearby quality of attraction in a material is estimated by its charge. 

 

An electromagnet is produced using a curl of wire that goes about as a magnet when an electric flow goes through it however quits being a magnet when the flow stops. Regularly, the loop is folded over a center of "delicate" ferromagnetic material, for example, mellow steel, which incredibly improves the attractive field created by the curl. 

 

Ancient people found out about attraction from lodestones (or magnetite) which are normally charged bits of iron mineral. The word magnet was embraced in Middle English from Latin magnetum "lodestone", at last from Greek μαγνῆτις [λίθος] (magnētis [lithos])[1] signifying "[stone] from Magnesia",[2] a piece of antiquated Greece where lodestones were found. Lodestones, suspended so they could turn, were the main attractive compasses. The soonest known enduring portrayals of magnets and their properties are from Greece, India, and China around 2500 years ago.[3][4][5] The properties of lodestones and their fondness for iron were composed of by Pliny the Elder in his reference book Naturalis Historia.[6] 

 

By the twelfth to thirteenth hundreds of years AD, attractive compasses were utilized in route in China, Europe, the Arabian Peninsula and elsewhere.[7] 

 

The attractive motion thickness (likewise called attractive B field or simply attractive field, for the most part signified B) is a vector field. The attractive B field vector at a given point in space is determined by two properties: 

 

Its bearing, which is along the direction of a compass needle. 

 

Its extent (additionally called quality), which corresponds to how emphatically the compass needle situated along that heading. 

 

In SI units, the quality of the attractive B field is given in teslas.[8] 

 

Attractive minute 

 

Primary article: Magnetic minute 

 

A magnet's attractive minute (additionally called attractive dipole minute and normally meant μ) is a vector that describes the magnet's general attractive properties. For a bar magnet, the bearing of the attractive minute focuses from the magnet's south shaft to its north pole,[9] and the size identifies with how solid and how far separated these posts are. In SI units, the attractive minute is indicated as far as A·m2 (amperes times meters squared). 

 

A magnet the two delivers its own attractive field and reacts to attractive fields. The quality of the attractive field it produces is at some random direct relative toward the size of its attractive minute. What's more, when the magnet is placed into an outside attractive field, created by an alternate source, it is dependent upon a torque tending to arrange the attractive minute parallel to the field.[10] The measure of this torque is relative both to the attractive minute and the outer field. A magnet may likewise be dependent upon a power driving it toward some path, as indicated by the positions and directions of the magnet and source. In the event that the field is uniform in space, the magnet is liable to no net power, in spite of the fact that it is dependent upon a torque.[11] 

 

A wire in the state of a hover with territory An and conveying current I has an attractive snapshot of size equivalent to IA. 

 

Polarization 

 

Fundamental article: Magnetization 

 

The polarization of a charged material is the nearby estimation of its attractive minute per unit volume, for the most part indicated M, with units A/m.[12] It is a vector field, as opposed to only a vector (like the attractive minute), in light of the fact that various territories in a magnet can be polarized with various headings and qualities (for instance, on account of spaces, see underneath). A decent bar magnet may have an attractive snapshot of extent 0.1 A•m2 and a volume of 1 cm3, or 1×10−6 m3, and in this way a normal charge size is 100,000 A/m. Iron can have a polarization of around a million amperes for every meter. Such a huge worth clarifies why iron magnets are so compelling at creating attractive fields. 

 

Demonstrating magnets 

 

Field of a tube shaped bar magnet registered precisely 

 

See additionally: Two meanings of minute 

 

Two distinct models exist for magnets: attractive shafts and nuclear flows. 

 

In spite of the fact that for some reasons it is helpful to think about a magnet as having unmistakable north and south attractive shafts, the idea of posts ought not be taken truly: it is only a method for alluding to the two distinct parts of the bargains. The magnet doesn't have particular north or south particles on rival sides. On the off chance that a bar magnet is broken into two pieces, trying to isolate the north and south shafts, the outcome will be two bar magnets, every one of which has both a north and south post. In any case, a rendition of the attractive shaft approach is utilized by expert magneticians to structure perpetual magnets.[citation needed] 

 

In this methodology, the difference of the polarization ∇·M inside a magnet and the surface ordinary part M·n are treated as a dissemination of attractive monopoles. This is a scientific comfort and doesn't suggest that there are really monopoles in the magnet. On the off chance that the attractive shaft circulation is known, at that point the post model gives the attractive field H. Outside the magnet, the field B is relative to H, while inside the polarization must be added to H. An augmentation of this technique that takes into consideration inside attractive charges is utilized in hypotheses of ferromagnetism. 

 

Another model is the Ampère model, where all charge is because of the impact of tiny, or nuclear, round bound flows, likewise called Ampèrian flows, all through the material. For a consistently polarized tube shaped bar magnet, the net impact of the infinitesimal bound flows is to cause the magnet to carry on as though there is a naturally visible sheet of electric flow streaming around the surface, with nearby stream bearing typical to the chamber axis.[13] Microscopic ebbs and flows in molecules inside the material are for the most part dropped by ebbs and flows in neighboring iotas, so just the surface makes a net commitment; shaving off the external layer of a magnet will

 

Medical issues and safety

Because human tissues have a very low level of susceptibility to static magnetic fields, there is little mainstream scientific evidence showing a health effect associated with exposure to static fields. Dynamic magnetic fields may be a different issue, however; correlations between electromagnetic radiation and cancer rates have been postulated due to demographic correlations (see Electromagnetic radiation and health).

If a ferromagnetic foreign body is present in human tissue, an external magnetic field interacting with it can pose a serious safety risk.[25]

A different type of indirect magnetic health risk exists involving pacemakers. If a pacemaker has been embedded in a patient's chest (usually for the purpose of monitoring and regulating the heart for steady electrically induced beats), care should be taken to keep it away from magnetic fields. It is for this reason that a patient with the device installed cannot be tested with the use of a magnetic resonance imaging device.

Children sometimes swallow small magnets from toys, and this can be hazardous if two or more magnets are swallowed, as the magnets can pinch or puncture internal tissues.[26]

Magnetic imaging devices (e.g. MRIs) generate enormous magnetic fields, and therefore rooms intended to hold them exclude ferrous metals. Bringing objects made of ferrous metals (such as oxygen canisters) into such a room creates a severe safety risk, as those objects may be powerfully thrown about by the intense magnetic fields.

Magnetizing ferromagnets

See also: Remanence

Ferromagnetic materials can be magnetized in the following ways:

  • Heating the object higher than its Curie temperature, allowing it to cool in a magnetic field and hammering it as it cools. This is the most effective method and is similar to the industrial processes used to create permanent magnets.

  • Placing the item in an external magnetic field will result in the item retaining some of the magnetism on removal. Vibration has been shown to increase the effect. Ferrous materials aligned with the Earth's magnetic field that are subject to vibration (e.g., frame of a conveyor) have been shown to acquire significant residual magnetism. Likewise, striking a steel nail held by fingers in a N-S direction with a hammer will temporarily magnetize the nail.

  • Stroking: An existing magnet is moved from one end of the item to the other repeatedly in the same direction (single touch method) or two magnets are moved outwards from the center of a third (double touch method).[27]

  • Electric Current: The magnetic field produced by passing an electric current through a coil can get domains to line up. Once all of the domains are lined up, increasing the current will not increase the magnetization.

Demagnetizing ferromagnets

Magnetized ferromagnetic materials can be demagnetized (or degaussed) in the following ways:

  • Heating a magnet past its Curie temperature; the molecular motion destroys the alignment of the magnetic domains. This always removes all magnetization.

  • Placing the magnet in an alternating magnetic field with intensity above the material's coercivity and then either slowly drawing the magnet out or slowly decreasing the magnetic field to zero. This is the principle used in commercial demagnetizers to demagnetize tools, erase credit cards, hard disks, and degaussing coils used to demagnetize CRTs.

  • Some demagnetization or reverse magnetization will occur if any part of the magnet is subjected to a reverse field above the magnetic material's coercivity.

  • Demagnetization progressively occurs if the magnet is subjected to cyclic fields sufficient to move the magnet away from the linear part on the second quadrant of the B-H curve of the magnetic material (the demagnetization curve).

  • Hammering or jarring: mechanical disturbance tends to randomize the magnetic domains and reduce magnetization of an object, but may cause unacceptable damage.

Types of permanent magnets

A stack of ferrite magnets

Magnetic metallic elements

Many materials have unpaired electron spins, and the majority of these materials are paramagnetic. When the spins interact with each other in such a way that the spins align spontaneously, the materials are called ferromagnetic (what is often loosely termed as magnetic). Because of the way their regular crystallineatomic structure causes their spins to interact, some metals are ferromagnetic when found in their natural states, as ores. These include iron ore (magnetite or lodestone), cobalt and nickel, as well as the rare earth metals gadolinium and dysprosium (when at a very low temperature). Such naturally occurring ferromagnets were used in the first experiments with magnetism. Technology has since expanded the availability of magnetic materials to include various man-made products, all based, however, on naturally magnetic elements.

Composites

Ceramic, or ferrite, magnets are made of a sinteredcomposite of powdered iron oxide and barium/strontium carbonate ceramic. Given the low cost of the materials and manufacturing methods, inexpensive magnets (or non-magnetized ferromagnetic cores, for use in electronic components such as portable AM radio antennas) of various shapes can be easily mass-produced. The resulting magnets are non-corroding but brittle and must be treated like other ceramics.

Alnico magnets are made by casting or sintering a combination of aluminium, nickel and cobalt with iron and small amounts of other elements added to enhance the properties of the magnet. Sintering offers superior mechanical characteristics, whereas casting delivers higher magnetic fields and allows for the design of intricate shapes. Alnico magnets resist corrosion and have physical properties more forgiving than ferrite, but not quite as desirable as a metal. Trade names for alloys in this family include: Alni, Alcomax, Hycomax, Columax, and Ticonal.[29]

Injection-molded magnets are a composite of various types of resin and magnetic powders, allowing parts of complex shapes to be manufactured by injection molding. The physical and magnetic properties of the product depend on the raw materials, but are generally lower in magnetic strength and resemble plastics in their physical properties.

Flexible magnets are composed of a high-coercivityferromagnetic compound (usually ferric oxide) mixed with a plastic binder. This is extruded as a sheet and passed over a line of powerful cylindrical permanent magnets. These magnets are arranged in a stack with alternating magnetic poles facing up (N, S, N, S...) on a rotating shaft. This impresses the plastic sheet with the magnetic poles in an alternating line format. No electromagnetism is used to generate the magnets. The pole-to-pole distance is on the order of 5 mm, but varies with manufacturer. These magnets are lower in magnetic strength but can be very flexible, depending on the binder used.[30]

Rare-earth magnets

Ovoid-shaped magnets (possibly Hematine), one hanging from another

Main article: Rare-earth magnet

Rare earth (lanthanoid) elements have a partially occupied felectron shell (which can accommodate up to 14 electrons). The spin of these electrons can be aligned, resulting in very strong magnetic fields, and therefore, these elements are used in compact high-strength magnets where their higher price is not a concern. The most common types of rare-earth magnets are samarium-cobalt and neodymium-iron-boron (NIB) magnets.

Single-molecule magnets (SMMs) and single-chain magnets (SCMs)

Main article: Single-molecule magnet

In the 1990s, it was discovered that certain molecules containing paramagnetic metal ions are capable of storing a magnetic moment at very low temperatures. These are very different from conventional magnets that store information at a magnetic domain level and theoretically could provide a far denser storage medium than conventional magnets. In this direction, research on monolayers of SMMs is currently under way. Very briefly, the two main attributes of an SMM are:

  1. a large ground state spin value (S), which is provided by ferromagnetic or ferrimagnetic coupling between the paramagnetic metal centres

  2. a negative value of the anisotropy of the zero field splitting (D)

Most SMMs contain manganese but can also be found with vanadium, iron, nickel and cobalt clusters. More recently, it has been found that some chain systems can also display a magnetization that persists for long times at higher temperatures. These systems have been called single-chain magnets.

Nano-structured magnets

Some nano-structured materials exhibit energy waves, called magnons, that coalesce into a common ground state in the manner of a Bose–Einstein condensate.[31][32]

Rare-earth-free permanent magnets

The United States Department of Energy has identified a need to find substitutes for rare-earth metals in permanent-magnet technology, and has begun funding such research. The Advanced Research Projects Agency-Energy (ARPA-E) has sponsored a Rare Earth Alternatives in Critical Technologies (REACT) program to develop alternative materials. In 2011, ARPA-E awarded 31.6 million dollars to fund Rare-Earth Substitute projects.[33]

Costs

The current cheapest permanent magnets, allowing for field strengths, are flexible and ceramic magnets, but these are also among the weakest types. The ferrite magnets are mainly low-cost magnets since they are made from cheap raw materials: iron oxide and Ba- or Sr-carbonate. However, a new low cost magnet, Mn-Al alloy,[34] has been developed and is now dominating the low-cost magnets field. It has a higher saturation magnetization than the ferrite magnets. It also has more favorable temperature coefficients, although it can be thermally unstable. Neodymium-iron-boron (NIB) magnets are among the strongest. These cost more per kilogram than most other magnetic materials but, owing to their intense field, are smaller and cheaper in many applications.[35]

Temperature

Temperature sensitivity varies, but when a magnet is heated to a temperature known as the Curie point, it loses all of its magnetism, even after cooling below that temperature. The magnets can often be remagnetized, however.

Additionally, some magnets are brittle and can fracture at high temperatures.

The maximum usable temperature is highest for alnico magnets at over 540 °C (1,000 °F), around 300 °C (570 °F) for ferrite and SmCo, about 140 °C (280 °F) for NIB and lower for flexible ceramics, but the exact numbers depend on the grade of material.

Electromagnets

Main article: Electromagnet

An electromagnet, in its simplest form, is a wire that has been coiled into one or more loops, known as a solenoid. When electric current flows through the wire, a magnetic field is generated. It is concentrated near (and especially inside) the coil, and its field lines are very similar to those of a magnet. The orientation of this effective magnet is determined by the right hand rule. The magnetic moment and the magnetic field of the electromagnet are proportional to the number of loops of wire, to the cross-section of each loop, and to the current passing through the wire.[36]

If the coil of wire is wrapped around a material with no special magnetic properties (e.g., cardboard), it will tend to generate a very weak field. However, if it is wrapped around a soft ferromagnetic material, such as an iron nail, then the net field produced can result in a several hundred- to thousandfold increase of field strength.

Uses for electromagnets include particle accelerators, electric motors, junkyard cranes, and magnetic resonance imaging machines. Some applications involve configurations more than a simple magnetic dipole; for example, quadrupole and sextupole magnets are used to focusparticle beams.

