A permanent magnet (also known as permanent magnet ) is a magnet made from a piece of hard magnetic material, for example alloys made from iron , cobalt , nickel or certain ferrites . It has and maintains a constant magnetic field , without having to use electrical power as with electromagnets . Permanent magnets have one or more north and south pole (s) on their surface.
The first man-made permanent magnet materials were made by John Canton around 1750 .
A permanent magnet can be generated by the action of a magnetic field on a ferrimagnetic or ferromagnetic material with a large-area hysteresis curve (so-called hard magnetic material). Early magnetic materials based on iron gave rise to the terms hard magnetic and soft magnetic : hard, carbon-rich steel can be made permanently magnetic, while low-carbon, soft iron (soft iron) can hardly be permanently magnetized and is therefore better suited for the production of iron cores for electromagnets . A permanent magnet can be demagnetized by a decaying alternating magnetic field, heating or impact .
The most popular form in everyday life are ferrite magnets, e.g. B. as a magnet or - provided with iron pole pieces - as a cabinet door lock.
- A permanent magnet acts on all ferromagnetic materials such as B. iron and on ferrimagnetic substances - such as ferrites - an attraction.
- Two permanent magnets attract each other with their opposite poles and repel poles with the same name.
Rings magnetized along the circumference have no poles (see e.g. core memory ) and do not exert any forces - although they are magnetized, they are not referred to as permanent magnets. Magnetized layers of magnetic tapes , magnetic strips or hard drives have poles, but are not referred to as permanent magnets either.
In contrast to the graphic shown, the hysteresis curves of magnetizable, hard magnetic materials are particularly wide and resemble a rectangle in which the almost vertical curves intersect the field strength axis at high field strengths at Hc. The graphic shown rather shows the hysteresis curve of a soft magnetic material, which is, for example, when recording the hysteresis curve in a transformer with only a small air gap or in an Epstein frame .
With soft magnetic materials, such as sheet metal or ferrites for transformer cores, the hysteresis curve is very narrow and intersects the field strength axis at low field strength values.
- Energy product
- The energy product, also known as the BH product, is the entire field energy stored in the magnet .
- Energy density
- The energy density is the magnetic energy related to the volume of the magnet.
- Coercive field strength H C
- The field strength must be applied to completely demagnetize the magnet (flux density B = 0) is the intersection of the hysteresis curve with the axis of the field strength H . The greater the coercive force , the greater the magnet's resistance to demagnetization by external fields.
- Maximum operating temperature
- While the Curie temperature indicates the point at which the ferromagnetic property of a material disappears, the macroscopic orientation of the Weiss domains and thus the permanent magnet properties disappear irreversibly even at significantly lower temperatures. In general, this macroscopic orientation is unstable at temperatures above absolute zero, but in practical use a temperature range can be specified for the relevant materials in which the unavoidable demagnetization proceeds imperceptibly slowly or is essentially determined by mechanical stress .
- Remanence B R
- With Retention refers to the flux density that occurs without an external field. Its value can be read off the hysteresis curve as the value of B at H = 0.
Permanent magnet materials
Permanent magnets used to be made of steel . But they are very weak and can be demagnetized very easily. The most famous form are horseshoe magnets. Permanent magnetizations can also form in steel tools through plastic mechanical deformation. This is an indication of their mechanical overload.
AlNiCo magnets consist of iron alloys with aluminum, nickel and cobalt as the main alloying elements. These materials can be used up to 500 ° C, but have a relatively low energy density and coercive field strength. The remanence is higher than with the ferrite magnets. They are manufactured by casting or powder metallurgical processes . They have good corrosion resistance, but they are fragile and hard.
Magnets made of hard magnetic ferrites are inexpensive, but relatively weak and have a maximum service temperature of 250 ° C. Typical applications are holding magnets and field magnets in DC motors and electrodynamic loudspeakers .
Rare earth magnets
- Neodymium-Iron-Boron (NdFeB) enables very strong magnets at an acceptable cost. For a long time, the operating temperatures were limited to a maximum of 60–120 ° C. For more recent developments with additives such as dysprosium , operating temperatures of up to 200 ° C are specified.
- Samarium cobalt
- Samarium-Cobalt (SmCo) with 20-25% iron content enables strong permanent magnets with high energy density and high operating temperature. The disadvantage is the high price.
A special type of magnet material are non-metallic, organic plastics with permanent magnetic properties, such as the plastic magnet material PANiCNQ , which has ferrimagnetic properties at room temperature .
Permanent magnets are mostly pressed into a mold from crystalline powder in the presence of a strong magnetic field. The crystals align themselves with their preferred magnetization axis in the direction of the magnetic field. The compacts are then sintered . If the sintering temperature is above 1000 ° C, the outwardly effective magnetization is lost because the thermal movement of the atoms leads to the largely anti-parallel alignment of the elementary magnets in the crystals. Since the orientation of the grains in the sintered composite is not lost, the parallel alignment of the elementary currents can be restored after the magnets have cooled down with a sufficiently strong magnetization pulse.
- Actuating magnets for reed contacts , e.g. B. spoke magnet for bicycle speedometer sensor
- Damping ( eddy current brake ) z. B. on roller coasters and in the electricity meter
- Transductor with permanent magnetic setting (historically, replaced by triac controls )
- Electric motors , e.g. B. self-excited DC motors , rotors of small synchronous motors
- Field generation in moving coil measuring mechanisms
- Field magnets from loudspeakers and dynamic microphones and headphones and earphones , permanent magnet synchronous motors , rotors of electronically commutated motors
- Runner of smaller generators , e.g. B. Bicycle dynamo
- Linear swivel motor for the read head arm of a hard disk drive
- Permanent magnet generator , e.g. B. in some modern wind turbines
- Permanent magnet motor
- Field magnets for circulators in high frequency technology
- Field magnets of magnetrons , e.g. B. two magnetic rings in the microwave oven
- Correction magnets on picture tubes
- Field magnet in coil-like components, which use the resulting asymmetrical saturation of a ferrite core (transducer effect)
- Particle accelerator technology : deflection and focusing magnets in ring accelerators as well as in undulators and wigglers for the generation of synchrotron radiation
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