Magnet, Magnets, Magnetism


Is the attraction between magnets as high as the repulsion?

The attraction between magnets is a little stronger than the repulsion. That is due to the alignment of the molecular magnets in the magnet. The attraction as well as the repulsion of magnets decrease significantly with increasing distance. The attraction as well as the repulsion of magnets decrease significantly with increasing distance. When two equal magnets touch each other, the attraction between two unequal poles is 5-10% stronger than the repulsion of equal poles. In a single magnet, the molecular magnets are aligned sort of parallel to each other.


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Description of magnetic materials

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, steel, nickel, cobalt, etc. and attracts or repels other magnets. A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include the elements iron, nickel and cobalt and their alloys, some alloys of rare-earth metals, and some naturally occurring minerals such as lodestone. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism. Ferromagnetic materials can be divided into magnetically "soft" materials like annealed iron, which can be magnetized but do not tend to stay magnetized, and magnetically "hard" materials, which do. Permanent magnets are made from "hard" ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a strong magnetic field during manufacture to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on coercivity of the respective material. "Hard" materials have high coercivity, whereas "soft" materials have low coercivity. The overall strength of a magnet is measured by its magnetic moment or, alternatively, the total magnetic flux it produces. The local strength of magnetism in a material is measured by its magnetization. An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it but stops being a magnet when the current stops. Often, the coil is wrapped around a core of "soft" ferromagnetic material such as mild steel, which greatly enhances the magnetic field produced by the coil..


Permanent magnet

Permanent magnets are materials where the magnetic field is generated by the internal structure of the material itself. Inside atoms and crystals you have both electrons and the nucleus of the atom. Both the nucleus and the electrons themselves act like little magnets, like little spinning chunks of electric charge, and they have magnetic fields inherent in the particles themselves. There's also a magnetic field that's generated by the orbits of the electrons as they move about the nucleus. So the magnetic fields of permanent magnets are the sums of the nuclear spins, the electron spins and the orbits of the electrons themselves. In many materials, the magnetic fields are pointing in all sorts of random directions and cancel each other out and there's no permanent magnetism. But in certain materials, called ferromagnets, all the spins and the orbits of the electrons will line up, causing the materials to become magnetic. This would be your normal iron, cobalt, nickel. Permanent magnets are limited by the structure of the material. And the strongest magnetic field of a permanent magnet is about 8,000 gauss. The strongest magnets here at the Magnet Lab are 450,000 gauss, which would be almost 50 times stronger than that.

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 crystalline 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 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 sintered composite 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. 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-coercivity ferromagnetic 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.

Rare-earth magnets

Rare earth (lanthanoid) elements have a partially occupied f electron 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)

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.

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.

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, 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.

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.


Units and calculations

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.

In all units, it is convenient to employ two types of magnetic field, B and H, as well as the magnetization M, 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

p>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.


Electromagnets

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. 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 focus particle beams.


Handling and Storage of Neodymium Magnets:

Neodymium Magnets are very strong and brittle, requiring appropriate handling and packing to ensure safety and prevent damage.
Neodymium magnets are very strong and brittle and appropriate handling and packing is required. Most receiving departments are not familiar with the strength of neodymium 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. We urge all customers to discuss handling techniques pertinent to their magnets with a Magnosphere team member.
The packaging methods of  neodymium magnetized alloys are dependent upon the magnet size and the customer requirement.

  • 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.

Neodymium Magnet Manufacturing Methods:

Neodymium Magnets are typically manufactured by a powdered metallurgical process utilizing rigid steel or rubber molds.
Fully dense Neodymium Magnets (also known as neo magnets, neodymium iron boron, neo, or rare earth magnets) are usually manufactured by a powdered metallurgical process. Micron size Neodymium and iron boron powder is produced in an inert gas atmosphere and then compacted in a rigid steel mold or in a rubber mold. The rubber mold is compacted on all sides by fluid and it is referred to as isostatic pressing. The steel molds will produce shapes similar to the final product, while the rubber mold will only create large blocks (loaves) of Neodymium magnet alloy.
The Neodymium alloy’s magnetic performance in both compacting methods is optimized by applying a magnetic field before or during the pressing operation. This applied field imparts a preferred direction of magnetization, or orientation to the Neodymium 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 rare earth magnet. After pressing, the Neodymium Magnets are sintered and heat treated until they reach their fully dense condition. The die pressed magnets are ground to the final dimensions, but the brick magnets from the rubber mold method are usually squared on large grinders and then sliced to the final geometry. Isostaticly pressed alloy has higher magnetic properties than the die pressed material, but it may lack the uniformity. The choice of manufacturing method to create Neodymium Magnets is usually application driven and is typically not a concern of the customer.
Table of magnetic properties of Neodymium Magnets:

Material

Remanence

Br (T)

Coercivity Hc (kA/m)

Max. energy product

(BxH)max (kJ/m3)

Max. working temp.*

Tw (°C)

Normal Hcb Intrinsic Hci
N30 1.08 - 1.13 >= 796 >= 1353 223 - 247 80 - 240
N33 1.13 - 1.17 >= 812 >= 1353 247 - 271 80 - 240
N35 1.17 - 1.22 >= 868 >= 955 263 - 287 80 - 200
N38 1.22 - 1.25 >= 899 >= 955 287 - 310 80 - 180
N40 1.25 - 1.28 >= 907 >= 955 302 - 326 80 - 180
N42 1.28 - 1.32 >= 915 >= 955 318 - 342 80 - 150
N45 1.32 - 1.38 >= 923 >= 955 342 - 366 80 - 150
N48 1.38 - 1.42 >= 923 >= 955 366 - 390 80 - 120
N50 1.40 - 1.45 >= 796 >= 796 382 - 406 60 - 100
N52 1.43 - 1.48 >= 796 >= 876 398 - 422 60
N54 1.45 - 1.51 >= 939 >= 875 405 - 437 60

*Maximum working temperature is differentiated by adding an alphabetical code after the code of material grade (e.g. N35EH = 200°C). Increasing the maximum working temperature results in gradual decrease of material grade:
no code - Tw <=80°C (for N52 and N54 max. 60°C)

M - Tw <=100°C

H - Tw <=120°C

SH - Tw <=150°C

UH - Tw <=180°C

EH - Tw <=200°C

AH - Tw <=240°C

Neodymium magnets are hard, brittle and sensitive to crack, so machining is problematic because the protective coating is corrupted. Machining can be carried out by grinding with diamond tools, but the coating must be repaired.
NdFeB magnets have excellent resistance to external magnetic fields and in normal conditions keep their magnetic properties during long time.
Other magnetic and mechanic parameters of neodymium magnets:

Temp. coefficient of Br (%/°C) Temp. coefficient of Hci (%/°C) Magnetizing field (kA/m) Curie temp. (°C) Density (g/cm3) Hardness (Hv)
- 0.12 - 0.6 2400 310 - 340 7.5 570

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