Units and calculations

Main article: Magnetostatics

For most engineering applications, MKS (rationalized) or SI (Système International) units are commonly used. Two other sets of units, Gaussian and CGS-EMU, are the same for magnetic properties and are commonly used in physics.[citation needed]

In all units, it is convenient to employ two types of magnetic field, B and H, as well as the magnetizationM, defined as the magnetic moment per unit volume.

  1. The magnetic induction field B is given in SI units of teslas (T). B is the magnetic field whose time variation produces, by Faraday's Law, circulating electric fields (which the power companies sell). B also produces a deflection force on moving charged particles (as in TV tubes). The tesla is equivalent to the magnetic flux (in webers) per unit area (in meters squared), thus giving B the unit of a flux density. In CGS, the unit of B is the gauss (G). One tesla equals 104 G.

  2. The magnetic field H is given in SI units of ampere-turns per meter (A-turn/m). The turns appear because when H is produced by a current-carrying wire, its value is proportional to the number of turns of that wire. In CGS, the unit of H is the oersted (Oe). One A-turn/m equals 4π×10−3 Oe.

  3. The magnetization M is given in SI units of amperes per meter (A/m). In CGS, the unit of M is the oersted (Oe). One A/m equals 10−3 emu/cm3. A good permanent magnet can have a magnetization as large as a million amperes per meter.

  4. In SI units, the relation B = μ0(H + M) holds, where μ0 is the permeability of space, which equals 4π×10−7 T•m/A. In CGS, it is written as B = H + 4πM. (The pole approach gives μ0H in SI units. A μ0M term in SI must then supplement this μ0H to give the correct field within B, the magnet. It will agree with the field B calculated using Ampèrian currents).

Materials that are not permanent magnets usually satisfy the relation M = χH in SI, where χ is the (dimensionless) magnetic susceptibility. Most non-magnetic materials have a relatively small χ (on the order of a millionth), but soft magnets can have χ on the order of hundreds or thousands. For materials satisfying M = χH, we can also write B = μ0(1 + χ)H = μ0μrH = μH, where μr = 1 + χ is the (dimensionless) relative permeability and μ =μ0μr is the magnetic permeability. Both hard and soft magnets have a more complex, history-dependent, behavior described by what are called hysteresis loops, which give either B vs. H or M vs. H. In CGS, M = χH, but χSI = 4πχCGS, and μ = μr.

Caution: in part because there are not enough Roman and Greek symbols, there is no commonly agreed-upon symbol for magnetic pole strength and magnetic moment. The symbol m has been used for both pole strength (unit A•m, where here the upright m is for meter) and for magnetic moment (unit A•m2). The symbol μ has been used in some texts for magnetic permeability and in other texts for magnetic moment. We will use μ for magnetic permeability and m for magnetic moment. For pole strength, we will employ qm. For a bar magnet of cross-section A with uniform magnetization M along its axis, the pole strength is given by qm = MA, so that M can be thought of as a pole strength per unit area.

Fields of a magnet

Far away from a magnet, the magnetic field created by that magnet is almost always described (to a good approximation) by a dipole field characterized by its total magnetic moment. This is true regardless of the shape of the magnet, so long as the magnetic moment is non-zero. One characteristic of a dipole field is that the strength of the field falls off inversely with the cube of the distance from the magnet's center.

Closer to the magnet, the magnetic field becomes more complicated and more dependent on the detailed shape and magnetization of the magnet. Formally, the field can be expressed as a multipole expansion: A dipole field, plus a quadrupole field, plus an octupole field, etc.

At close range, many different fields are possible. For example, for a long, skinny bar magnet with its north pole at one end and south pole at the other, the magnetic field near either end falls off inversely with the square of the distance from that pole.

Calculating the magnetic force

Main article: force between magnets

Pull force of a single magnet

The strength of a given magnet is sometimes given in terms of its pull force— its ability to move (push/ pull) other objects. The pull force exerted by either an electromagnet or a permanent magnet at the "air gap" (i.e., the point in space where the magnet ends) is given by the Maxwell equation:[37]

{\displaystyle F={{B^{2}A} \over {2\mu _{0}}}}

,

where

F is force (SI unit: newton)

A is the cross section of the area of the pole in square meters

B is the magnetic induction exerted by the magnet

Therefore, if a magnet is acting vertically, it can lift a mass m in kilograms given by the simple equation:

{\displaystyle m={{B^{2}A} \over {2\mu _{0}g_{n}}}}

.

Force between two magnetic poles

Further information: Magnetic moment § Forces between two magnetic dipoles

Classically, the force between two magnetic poles is given by:[38]

{\displaystyle F={{\mu q_{m1}q_{m2}} \over {4\pi r^{2}}}}

 

where

F is force (SI unit: newton)

qm1 and qm2 are the magnitudes of magnetic poles (SI unit: ampere-meter)

μ is the permeability of the intervening medium (SI unit: teslameter per ampere, henry per meter or newton per ampere squared)

r is the separation (SI unit: meter).

The pole description is useful to the engineers designing real-world magnets, but real magnets have a pole distribution more complex than a single north and south. Therefore, implementation of the pole idea is not simple. In some cases, one of the more complex formulae given below will be more useful.

Force between two nearby magnetized surfaces of area A

The mechanical force between two nearby magnetized surfaces can be calculated with the following equation. The equation is valid only for cases in which the effect of fringing is negligible and the volume of the air gap is much smaller than that of the magnetized material:[39][40]

 

Neodymium magnet

 

Description

 

Neodymium magnet (also known as NdFeB) is the type of permanent magnet most widely used in various industrial and household products due to its (1) large magnetic strength, (2) high resistance to demagnetization, and (3) relatively cheap price.  Though it is discovered in 1984, later than most of other types of permanent magnets, neodymium magnet is replacing alnico and ceramic magnets in many applications.

 

Neodymium is a rare earth metal and neodymium magnet is an alloy composed of neodymium, iron and boron atoms that are organized in a microcrystalline structure with a chemical formula of Nd2Fe14B. Because of the low temperature (-254.2/-425.5 ºC/ºF) at which the ferromagnetism of neodymium metal disappears (or called Curie temperature), a certain amount of iron is present in neodymium magnet in order to increase the Curie temperature well above room temperature.

 

During manufacturing and under an external magnetic field, the spin of 4 unpaired electrons in neodymium atoms (compared 3 in iron) can align along the same direction giving rise to a strong magnet with a magnetic energy 18 times larger than traditional ferrite magnets.  The difficulty of its reorientation of magnetization leads to a high level of resistance to being demagnetized. The boron atoms in neodymium magnets do not contribute directly to the magnetism, but improve cohesion by strong covalent bonding. The relatively low content of rare earth  (12% by volume) in neodymium magnets lower their price compared with other rare earth magnets like samarium-cobalt magnets. 

 

Our company (CMS & Magnets For Sale) is among leading magnetic product suppliers carrying a wide variety (size, shape, grade, assembly) of high-quality neodymium magnetic products.  They are protected by triple layers of nickel-copper-nickel.

 

Properties

 

Demagnetization 

Demagnetization can occur 

(1) in the presence of an external magnetic field (e.g. in the vicinity of a motor)

Depending on the strength of the demagnetizing field, the magnetic flux of the Neodymium magnet may remain the same or loss partially, and this process can be reversible or irreversible.  When the demagnetizing field exceeds a critical value, the magnet's coercivity, the Neodymium magnet will be demagnetized and re-magnetized depending on the magnetic direction of the external field.

