Image courtesy of

Permanent magnets have fascinated and aided human kind for centuries, from the first magnetic compasses to the toy magnets many of us played with as children. Most people know the basic properties of permanent magnets; they have a north and a south pole, they attract certain metals, and opposite poles attract. However, relatively few people understand why magnets act this way. In most cases, it is due to a property called ferromagnetism. Ferromagnetism is a found in Iron, Cobalt, Nickel, and certain rare earth metals and is a consequence of the electron configuration of those elements.

Electron Configuration and Spin

The electrons that orbit an atom have a property that physicists refer to as spin, or intrinsic magnetic moment. Simply put, each electron acts like a small permanent magnet with a north and a south pole. When electrons orbit an atom, they tend to come together in pairs with opposite spin according to the Pauli Exclusion Principle. This pairing means that the spin of each electron cancels out the other so that the atom has no net magnetic moment. However, in some materials, namely those mentioned above, there are unpaired electrons that do result in a net magnetic moment. In ferromagnetic materials, these unpaired electrons tend to line up facing the same way so that a large chunk of material has a net magnetic moment. These chunks of aligned magnetic moments are called magnetic domains.

Magnetic Domains

Magnetic domains are small sections of a larger piece of ferromagnetic material where the magnetic moments of the unpaired electrons within them are all aligned the same way. Each of these domains has a magnetic moment in the same direction as that of the aligned electrons inside it. The overall magnetic moment of any ferromagnet is the sum of the magnetic moments of the magnetic domains which make up the magnet. In an unmagnetized piece of ferromagnetic material, the domains all have random magnetic moments that point in different directions so that the sum of those moments adds up to be zero. In a ferromagnet, the moments of the domains do not sum to zero, resulting in some net magnetic moment for the whole magnet. This doesn't mean that all of the domains point in the exact same direction, only that there is some overall trend toward one direction. The strength of the magnet depends on how well the domains are aligned.


Under normal conditions, magnetic domains do not readily change direction. If left in a magnetic field for a long time, they will realign, but the overall effect is relatively small compared to some other methods of magnetization, so a very strong external magnetic field is required to make a strong magnet. There are a few different methods for weakening the bonds that hold magnetic domains in place. One is to vibrate the material. This effect can be seen in some pieces of fixed machinery that have operated for a long time. They are oriented the same way in a magnetic field (the magnetic field of the Earth) for a long time and vibrate as they operate. The end result is that the machinery shows a certain degree of magnetization. The effect is small, however, because the earth's magnetic field is not very strong. A more effective way to magnetize something is to heat it. Every ferromagnet has a property called a Curie temperature, defined as the temperature at which the random motions of its particles due to heat overcomes the tendency to align in domains. If a piece of material is heated past this point and then cooled in a strong magnetic field, the particles will align themselves with the external field so that when the material is below its Curie temperature again, they will mostly be aligned in the same direction, giving the material a considerable overall magnetic moment. Heating a ferromagnet past its Curie temperature and cooling it in the absence of a magnetic field will result in a random alignment of magnetic domains within the material, in effect demagnetizing it.

Magnetic Cores and Solenoids

A Solenoid with no core

A solenoid (pictured above) is a cylinder of many loops of wire all wound the same direction so that when current runs through the wire a magnetic field is produced inside the cylinder. Solenoids are capable of creating magnetic field much stronger than those commonly found in nature, so they are useful for making permanent magnets and in many mechanical and electronics applications. Often they are made by wrapping the wire around a cylindrical piece of magnetic material, making them stronger. To understand why, first imagine a solenoid with no core. With a constant current running through it, it would have some magnetic field H. Now imagine that you put a piece of iron in that field. The iron would become magnetized as described above, and it would have a magnetic field of its own (M). But the H field that the solenoid had without the core would still be there, so the total magnetic field inside the solenoid with a core would be the sum of H and M combined. Depending on how readily the magnetic domains in the core material realign themselves, the M could be considerably greater than H, so if the material is something like iron, which very readily realigns, the introduction of a core can greatly increase the effectiveness of a solenoid. This ability to realign is called permeability, and is denoted by the Greek letter µ.

Some Helpful Sites: