Magnetic properties of materials

Examples of materials with remarkable magnetic properties are iron, cobalt and nickel, as well as some iron oxides such as magnetite and maghemite, chromium oxides, nickel oxides and alloys such as alnico (aluminum, nickel and cobalt).

Its magnetism is manifested through the attraction that the bars made of these materials exert on iron filings, metal clips, coins and other small metal objects. magnetic properties of materials

If iron filings are placed on a sheet of paper and a bar magnet is passed underneath, it will quickly be seen that the filings are arranged in a pattern of closed, curved lines, emerging from one end of the bar and ending at the end of the bar. other.

The origin of magnetism in matter is the movement of electrons inside the atom. Electrons have a movement driven by the electrostatic attraction exerted by the nucleus on them and also possess spin, an entirely quantum quality, analogous to the rotation of the electron around its own axis.

As a result, the electron behaves like a tiny loop of current that produces its own magnetic field. magnetic properties of materials

Magnetic response magnetic properties of materials

All substances, indistinctly, respond to an external magnetic field. This is because, in any atom, the orbital motion of the electrons creates a vector called the orbital magnetic moment , and the spin creates the spin magnetic moment.

Between them they generate the magnetic moment of the electron and this in turn contributes to the net magnetic moment of the atom.

By the way, protons, which like electrons, are charged particles in motion, make a very small contribution to the net magnetic moment of the atom. So the atomic magnetic moment can be considered to be almost entirely dependent on its electrons.

Now suppose that a material is placed in the presence of an external magnetic field, which could align the disordered magnetic moments in the material, and create a net magnetic moment other than 0. This would elicit a magnetic response from the substance in question. magnetic properties of materials

There are three kinds of responses:

• Diamagnetism
• Paramagnetism
• Ferromagnetism

Magnetic susceptibility

To characterize each of these responses, there is a dimensionless physical quantity called the magnetic susceptibility . Its value informs about the degree of magnetization that the substance is capable of showing in the presence of the external magnetic field.

If M is the magnetization vector created by the net magnetic moment vector per unit volume within the material, H the external magnetic field and χ the magnetic susceptibility, it follows that, for many substances:

M = χ ∙ H

That is, the magnetization created in the material is directly proportional to the external field applied.

Main magnetic properties of materials

1. Diamagnetism magnetic properties of materials

All materials, without exception, have a diamagnetic response, which is always repulsive to the external magnetic field. If this is the only effect the external field has on material, it is considered diamagnetic.

The repulsion originates from the Faraday-Lenz law, since the external field induces a current in the material that always opposes the cause that causes it.

The materials with the most pronounced diamagnetic response are bismuth and antimony. Diamagnetism can also be observed with wood, water, salt, in metals such as gold, silver, and copper, and in some gases such as helium.

The magnetic susceptibility of these materials is always negative, for example, that of bismuth is -16.6 (unitless, since it has no dimensions). magnetic properties of materials

2. Paramagnetism magnetic properties of materials

There are atoms with a small net magnetic moment. When exposed to an external magnetic field, it exerts a torque that tends to align the individual magnetic moments with that field.

The response of the material to the field is attraction, generating a net magnetization vector M in its interior. Therefore the magnetic susceptibility of a paramagnetic material is always positive.

As the material is heated, the alignment of the acquired magnetization with the external field is counteracted by thermal agitation, which tends to destroy it.

Experimentally it is known that the magnetic susceptibility χ of paramagnetic materials depends on the temperature T as:

Examples of paramagnetic materials are: uranium, platinum, aluminum, sodium, copper sulfate, and rare earths.

3. Ferromagnetism 

In ferromagnetic materials, such as iron, nickel, cobalt, and alloys, the magnetic moments of each atom tend to align much more, forming micro regions called magnetic domains .

The domains are randomly oriented when the material is not magnetized, such as an iron nail, making the potential energy within the material minimal.

But when applying an external magnetic field, the limits of the domains are modified, gaining size those that manage to align with the external field. If this is strong enough, all the domains acquire the same direction and the material becomes magnetized in it.

Iron, nickel or cobalt objects, with high magnetic susceptibility, can acquire strong magnetization when subjected to the influence of a strong external field, and retain it to a large extent when the field is suppressed. In this way permanent magnets can be manufactured. magnetic properties of materials

As with paramagnetic materials, ferromagnetism decreases with temperature, disappearing at a critical temperature called the Curie temperature.

Another way to weaken the magnetization is by dropping or hitting the magnet, as impacts tend to undo the magnetic domains. magnetic properties of materials

Ferrimagnetism

In ferrimagnetic materials there is also an ordering in the individual magnetic moments of each atom. They are all aligned in the same direction, but alternating direction, which means that some can be canceled, but not all, so the result is a net magnetization in the material.

An example of a ferrimagnetic material is maghemite, an iron oxide that under certain conditions forms from magnetite and exhibits strong magnetism. magnetic properties of materials

4. Antiferromagnetism 

Another way in which the magnetic moments are arranged is in an antiparallel way, that is, by alternating their senses, as in manganese oxide, for this reason they do not respond in the same way to external fields as ferromagnetic materials.