(2) by increasing temperature.

when temperature is raised, thermal (random) motion exerts more force to re-orientate the initially aligned (domain atoms) spins causing demagnetization of the Neo magnet.   This demagnetization can be permanently or temporary depending on the level of the elevated temperature. The ability of an neodymium magnet to resist demagnetization by increasing temperature is measured by  two parameters, the magnet's maximum operating temperature (MaxOpTemp) and Curie temperature as shown by the table below for different grades of Neodymium magnets. For a standard N grade of Neodymium magnets, their magnetic flux will permanently loss a fraction of the strength at their maximum operating temperature and loss all of their magnetic strength at their Curie temperature.  For example, most Neodymium magnets start to loss their magnetization above 80/176 ºCF.  Special grades of Neodymium magnets with a higher Curie temperature (up to 220/428 ºCF) have been developed to work at a high temperature such as in windmills, hybrid motors, etc.  Thus, in choosing neodymium magnets it is imperative to consider which grade of Neodymium magnets is best suited to your needs in terms of the operating temperature setting.

grades

Neodymium magnets are graded in terms of (1) their strength of magnetic field as measured by the remanence (Br) or the energy product (BHmax) and their resistance to being demagnetized by temperature changes as measured by the maximum operating temperature or the Curie temperature (cf. table below).  Some grades of neodymium magnets are designated as 

N35-N55

N33M-N48M

N30H-N45H

N30SH-N42SH

N30UH-N35UH

N28EH-N35EH

 

where the  two digit numeric value in the unit of MGOe indicates the strength of the magnet (the higher the number the stronger the magnet) and the last one or two letters indicates the temperature sensitivity as described in the table below.

 

Corrosion

Neodymium magnets are manufactured in the form of powered porous metal in which the irons are more easily subjected to oxidation or rust, especially under humid conditions, though recently certain grades of Neodymium magnets have been made that exhibit higher resistance to oxidation.  Corrosion can be at least partly prevented by applying a suitable coating or plating. Neodymium Magnets can be coated by many different materials including nickel, copper, zinc, tin, epoxy, silver and gold, though nickel is the most commonly used one and a multi-layer coating method (e.g. nickel-copper-nickel) is also usually applied to make the product more resistant to corrosion.  The performance of a coating or plating could be evaluated with a Salt Spray/Salt Fog Test (SST) and this is executed in accordance with ASTM B117. 

Mechanical stress and handling care

Neodymium magnets are made of powered porous metal and thereby inherently bristle and prone to chipping and breaking when handled inappropriately.  Neodymium magnets are also very strong magnets. Thus appropriate handling and packing are required to ensure safety and prevent damage. It is advised not to (1) machine the magnets, which could also generate heat and demagnetizing the magnets, (2) put magnets in conditions of mechanical stress e.g. in load bearing situations, and (3) place any body part (e.g. hand) between attracting magnets or between a magnet and a ferrous metal.

Neodymium powder is very fine and when dry can ignite spontaneously.  Thus, care must be taken in handling neodymium powder. 

Applications

Popular uses

Neodymium magnets are now widely used in various industrial and commercial products.  They have replaced other types of magnets in many applications that require strong permanent magnets  including but not limited to:

  • Electric motors (e.g. powerless tools, hybrid/electric vehicles)

  • Computer and cellphone accessary (e.g. hard disks, loudspeakers, headphones)

  • Office supplies (e.g. message boards, name badges, business cards)

  • Small household items (e.g. hooks, toys, crafts. jewelries)

  • Green energy devices (e.g. wind turbines, rechargeable batteries and biofuel catalysts)

  • Magnetic therapy (e.g. magnetic therapy bracelets, necklaces, beddings)

  • Scientific instruments (e.g. NMR spectrometers and MRIs)

 

Here is a list of major different applications 

  • Medical equipment

  • Aerospace

  • Consumer electronics

  • Home applications

  • Industrial applications

  • Wind turbines

  • Automotive

 

New trends

  • Neodymium is right now responsible for most of the growth in rare earth demand with the Neodymium magnetic market worth $11 billion in 2017 according to market research group IMARC.  With rapid development of electric vehicles and other renewable energy applications in the next couple of decades, this demand for Neodymium magnets will suely surge (e.g. Tesla has recently used motors with Neodymium magnets in its Tesla Model 3). 

  • Because of the strong and/or homogenous magnetic field Neodymium magnets can generate, they have found new applications in some medical fields such as magnetic resonance imaging and magnetic therapy.  Neodymium magnets are also used as a surgically implanted anti-reflux system (GERD) around the lower esophageal sphincter to treat GERD disease.

  • The N52 grade of Neodymium magnets is an emerging market, and with the very strong magnetic force, it has been finding new applications that require small-sized and/or light-weighted magnets (e.g. sensors for medical diagnosis). Our company offers a huge selection of shapes and sizes of N52 Neodymium magnets and the list is growing!  More recently, the strongest neodymium magnets, N55 magnets, are developed for commercial uses, which are ideal for fine and delicate work where small size really matters.  

  • Neodymium magnets having many combinations of partial alloying substitutions for Nd and Fe are being invented, leading to a wide range of available properties and opening up new application avenues. 

 

Ceramic magnets

 

Description

 

Ceramic magnets are made by mixing strontium carbonate and iron oxide and subjecting them to a chemical conversion at a very high temperature (982-1093/1800-2000 ºCF).  The resulting material, also known as hard ferrite, is then reduced to small particles by wet milling.  The milled powders are sintered through a dry or wet compacting (hydraulic press) process in the presence of a magnetic field and at an elevated temperature (1093/2000 ºC/ºF).  

 

Ceramic (ferrite) magnets are least expensive permanent magnets compared to rare-earth and Alnico magnets. As such, they have found widespread applications (e.g. DC motors) taking >75% market shares by weight of magnetic products. However, ceramic magnets are relatively weak magnets, making them unsuitable for applications requiring very powerful magnets such as powerful turbines and generators. 

 

Our company (CMS & Magnets For Sale) carries a large inventory of ceramic magnets varying in sizes & shapes (disc, block, ring, etc.) and many assemblies made with ceramic magnetic parts.  We can also custom manufacture ceramic magnets to meet your specifications using our in-house or partner's manufacturing facilities and experienced teams of engineers. Just let us know what you are looking for by sending us a request for quote or contact us today.  We'll response immediately to your request.

 

Properties

 

Demagnetization and temperature effects

 

Ferrite magnets can be made to possess low or high coercivity.  The former is also called the soft ferrite and the latter the hard ferrite, or also referred to as ceramic magnets.  

 

Ceramic magnet becomes demagnetized when exposed to a temperature higher than 204/400 ºCF. Thus, ceramic magnets have an operating temperature lower than Alnico magnets but higher than rate earth magnets, the latter being another reason for ceramic magnets' popularity among various types of permanent magnets.   

 

Mechanical stress and handling care

 

Ceramic magnets are sintered powders and are thereby brittle and easily broken requiring special machining processes.  As a result, ceramic magnets are available only in simple shapes and tooling can be expensive. When in use, care needs to be excised to avoid it being dropped or allowing it to hit objects it is attracted to.

 

Corrosion

 

Ceramic magnets have a strong resistance to corrosion.  As such, they require little protection measures such as coating or plating, though coating can be applied for clean and non-contaminated applications since there exists a thin film of fine magnetic powder on the surface or for cosmetic reasons.

 

Grades

 

The grade of ceramic magnets ranges from 1 to 10, among which 1,5,and 8 grades are the most common ones and the magnetic strength increases from 1 to 10.  The following table describes some common properties of different grades of ceramic magnets.

