(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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Température de Curie

La température de Curie est ainsi nommée en l'honneur de Pierre Curie (1859-1906).

magnétism Curie température

La température de Curie est la température au-dessus de laquelle les matériaux ferromagnétiques n’ont plus de champ magnétique autour d’eux. Au-delà de cette température, le matériau est paramagnétique. Lorsque la température monte, la fluctuation de température provoque une rupture progressive de l’ordre de spin. Arrivé à la température de Curie, l’ordre s’effondre complètement parce que l’énergie thermique est devenue plus importante que l’énergie de l’interaction magnétique.

Il est difficile de mesurer exactement la température de Curie. Premièrement, le champ magnétique permanent qui se trouve autour du matériau ne disparaît que progressivement. Deuxièmement, la température de Curie dépend fortement des petites impuretés dans le matériau.

Par exemple, lorsqu’un aimant AlNiCo est réchauffé au-dessus de la température de Curie de 850°C, il n’est plus ferromagnétique. Il devient alors paramagnétique. Lorsque l’aimant se refroidit, le champ magnétique ne revient pas. Même si un champ magnétique réapparaît dans des petites parties du matériau, les fameux domaines de Weiss (Weiss 1865-1904), ces domaines indiquent des directions au hasard, et aucun champ magnétique externe n'est engendré. Il est cependant possible de remagnétiser l'aimant.

Les éléments et alliages ferromagnétiques avec leurs températures de Curie

Materiau     Température de Curie.
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

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