Ferromagnetic Materials

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

  • Define what ferromagnetic materials are.
  • Explain the differences between a permanent magnet and a temporary.
  • Explain why some materials have magnetic properties only when a permanent magnet is near them.

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 - 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.

A Ferromagnetic material can be magnetized by placing the material in a strong external magnetic field or by passing electrical current through the material. When a ferromagnetic material is magnetized, some or all of the magnetic domains align parallel to each other to produce a large net magnetic field strength in the material and the material becomes magnetic. The more domains that are aligned, the stronger the magnetic field in the material. When all of the domains are aligned, the material is said to be magnetically saturated. When a material is magnetically saturated, no additional amount of external magnetization force will cause an increase in its internal level of magnetization.

When magnetic domains are not aligned, there is no net magnetic field produced by the object. Once all of the domains of an object are aligned, the object is considered magnetized and produces a magnetic field.
Unmagnetized Material
 
Magnetized Material

Magnetic domains can be detected using Magnetic Force Microscopy (MFM) and images of the domains like the one shown below can be constructed.

Magnetic domains (on the order of 50 microns in length and width) can be imaged using magnetic force microscopy.

Magnetic Force Microscopy (MFM) image showing the magnetic domains in a piece of heat treated carbon steel.

Permanent and Temporary Magnets

We have determined in previous discussions that magnets can be permanent or temporary. A permanent magnet is more difficult to magnetize but will retain the properties of magnetism indefinitely. A temporary magnet is generally made of soft iron and will remain magnetized only as long as the magnetizing cause is present. From previous experiments you saw how the difference in magnetized and unmagnetized material depends on the motion and arrangement of the material's molecules. Bringing a ferromagnetic object, like a nail, into the magnetic field of a strong magnet can cause the molecules of the iron material to line up and the nail to become a temporary magnet. As long as it is in the magnetic field of the bar magnet, the nail acts like a magnet and picks up other ferromagnetic materials. In this case it is the paper clip. Then, the paper clip becomes a magnet and can pick up another paper clip, and so forth.

Review:

  1. Magnetic domains are always present in ferromagnetic materials due to the way the atoms bond to form the material.
  2. When all of the domains are aligned, the ferromagnetic material is said to be magnetically saturated.
  3. A permanent magnet is more difficult to magnetize but will retain the properties of magnetism indefinitely.