 

 

Applications

 

Ceramic magnets have found widespread applications (see below) where more powerful rare-earth magnets like neodymium magnets are not required, where cheap cost, strong resistance to demagnetization and corrosion, or/and stably functioning at a high temperature is major consideration.  

 

  • DC motors (the first choice for most types of DC motors such as DC brushless motors)

  • Generators (e.g. wind turbines)

  • Office & house (e.g., crafts, refrigerator magnets, badge holders, display boards, etc)

  • Loudspeakers/microphones/headphones

  • Magnetic sweepers

  • Magnetic resonance imaging (MRI)

  • Security systems

  • Automotive sensors

  • Magnet assemblies (e.g.,lifting, holding, retrieving and separating)

 

Soft ferrites, on the other hand, are easy to be demagnetization.  As such, they can act as conductors of magnetic field. This feature is useful for alternating current applications such as high frequency transformers and switched power supplies.

 

Samarium Cobalt magnets

 

Description

 

Samarium-cobalt (abr. SmCo) magnet is another type of rare-earth magnets.  It is made of an alloy of samarium and cobalt. Samarium-cobalt magnets have magnetic strength comparable to neodymium magnets, ranging in the maximum energy product (BHmax) of 16-33 MGOe,

 

Samarium-cobalt magnets are slightly more expensive than the popular neodymium magnets, but they have higher operating temperature, magnetic stability over a widely changing temperature environment, and strong resistant to corrosion or oxidation. 

 

Our company carries a wide selection of Samarium-cobalt magnets varying in shape and size (e.g. disc 2mm-1'and block 3/8-2').

 

Properties

 

Temperature and demagnetization

 

Samarium-cobalt magnets have a substantially higher Curie temperature (~800 ºC) than neodymium magnets (320 ºC). Since they also have a much lower reversible temperature coefficient (-0.03 %/K compared to -0.12 %/K for neodymium magnets), SmCo magnets have also superior ability to resist changes in the magnetic field upon changes in temperature.  Thus, SmCo magnets are a better choice when a very cold or hot operating temperature or an environment of changing temperature is required.  

 

Grades

 

Samarium-cobalt magnets are available in two "series":  Series 1:5 and Series 2:17. Series 1:5 is composed of ~37% samarium and 63% cobalt by weight.  The composition for Sm2Co17 materials is more complex, having ~25% samarium, 5% copper, 18% iron, 2% hafnium or zirconium, with the remainder being cobalt. These small amounts of zirconium and hafnium etc. are added in order to improve heat treatment.

 

Generally, the magnetic strength for series 1:5 resides in a lower range (BHmax ~16-25 MGOe) than Series 2:17 (BHmax ~20-32 MGOe), though the former series is easier to calibrate to a specific magnetic field than Series 2:17 magnets.

 

Mechanical stress and handling care

 

Similar to other permanent magnets, Samarium-cobalt magnets are manufactured from sintered powder of Sm-Co alloy.  As such, they are bristle and easy to break. Samarium-cobalt Magnets are also very strong. When handling them, eye protection must be worn and avoiding the magnets to snap together that could shatter magnets to shatter and cause injury,

 

Corrosion

 

Samarium-cobalt magnets have better resistance to corrosion than Neodymium magnets and require no coating or plating in many applications.  However, coating (e.g., parylene and cladding) or plating (nickel) may be desirable when operating in acidic, high moisture, or very cleaning (e.g. vacuum and medical applications) environment.  

 

Applications

 

Popular uses

 

Samarium-cobalt magnets, because of their unique temperature characteristics, are appropriate for applications where a very cold or hot operating temperature or an environment of changing temperature is required.  They are also used in

 

  • High-end electric motors (e.g. used in the more competitive classes in slotcar racing)

  • Electric guitar pickup

  • Turbo machinery

  • Traveling-wave tube field magnets

  • Bench-top NMR spectrometers

  • Rotary encoders where it performs the function of magnetic actuator 

  • Servo motors

  • Space probes and satellites

  • Magnetic couplings and separators. 

  • Switches

  • Sensors, 

  • head- and microphones

 

New trend

 

  • Electric vehicles are a growing market for rare earth magnets because of the energy and environment concerns. Consumption of rare earth magnets in transportation is estimated to rise from 7,000 tons in 2015 to 17,000 tons in 2020.  Permanent magnet machines are currently the leading choice for high performance automotive applications. The rotor’s well-balanced magnetic field and the lower stator current requirement, paired with very strong positional control, result in a higher motor efficiency.

 

Alnico magnets

 

Description

 

Alnico refers to a family of iron alloys which in addition to iron are composed of 8–12% Al, 15–26% Ni, 5–24% Co, and small amounts of metals (< 6% Cu and < 1% Ti) with the remaining of Fe.  Although their magnetic strength is lower than rare-earth magnets, they are the strongest magnets before the invention of rare earth magnets. At present, Alnico magnets are excellent choice for high-temperature or corrosion-prone applications.

 

Our company (CMS & Magnets For Sale) carrier different grades (alnico 5, alnico 7 etc.), shapes (disc and block), and sizes of Alnico magnets to optimize their magnetic performance, operational temperature and cost.

 

Properties

 

Demagnetization and re-magnetization

 

Alnico magnets have a high Residual induction or Remanence (Br) but relatively lower Coercive Force,  which means they are capable of possessing a large magnetic strength, but more easily self-demagnetized when the external magnetic field is removed.  Thus special care must be taken to prevent demagnetization of Alnico magnets by a nearby repulsive magnet or electrical current once they are magnetized. On the plus side, partially demagnetized Alnico magnets can be easily re-magnetized.  Taking advantage of this property, Alnico magnet can be efficiently utilized by magnetizing it after the magnet is assembled in a final magnetic circuit.

 

Temperature on demagnetization

 

Alnico magnets have a very high Currier temperature (799/1470 ºCF) and maximum operating temperature (538/1000 ºCF) and the lowest temperature coefficient (0.02% per degree) of all permanent magnets, which makes them very stable against demagnetization over a wide range of temperature and only magnets to maintain useful magnetism when heated red-hot.

 

Grades

 

Alnico magnets are classified using (1) numbers assigned by the Magnetic Materials Producers Association (e.g., Alnico 2, Alnico 5, Alnico 5-7 etc.) and (2) manufacturing method, cast or sintered.  As a general rule, cast Alnico magnets, because of their anisotropic property, or can be magnetized only in one direction, have a higher magnetic strength (5.0-9.0 MGOe) than sintered Alnico magnets (1.5-5.3 MGOe) of equivalent grade, some of which are isotropic, or can be magnetized in any direction.  Among various grades of Alnico magnets, cast Alnico 5 is the most widely used Alnico magnet.

 

Mechanical stress and handling care

 

Most Alnico magnets are manufactured using foundry casting techniques, where the molten alloy is poured into sand molds. Thus, these magnets can be cast in complex shapes (e.g. horseshoes) not possible with other magnet materials. Very small magnets, usually one ounce or less, are produced using sintering techniques. 

 

Alnico magnets are hard and brittle, and not suitable for drilling, tapping, or conventional machining operations. Thus, for mechanical-stress protection and cost consideration, use of cast or sintered Alnico magnets in simple shapes and not too small cross-sectional area (> .125") are recommended.  For further protection in cases of high mechanical stress such as holding and rotors, the Alnico magnets can be housed in an aluminum housing. 

 

 Also, when handling these strong and brittle magnets, proper handling and packing procedures are required to ensure safety.

 

Corrosion

 

Alnico magnets are highly resistant to corrosion.  As such, they are typically used in various applications with no coating or plating.

