Some materials are inherently magnetic (Natural Magnets). Other materials, such as Iron, are not inherently magnetic but can become magnetized if placed into a strong magnetic field. Some of these materials which do not start as magnets will retain much of their magnetism long after an initial magnetizing force has been applied and removed. These materials or bodies are called permanent magnets.
What is the source of magnetism in these materials, and what causes a permanent magnet to form, long-term, in some materials, but only for a short time in others?
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The property of magnetism is currently believed to be related to sub-atomic characteristics of the materials which can become magnetized:
Moving electrical charges and magnetic fields are inseparable. A moving electrical current forms a magnetic field, and a moving magnetic field will induce an electrical current in a conductor. The smallest particle with an electrical charge is the electron, which normally moves about in the space around the atom’s nucleus. In neutral atoms at rest, electrons and protons exist in equal numbers. To create currents in a conductor, electrons are forced to move. When they move, they create a magnetic field. But electrons are never still, even when they are not participating in the current. Electrons ‘spin’ about in the nucleus, and each spinning electron is moving, so it has a magnetic field. In each shell or energy band, some of the electrons revolve clockwise and others counterclockwise around the nucleus. These groups of electrons each produce a magnetic field. The strength of these fields and their directions depend upon the number of electrons and their direction of rotation.
Since iron is a common magnetic material let us examine its atomic structure to see why it has magnetic properties. There is a total of 26 electrons in the iron (Fe) atom. The two in the first shell neutralize each other because of opposite rotations. The same is true for second shell four revolve in one direction and four in the other. The third shell has 14 electrons nine revolve in one direction and five in the other. The outermost shell of the iron has only two electrons, which revolve in opposite direction. Because the fourth shell has a smaller radius than the third shell’s unbalanced electrons, the magnetic properties of iron are determined by the four unbalanced electrons in the third shell.
Nickel and Cobalt have fewer magnetic properties than iron. For example, nickel has two unbalanced electrons, and cobalt has three.
When a material has the same number of electrons revolving in each direction, their magnetic fields cancel and the material is said to be non-magnetic. An example of these materials is paper, wood, air, glass, aluminum, and copper.
When a piece of hard steel or alloy has not been exposed to a strong magnetic field, the fields of each atom are arranged randomly, and the number of atoms which are aligned in any direction has a high probability of being neutralized by another such group aligned in the opposite direction. When that piece of magnetizable material is subjected to a strong magnetic field, its atoms align themselves with the external field. Once the metal is removed from the external field, nearly all the atoms remain in their magnetizing position. The magnetism remaining after the external magnetizing field is removed is called residual magnetism.
The ability of a magnetic material to retain its magnetism is called retentivity.
Permanent magnets have high retentivity and therefore a large amount of residual magnetism. This can be disrupted by heating above a certain temperature or may be disrupted by a sharp impact. Soft iron has very little retentivity and can only be a temporary magnet.
Magnetizing by Induction
Soft iron becomes temporarily magnetized when brought into contact with a permanent magnet or another magnetic field. This temporary process is called magnetizing by induction. For example, small nails become temporarily magnetized when attracted to the ends of a magnet. Each nail becomes a small magnet by induction and will attract other small nails to their own ends. When removed from the magnet, they lose their magnetism very rapidly.