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(Ferro) magnetism

Back in ancient times, people discovered that magnetite crystals attract or repel each other depending on their orientation. This physical phenomenon is referred to as magnetism. The words magnetite and magnesium are both derived from Magnesia, the name of an area in the Thessaly region of Greece where magnetic stone can be found in abundance.Magnetic separation

It is the iron in the rock that is responsible for the magnetic properties of magnetite. Many iron alloys possess magnetic properties. In addition to iron, we find magnetic properties in nickel, cobalt and gadolinium as well.

Although ferromagnetic (and ferrimagnetic) materials are the only kinds with strong enough magnetic properties to be drawn to a magnet (which is why they are called ‘magnetic’), all other substances also respond weakly to a magnetic field, via one or more other types of magnetism.

Ferromagnetic materials can be divided into magnetically 'soft' materials, such as annealed iron, which can be magnetized but usually do not retain the magnetization indefinitely, and magnetically 'hard' materials that do remain magnetized. Permanent magnets are made of 'hard' ferromagnetic materials such as Alnico and ferrite, which undergo special processing in a powerful magnetic field during production to 'align' their internal microcrystalline structure, making them very resistant to demagnetization.

Objects which strongly exhibit this behaviour are called magnets. There are natural and man-made magnets (e.g. Alnico, Fernico, ferrites). All magnets have two poles, which are referred to as the north pole and the south pole. The north pole of a magnet repels the north pole of other magnets and attracts the south pole of other magnets. Two south poles also repel.

Magnetism Gripper

Because the earth has a magnetic field as well, with its magnetic south pole close to the geographic north pole and its magnetic north pole close to the geographic south pole, a free-spinning magnet will always take on a north-south orientation. The names of the poles of a magnet are derived from this phenomenon. For the sake of convenience, but nevertheless slightly confusing, the south pole of “the 'earth magnet' is called the magnetic north pole and the north pole of the 'earth magnet' is called the magnetic south pole.

Magnetic field - North and South pole

A related phenomenon is electromagnetism, which is magnetism generated by an electric current. In essence, all magnetism is caused by either rotating or revolving electrical charges in eddy currents.


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De Curietemperatuur is genoemd naar Pierre Curie (1859-1906).

magnetisme Curietemperatuur
De Curietemperatuur is de temperatuur waarboven ferromagnetische materialen ophouden een permanent magneetveld om zich heen te bezitten. Dit gebeurt doordat de atomen zo hevig trillen dat er geen globale richting meer is: de stof verliest zijn magnetisme.

Boven de Curietemperatuur gedraagt het materiaal zich paramagnetisch. Bij het verhogen van de temperatuur zorgt de temperatuurbeweging voor een geleidelijk doorbreken van de spinorde. Bij de Curietemperatuur stort de ordening ineen doordat de thermische energie groter is geworden dan de energie van de magnetische wisselwerking.

De Curietemperatuur is moeilijk exact te meten. Ten eerste verdwijnt het permanente magneetveld rond het materiaal slechts geleidelijk. Ten tweede is de Curietemperatuur sterk afhankelijk van kleine verontreinigingen in het materiaal.

Als bijvoorbeeld een AlNiCo magneet wordt verwarmd boven de Curietemperatuur van 850°C dan is hij niet meer ferromagnetisch. Hij wordt dan paramagnetisch. Als de magneet weer afgekoeld is, komt het permanente magneetveld niet terug. Wel is er dan weer een magneetveld aanwezig in kleine gebiedjes in het materiaal, de zogenaamde Weissgebiedjes (Weiss 1865-1904), maar deze velden wijzen in willekeurige richtingen, zodat er geen resulterend extern magneetveld aanwezig is. Het is wel mogelijk de magneet opnieuw te magnetiseren.

De ferromagnetische elementen en legeringen met hun Curie-temperaturen:

Materiaal     Curietemp.
Fe     770°C
Co     1115°C
Ni     354°C
Gd     19°C
AlNiCo     850°C
Ferriet     450°C
Sm Kobalt     750-825°C
Nd-Fe-B     310-340°C

Magnet materials


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Samarium-cobalt magnets

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Aluminum-nickel-cobalt magnets

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