 

Applications

 

Alnico magnets are used in many industrial and consumer applications where strong permanent magnets are needed such as

 

  • Electric motors (most automotive engines and electric motors use alnico magnets as they are highly coercive in nature)

  • Electric guitar pickups 

  • Microphones

  • Sensors

  • Loudspeakers

  • Magnetron tubes

  • Security systems

  • Relays

 

In the past, Alnico magnets are superseded by rare-earth magnets, whose stronger magnetic fields allow smaller-size magnets to be used for a given application.

 

New Trends

 

  • Because of increasing shortage of rare earth supply, an intensive search for alternative permanent magnets are under way.  Alnico magnets, because of their stable temperature characteristics, have shown great potential for replacing Neodymium magnets for applications above 200 ºC.  

 

  • The Alnico magnet market is projected to grow by ~9.6% in terms of revenue by 2022 due to the innovation efforts to advance increasing number of applications among many industrial sectors such as automobiles, medical devices, household equipment, etc.  Environmental protection has also stimulated the use of non-rare-earth Alnico magnets.

 

Glossary

Diamagnetism and Paramagnetism

Electrons in any atom have two spin states (1/2 or -1/2) with opposite spin direction (magnetic dipole moment) and move within an atomic orbital.  A maximum of two electrons can occupy a single atomic orbital, but they must have opposite spins giving rise to a total spin of zero. These paired electrons are called diamagnetic electrons.   An unpaired electron occupying an atomic orbital has a non-zero net spin and is called a paramagnetic electron. An atom is considered paramagnetic if even one orbital has a non-zero net spin or magnetic dipole moment, and otherwise the atom is considered diamagnetic.  Paramagnetic atoms are attracted to an external magnetic field, and the opposite is true for diamagnetic atoms. These phenomena are called paramagnetism or diamagnetism, respectively. 

Permanent magnets

Magnets are materials that are capable of generating a magnetic field.  Permanent magnets, also known as ferromagnetic materials, are consist of paramagnetic atoms that are organized into domains with a net magnetic moment.  In the absence of an external magnetic field, these domains are randomly oriented, yielding a zero magnetic field. When an external magnetic field is imposed, these domains start to line up, generating a net magnetic field even when the external magnetic field is withdrawn. There are typically four types of permanent magnets: neodymium iron boron, samarium cobalt, alnico, and ceramic (or ferrite) magnets. Two of the most popular types of magnets are neodymium and ceramic ones.  Their pros and cons are mostly compensated for each other, making it imperative to select wisely different types of magnets to meet your special needs.

Other than permanent magnets, there are two additional classes of magnets: temporary magnets and electromagnets.  

Temporary magnets

Temporary magnets usually refer to some iron and iron alloys that can be easily magnetized even by a week magnetic field, but their magnetic strength would gradually get lost when the external magnetic field is removed.  

Electromagnets

Electromagnet is a device that is consisted of a core of magnetic material (e.g. iron) surrounded by a coil through which an electric current is passed to magnetize the core.The magnetic field disappears when the current is turned off.  Thus, this types of magnets are soft, and easily been demagnetized. On the plus side, the strength/pole direction of an electromagnet can be changed by simply changing the amount and direction of electric current that flows through the coil.   Electromagnets are widely used in motors, hard drives, TVs and many other applications.

Magnetic poles

Magnetic poles are one of the fundamental physical properties of permanent magnets. The magnetic field generated by a permanent magnet is a vector field and is usually visualized by the magnetic lines of force that flows from one end of the magnet to the other end of the magnet.  These two ends of a magnet are conventionally called north (N) and south (S) poles, respectively.  

One common feature of magnetic poles is the fact that two different magnets with like poles (N-N or S-S) close to each other will repel each other while the opposite holds with opposing poles (N-S or S-N). 

Anisotropic and isotropic magnetization

Materials that have a preferred direction of magnetization are said to be anisotropic while those that have no preferred direction are said to be isotropic. The anisotropic property is determined by material's molecular structure, crystal structure, grain shape, applied or residual stresses and temperature.  The anisotropic materials are typically manufactured in the presence of a strong magnetic field, and can only be magnetized through the preferred axis. The rare-earth magnets (Neodymium and Samarium-cobalt magnets) and some ceramic and Alnico (cast) magnets are anisotropic while some ceramic and Alnico (sintered) magnets are isotropic.  Anisotropic magnets are generally stronger than isotropic ones, but they can only be magnetized in the specified direction.

Magnetic field strength

How strong of the magnetic field generated by a permanent magnet can be measured by the magnitude of the generated magnetic field or the energy density stored in the magnetic field.  Both concepts are commonly represented in terms of Remanence and Maximum energy product as described in more details below. In the CGS unit system, the magnetic strength is measured in the unit of Gauss and the energy density in the unit of Gauss-Oersted or a million Gauss-Oested (MGOe) for convenience.  In the more widely used international system of units (SI), the magnetic field strength is measured in the unit of Tesla and the energy density in the unit of kiloJoule per cubic meter (kJ/m3).

Magnetic field strength B can be expressed as (in SI system)

B0H+J(1)

where µ0 is a constant (4π x 10-7 webers per ampere and meter) called the magnetic permeability in vacuum  H is the strength of an external magnetic field, and J is the polarization or the vector sum of the magnetic dipole moments of a unit material.

Remanencec (Br)

When all the spins in a ferromagnetic material are aligned along the same direction dictated by the orientation of the external magnetic field, the magnet reaches its maximum magnetization (also called the saturation magnetization), which is usually measured by the magnetic energy value. When the imposed magnetizing field is removed (H=0 in Eq. 1), the magnetization in the ferromagnetic material is not relax back to zero magnetization. This remaining magnetic field is called its remanence or residual induction (Br), and is expressed in the unit of Tesla (T) or Gauss (Gs).

Maximum Energy product (BHmax)

The maximum energy product is the maximum amount of magnetic energy density stored in a magnet. It concerns the product maximally attainable with a material made out of flux density B and field strength H.  The standard unit of measurement is kJ/m³ (Kilojoule per cubic meter) or MGOe (Mega-Gauss-Oersted). Since it is the maximum energy per unit volume, a small magnet with a higher BHmax can generate similar magnetic energy to a larger magnet with a lower BHmax value.

Coercivity

When an external magnetic field is imposed on a permanent magnet in the opposite direction after the magnet is magnetized to a saturation level, the amount of this reversing magnetic field to demagnetize the permanent magnet to zero (B=0 in Eq. 1) is called the permanent magnet's coercivity or coercive force (Hcb = J/µ0 when B=0 according to Eq. 1).  As noted, the polarization J is not zero and thereby the magnet is not completely demagnetized and the magnetic strength of the magnet will spontaneously regain once the demagnetizing force is removed.  Thus, another coercivity property, the intrinsic coercive force (Hcj), is introduced, which indicates the demagnetizing field that is required to completely demagnetize the magnet material.  Clearly, Hcj is higher than Hcb.  Both Hcb and Hcj are commonly reported for magnets of different types and grades, measured in the unit of Oersteds (Oe) or A/m.   

Hysteresis Loop 

A plot of the magnetizing force (H) versus the induced magnetic flux density (B) for a ferromagnetic material as it is successively (1) magnetized to saturation (where almost all magnetic domains in the material are aligned), (2) demagnetized, (3) magnetized in the opposite direction and (4) finally re-magnetized to saturation and fulfilling a closed loop. The size and shape of this “loop” are important properties of a ferromagnetic material  The first quadrant of the loop shows how much magnetizing force must be applied to achieve saturation. The second quadrant is the demagnetization phase that can reveal the coercivity of the material. The induced magnetic flux at H = 0 between the first and second quadrants indicates the amount of residual magnetic field the material maintains when the magnetizing force is removed after achieving saturation, or known as Remanence or Retentivity.

