Can keep for a long time its magnetic magnet said permanent magnets.
Such as natural magnet (
And artificial magnet steel (
Iron nickel and cobalt magnet steel)
And so on.
Permanent magnet is hard magnet, excitation-loss not easily, also not easy to be magnetized.
And as the material of magnetizer and electromagnet are mostly soft magnet.
Permanent magnet polarity will not change and soft magnet polarity is changed with the applied magnetic field polarity.
Just like that you normally see with magnetic steel rods.
Permanent magnets is after plus a magnetic field to remove, sparing the remanent magnetization of the object.
To make such an object, remanent magnetization is zero, the magnetic completely eliminate, must reverse magnetic field.
Ferromagnetics completely demagnetization need the size of the reverse magnetic field, called ferromagnetic properties of coercive force.
Steel and iron are ferromagnetics, but they often force different, handsomer steel has great power, and iron coercive force is small.
This is because in the process of steelmaking, add the carbon in iron, tungsten, chromium element, refined into carbon steel, tungsten steel, chrome steel, etc.
Carbon, tungsten, chromium element to join, make steel at room temperature conditions, within all sorts of inhomogeneity, such as the uneven of crystal structure, uneven of internal stress, magnetic strength uneven and so on.
The physical properties of inhomogeneous, make steel often force increases.
And the greater the degree of uneven within a certain scope, the greater the coercive force.
But the inhomogeneity are not steel or in any case have have to reach the best state, in order to make the steel internal optimal nonuniformity, must be properly heat treatment or mechanical machining.
Carbon steel, for example, in the condition of smelting, magnetic and ordinary iron.
It from the high temperature after refinement, uneven to rapid growth, to become a permanent magnetic material.
If the steel from the high temperature cooling down slowly, or have been refined through the steel smelting six or seven hundred c, its internal atoms have sufficient time to arrange into a stable structure, all kinds of nonuniformity is reduced, and there is less coercive force is, it ceases to become a permanent magnetic material.
Steel or other materials can become a permanent magnets, is because they after proper handling, processing, within the nonuniformity in the best condition, coercive force is the largest.
Iron the crystal structure, the internal stress nonuniformity is very small, such as nature of coercive force is small, make it magnetization and demagnetization don't need a strong magnetic field, as a result, it cannot become a permanent magnet.
Usually the magnetization and demagnetization are easy to material, known as 'soft' magnetic materials.
'Soft' magnetic materials cannot be permanent magnets, iron is belongs to the material