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Magnetic materials properties

    All materials show some magnetic effects. In engineering, taking by definition r = 1 for a free vacuum, materials are divided into two main groups:

    - Non-magnetic materials - r 1
    - Magnetic materials - r >> 1

    From the physics course it is also known that non-magnetic materials are divided in addition into diamagnetics (such as copper, silver, lead), where r < 1, and paramagnetics (such as air, aluminium) where r > 1. But of main interest are the so-called ferromagnetics (such as iron, nickel, cobalt and their alloys), where r may reach value of several thousand.
    The effect of ferromagnetism is explained by the domain theory. Not going into details the main results and conclusions are specified below:

Due to the spinning of the electrons and their orbital movement around the atom nucleus, a vector quantity "magnetic moment m" is introduced as product of the electron micro current and the area enclosed by the electron orbit;

In absence of an external magnetic field the magnetization M of the material (per unit volume) is practically zero, due to different orientation of the individual magnetic moments m of each atom;

When an external magnetic field H is applied, the individual magnetic moments m tend to turn: in opposite direction to the field for diamagnetics, and in the field direction for the paramagnetics. However for these materials this effect is weak enough, as magnetization M is small compared with the external field intensity H. The magnetic flux density B only slightly changes;

For ferromagnetics the situation is quite different, due to a special effect - the domains formation. The domains are subcristaline structures, each one of them containing about 1015 atoms in a volume of approximately 10-6 cubic millimeters. The main distinguished feature of a domain is that the magnetic moments of its constituent atoms are aligned in one and the same direction.

As domains are independent from each other, in absence of external magnetic field the total magnetization M is practically zero. This is so, as in the metal crystal lattice of iron, there exist 6 "easy directions" of domain alignment - up, down, left, right, in and out, as depicted in figure:

permanent magnet theory

    In figure (b) a result is presented when a strong external field H is applied in direction from left to right. As seen all the domains magnetic moments have aligned into external field H direction. Such condition for the iron is called "saturation". It means that the magnetic flux density B inside the material could increase over again only on account of additional external field applied.

It should be noted that above certain temperature called "Curie point", alignment of domains is totally destroyed, which results in the fact that material looses its magnetic properties. For the iron this temperature point is 770oC.

    Magnetization of materials in the presence of external field is called induced magnetization. It differs from the permanent magnetization which is present in absence of external field. Materials exerting permanent magnetization are called permanent magnets.

MODEL GALLERY - Permanent Magnet

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permanent magnet modeling FEM

permanent magnet modeling FEM

permanent magnet modeling FEM

permanent magnet modeling FEM

permanent magnet modeling FEM

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