Hysteresis Loop reveals many important properties of a magnetic material.Along with every custom order, we can attach the corresponding hysteresis curve for the exact batch of material used.

The following figure shows some hysteresis loops (only in the second quadrant) of Neodymium magnets (after www.neorem.fi) from https://www.researchgate.net/figure/some-hysteresis-cycles-of-Neodymium-magnets-after-wwwneoremfi_fig3_316240964.

 

Hard and soft magnetic materials

Materials which retain their magnetism after the removal of the applied magnetic field and are difficult to demagnetize are called hard magnetic materials.  Soft magnetic materials, on the other hand, are easy to magnetize and demagnetize, and are used for making temporary magnets. A comparison of the hysteresis loops for hard and soft magnetic materials is shown in the figure below.  As noted, hard magnetic materials have a large area inside the hysteresis loop. In other wards, they have large magnetic energy and coercivity, or strong resistance to demagnetization. Permanent magnets such as rare-earth, ceramic, and Alnico magnets are hard magnetic materials.

Permeance Coefficient

Permeance Coefficient of a magnet is referred to as the "operating slope", load line or B/H of the magnet. When no other magnetic material is nearby, the Pc can be calculated solely from the dimensions of the magnet.  If the permeance coefficient is p, the magnet's working point is the intersection of a straight line (the permeance line) drawn from the origin and having a slope of μ0p and the B-H demagnetization curve. 

Curie temperature

Curie temperature is the temperature at which the magnetic atoms can no longer be aligned in their spin direction permanently, and the ferromagnetic (having magnetic attraction) property of the magnet material is lost and becomes paramagnetic (having magnetic attraction only when there are external magnetic fields). This paramagnetic conversion or demagnetization is irreversible even when the permanent magnets have been cooled. Neodymium magnets have relatively lower Curie temperatures (310-340/590-644 ºC/ºF) as compared to other kinds of magnet materials (800/1472 ºC/ºF for Sm2Co17, 450/842 ºC/ºF for ceramic magnets). Keep this magnetic property in mind when you design and select materials for your high temperature applications.

Maximum operating temperature

A magnet's maximum operating temperature (MaxOpTemp) is quite different from and usually much lower than the magnet's Curie temperature.  It refers to the temperature at which a magnet begins to loss its magnetic strength if it is further heated. This loss of magnetic strength may be minimal (< 5%) when the temperature is returned to room temperature.  It should be noted that permanent magnets of different types and grades may possess different Curie temperature and maximum operating temperature. It is thus imperative to check for these properties when ordering magnets that require special temperature characteristics.  

Adhesives

Permanent magnets in a magnetic assembly, whether manufactured in a factory or prepared in a DIY project, are usually organized with other parts of the magnetic assembly using a certain adhesive to hold the magnets in place.  While epoxy adhesives work best for majority of surface between a permanent magnet and another part made of wood, metal, or plastics, other types of adhesives can also be used such as urethane adhesives, liquid nails, silicone adhesives, JB weld, etc. 

 

In a DIY project, read carefully the instruction sheets attached to an adhesive product for the range of applying surfaces, handling procedure, and safety precaution.   Avoid using a hot glue gun on neodymium magnets as they may loss their magnetic field at a higher temperature.

 

FAQs

How do permanent magnets cost?

Different types of magnetic materials can have significantly different cost as measured by weight or magnetic strength.  The following table demonstrates a rough guidance of the relative cost. While rare earth magnets are more expensive than Ceramic and Alnico magnets per unit weight but their cost is substantially reduced when measured in terms of their magnetic strength, especially for neodymium magnets.  Furthermore, when choosing different types of permanent magnets, other factors that may potentially important to your application should also be considered, including but not limited to temperature effects, resistance to corrosion, and mechanical stress. 

 

What magnet properties one should consider when ordering magnets except cost?

These four properties that characterize a permanent magnet may need to be carefully considered when ordering magnets:

  • The strength of magnetic field a permanent magnet can generate as measured by Remanence (Br) or Maximum Energy product (BHmax

  • The ability of a magnet to resist demagnetization as usually measured by coercive force (Hcb) or intrinsic coercive force (Hcj

  • The stability of a magnet's strength in changing temperature environment as measured by the maximum operating temperature (MaxOpTemp)

A more detailed discussion of these magnetic properties can be found in the Glossary section.  As a quick guidance, the following table provides estimates of these properties for some popular magnets.

 

 

Will permanent magnets lose their magnetic strength over time? 

Depending on the type of magnetic material, permanent magnets do lose a very small amount of their magnetism over time. With Samarium-cobalt magnets, for example, this has been shown to be < 1% over a period of ten years.  Other factors may adversely affect a magnet's strength such as

  • Heat

  • Radiation

  • Electrical current nearby (e.g. powerlines)

  • Other magnets nearby

  • Corrosion

  • Mechanical damage

 

In particular, to prevent loss of magnetism for a magnet, don't use the magnet in places where it is exposed to extreme heat, especially for neodymium magnets which have a lower maximum operating temperature than other major types of permanent magnets.

How to choose different types of permanent magnets?

There are four main types of permanent magnets: Neodymium, Samarium cobalt, ceramic and Alnico magnets.  The former two are rare-earth magnets and are generally more expensive as gauged by magnet weight. However, these rare-earth magnets, in particular Neodymium magnets, are the magnets with strongest magnetic strength, and may be more economic and efficient when small magnet size is a primary concern in application design.  Other concerns such as temperature stability, resistance to demagnetization, corrosion, and mechanical stress should also be considered when the need comes. In brief, neodymium magnets have the lowest maximum operating temperature among these four types of magnets, mechanical strength, and prone to corrosion if left unprotected by coating or plating but high resistance to demagnetization.  SmCo magnets have higher resistance to heating and corrosion but low mechanical strength and are more expensive. Ceramic magnets are generally cheap and resistance to corrosion but brittle and chip or crack easily. Alnico magnets have outstanding temperature stability even at very high temperature, though they are more easily demagnetized.

The following is a comparison table to get a glance of the pros and cons of different types of permanent magnets

  

Which permanent magnet is the strongest type of magnet? 

Neodymium magnets are the strongest permanent magnets in the world.  Neodymium magnets are divided into many different grades varying in their magnetic strength.  At present, the grade N55 is known to be the strongest among all grades of neodymium magnets.

What magnetization directions of simple shaped magnets may possess?

 The shapes and magnetization directions of magnets to choose is usually governed by  the design of your specific application. Since the magnetic field a permanent magnet generates is not isotropic and is the strongest at the poles of the magnet,  it is necessary to know the magnetization direction of a given magnet to fit into your application. Most simple shaped magnets are circular or rectangular in shape.  For circular shaped magnets such as discs, rings, and cylinders, there are two types of magnetization direction, one is oriented axially with the poles located on the flat sides, and the other is oriented diametrically with the poles located on the round ends.  For rectangular shaped magnets (i.e. bars or blocks), their magnetization direction is usually oriented along the thru thickness direction by convention.

What shape of permanent magnets should one consider?

