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. 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 This impresses the plastic sheet with the magnetic poles in an alternating line format.
No electromagnetism is used to generate the magnets. These magnets are lower in magnetic strength but can be very flexible, depending on the binder used. 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. In the s, 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:. 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. Some nano-structured materials exhibit energy waves , called magnons , that coalesce into a common ground state in the manner of a Bose—Einstein condensate. 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 current [update] 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 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.
Introduction to Magnetic Materials
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. 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. 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. Both hard and soft magnets have a more complex, history-dependent, behavior described by what are called hysteresis loops , which give either B vs.
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.
For pole strength, we will employ q m. 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. The pull force exerted by either an electromagnet or a permanent magnet at the "air gap" i.
Therefore, if a magnet is acting vertically, it can lift a mass m in kilograms given by the simple equation:. Classically , the force between two magnetic poles is given by: . 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. 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:  . Note that all these formulations are based on Gilbert's model, which is usable in relatively great distances. In other models e. In these cases, numerical methods must be used.
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From Wikipedia, the free encyclopedia. Electrical network. Covariant formulation. Electromagnetic tensor stress—energy tensor. Main article: History of electromagnetism. See also: Magnetism history. Play media. Main article: Magnetic field. Main article: Magnetic moment. Main article: Magnetization. See also: Two definitions of moment. Main article: Magnetism. See also: Remanence. Main article: Rare-earth magnet. Main article: Single-molecule magnet. Main article: Electromagnet. Main article: Magnetostatics.
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Magnetic Materials for Nuclear Magnetic Resonance and Magnetic Resonance Imaging
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