All substances show some kind of magnetic behavior. After all, they are made up of charged particles: electrons and protons. It is the way in which electron clouds arrange themselves in atoms and how groups of these atoms behave that determines the magnetic properties of the material. The atom (or group of atoms) in effect becomes a magnetic dipole or a mini bar magnet that can align according to the magnetic field applied. The net effect of all these dipoles determines the magnetic properties of the Magnetic Materials.
To study magnetic properties of Magnetic Materials, the material is usually placed in a uniform magnetic field and then the magnetic field is varied. There are three major kinds of magnetic behavior:
- Diamagnetic materials:
These materials are barely magnetized when placed in a magnetic field. In fact, magnetic dipoles in these substances tend to align in opposition to the applied field. In effect, they produce an internal magnetic field that opposes the applied field and the substance tends to repel the external field around it.
This opposing field disappears as soon as the external field is removed.
Ex: Gold, water, mercury and even animals!
- Paramagnetic materials:
In these materials the magnetic dipoles in the Magnetic Materials tend to align along the applied magnetic field and thus reinforcing the applied magnetic field. Such substances are attracted by a magnet if it applies a sufficiently strong field. It must be noted that such materials are still feeble magnetized and the magnetization disappears as soon as the external field is removed. The magnetization (M) of such materials was discovered by Madam Curie and is dependent on the external magnetic field (B) and temperature T as:
Where C= Curie Constant
Ex: Liquid oxygen, sodium, platinum, salts of iron and nickel.
- Ferromagnetic materials:
We are most familiar with these materials as they exhibit the strongest magnetic behavior. Magnetic dipoles in these materials are arranged into domains where the arrangements of individual magnetic dipoles are essentially perfect that can produce strong magnetic fields. Normally, these domains are usually randomly arranged and thus the magnetic field of each domain is canceled by another and the entire material does not show any magnetic behavior.
However when an external field is applied, the domains reorient themselves to reinforce the external field and produce a strong internal magnetic field that is along the external field. Upon, removal of the external field, most of the domains stay put and continues to be aligned in the direction of the (erstwhile) magnetic field. Thus, the magnetic field of the Magnetic Materials persists even when the external field disappears. This property is used to produce Permanent magnets that we use every day.
Iron, cobalt, nickel, neodymium and their alloys are usually highly ferromagnetic and are used to make permanent magnets.
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