Permanent magnets can be manufactured into a variety of shapes.  Most commonly used magnets have simple shapes such as bar, disc, cylinder, or ring, There are many factors to consider when choosing magnets of different shape, which is often dictated by the functioning in a specific application.  One factor to consider is the magnetic strength for magnets of different shapes but similar size. Since the magnetic flux is focused on the poles of a magnet, magnets of similar size but different shape may exhibit different magnetic pull force.  For example, bar magnets where the magnetization is directed along perpendicular to the width/height surface, have smaller pole area than a similar sized disc magnet, Horseshoe magnets have the two poles close to each other and along the same direction making them much stronger than simple bar magnets.  

One should remember that the effects of shape will quickly diminish with distance from the magnet of interest.   

How to make a magnet stronger?

A permanent magnet may loss its strength over time due to many different reasons.  One way of making its stronger is to find a very strong magnet and repeatedly rub it across your weakened magnet.  This will force the magnetic domains in the weakened magnet to realign and increase its magnetic strength.

How to identify the north and south poles of a magnet?

One simple way to identify the two poles of a magnet is to use a compass, which is really just a small bar magnet, and place it close to but not touching your magnet.  The needle of the compass that points to the north of the earth is the north pole of the compass and will move toward the south pole of the magnet (or vice versa).  

What safety measures should be taken when handling permanent magnets?

Magnetic products with a relatively strong magnetic field and large size such as Neodymium magnetics larger than a few cubic centimeters may cause injury when operated in an improper manner.  Personnel receiving or working on strong magnets should be warned about the dangers of handling these materials. In particular, 

  • Avoid to place any body part (e.g. hand) between attracting magnets or between a magnet and a ferrous metal

  • Keep sufficient distance from magnets if you wear a pacemaker 

  • Since most permanent magnets are brittle, sharp splinters could be catapulted away for several meters and injure your eyes, so always wear safety goggles when handling permanent magnets

  • Keep children away from magnets

  • Always wear gloves or any other form of hand protection when handling magnets.

  • Keep a sufficiently safe distance between your magnets and other objects that can be adversely affected (damaged) by the magnets (e.g. watches, televisions, computer monitors, credit/bank cards, data storages, video tapes, & hearing aids etc).

How to prevent damage to permanent magnets and other devices?

 Most permanent magnets, except perhaps Alnico magnets, are brittle and prone to chipping and cracking.  Thus, care should be taken to ensure that they do not snap towards each other. A strong magnetic field generated by a permanent magnet could also affect the functioning, even damage, some nearby electronic devices (e.g. TVs,, computer hard drives, data storage, mechanical watches, hearing aids and speakers).

What is a magnetic assembly?

For many applications permanent magnets of different structures (e.g. size and shape) are combined with other non-magnet components of interest to form various magnetic assemblies. There exist virtually countless of magnetic assemblies.  Our company (CMS & Magnets For Sale) sales a broad range of high-quality magnetic assemblies for industrial, office, and household usage, including but not limited to 

 

  • Magnetic sweepers 

  • Magnetic lifters

  • Magnetic building toys 

  • Magnetic holders/hooks

  • Name Tags/Business Cards

  • Message Board magnets

  • Educational magnets

  • Wind Turbines

 

How to measure a magnetic field?

 

Magnetic field can be measured by a magnetometer.  Depending on the unit it uses, a magnetmeter is also called a Gaussmeter or Teslameter.  Since magnetic field is a vector quantity, some magnetometers (vector magnetometer) are dedicated to measuring vector components of a magnetic field while others (total field magnetometer) to measuring the magnitude of the magnetic field at various locations in space.

 

Can a magnetic field be shielded or blocked?

 

There is no material that is capable of removing a magnetic field.  However, magnetic field can be redirected by a ferromagnetic material that is attracted to the magnetic field to the effect of directing the magnetic field to itself .  Depending on how thick the shielding material is, it will partially or completely shield the magnetic field. 

 

Is there monopole magnet?

 

While the concept of monopole magnets have been hypothesized, there is no evidence that they are ever exist.

 

How to evaluate the temperature effect on demagnetization of a magnet?

 

It is important to note that the temperature effect on demagnetization (e.g. MaxOpTemp) of a given magnet is determined not only by the nature of the magnetic material but also by how it is used, "in circuit".  For a magnet in free space, it depends on the shape of the magnet as measured by the Permeance Coefficient (Pc). As an example, the following figure 1 shows the demagnetization curves (intrinsic and normal curves) for N42 neodymium magnets along with the loading line for N42 disk magnets of ½” diameter x 1/8” thickness.  (https://www.kjmagnetics.com/blog.asp?p=temperature-and-neodymium-magnets) .   The slope of the loading line, also called the Permeance Coefficient, is calculated to be 0.61.

 

At 80°C, the operating point is right at the knee of the normal curve.  Therefore, 80°C is the maximum operating temperature. Above 80°C, the magnet begins to irreversibly demagnetize.  Let's consider a magnet that's gone up to 100°C. The operating point shown as a blue dot below is at the intersection of the green load line, and the 100°C normal curve.  It is below the knee of the normal curve. The operating point has dropped down by a distance shown as "B" in the figure.

 

When you cool this magnet back to room temperature, it won't climb back up the knee.  It will drop by an amount shown graphically in purple, as "A". The red dot represents the new operating point once the magnet cools back to 20 °C.  The new operating point is dropped from the 20°C normal curve by the amount lost when the magnet went up to 100 °C.;How much pull strength as measured by the product of B and H (ignoring the negative sign of H) is lost?  From the figure, it is ~38.6 MGOe at 20° C before heating to 100 °C. Doing the same for the red dot in the figure, it is ~26.3 MGOe.  Thus, the strength is dropped to ~60% of the original strength.

 

We now consider N42 magnetic cylinders with equal diameter and height (1/4’’), which has a much larger Pc value of 3.46 or a much steeper loading line.  As noted in the figure 2, even at a high temperature of 140 °C, the operating point shown in red still resides above the knee of the curve, or no demagnetization.

 

 

How much force will a magnet pull?

 

This depends on your specific situation.  There are three common configurations of determining pull force of a magnet: (1) the most basic is between a flat and sufficiently thick steel plate and a given magnet; (2) a given magnet sandwiched between two steel plates, (3) between two magnets of same kind.  There are many other factors that affect the pull force, including the size, shape, magnetic and steel grades, surface condition, direction of pull. The pull force estimated can only serve as an approximate guide to product design.

 

Note that all these formulations are based on Gilbert's model, which is usable in relatively great distances. In other models (e.g., Ampère's model), a more complicated formulation is used that sometimes cannot be solved analytically. In these cases, numerical methods must be used.

See also

Notes

References

  • "The Early History of the Permanent Magnet". Edward Neville Da Costa Andrade, Endeavour, Volume 17, Number 65, January 1958. Contains an excellent description of early methods of producing permanent magnets.

  • "positive pole n". The Concise Oxford English Dictionary. Catherine Soanes and Angus Stevenson. Oxford University Press, 2004. Oxford Reference Online. Oxford University Press.

  • Wayne M. Saslow, Electricity, Magnetism, and Light, Academic (2002). ISBN0-12-619455-6. Chapter 9 discusses magnets and their magnetic fields using the concept of magnetic poles, but it also gives evidence that magnetic poles do not really exist in ordinary matter. Chapters 10 and 11, following what appears to be a 19th-century approach, use the pole concept to obtain the laws describing the magnetism of electric currents.

  • Edward P. Furlani, Permanent Magnet and Electromechanical Devices:Materials, Analysis and Applications, Academic Press Series in Electromagnetism (2001). ISBN0-12-269951-3.