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MAGNETIC DOMAIN

After completing this section you will be able to do the following:

• Define a magnetic domain.
• Explain one way an object can be magnetized.

A magnetic domain is region in which the magnetic fields of atoms are grouped together and aligned. In the experiment below, the magnetic domains are indicated by the arrows in the metal material. You can think of magnetic domains as miniature magnets within a material. In an unmagnetized object, like the initial piece of metal in our experiment, all the magnetic domains are pointing in different directions. But, when the metal became magnetized, which is what happens when it is rubbed with a strong magnet, all like magnetic poles lined up and pointed in the same direction. The metal became a magnet. It would quickly become unmagnetized when its magnetic domains returned to a random order. The metal in our experiment is a soft ferromagnetic material, which means that it is easily magnetized but may not retain its magnetism very long.

Click and drag the magnet across the metallic strip.
The arrows represent the alignment of the atoms in the metallic strip.

Questions

1. What happened to the piece of metal when you rubbed a strong magnet across it the first time? The second time?
2. What do the arrows in the material represent?
3. Why do they become lined up when the magnet is brought in contact with the metal?
4. If you wanted to turn a paper clip into a magnet, how do you think you could do it?

How can you turn a paper clip into a magnet?

You can turn a paper clip into a magnet by rubbing a strong magnet several times over the surface of the paper clip. The more you drag the magnet over the paper clip, the stronger the paper clip will become magnetized. The same thing happened with the metal in the experiment. When we rubbed the magnet over the surface of the metal, some of the magnetic domains aligned and the metal became partially magnetized. When we rubbed the magnet over the metal a second time, more of the magnetic domains became aligned and the metal became a stronger magnet.

What is different about ferromagnetic materials that make them strongly magnetic?

In ferromagnetic materials, the magnetic moments of a relatively large number of atoms are aligned parallel to each other to create areas of strong magnetization within the material. These areas, which are approximately a millimeter in size, contain billions of aligned atoms and are called magnetic domains. Magnetic domains are always present in ferromagnetic materials due to the way the atoms bond to form the material. However, when a ferromagnetic material is in the unmagnetized condition, the magnetic domains are randomly oriented so that the magnetic field strength in the piece of material is zero.

In the unmagnetized condition, the material will be attracted to a magnet but will not act as a magnet. That is to say, two unmagnetized pieces of ferromagnetic material will not be attracted to each other. When a ferromagnetic material is magnetized, the magnetic domains align parallel to each other to produce a large net field strength in the material and the material becomes magnetic.