Classifications of Magnetic Materials
by Ron Kurtus (revised 23 March 2012)
Magnetic properties of Steel and Iron
Iron gets magnetized faster but loses its magnetism as soon as the inducing magnet is removed. Hence soft iron is said to have high susceptibility but low retentivity. This property of soft iron is very useful in making temporary electromagnets where we need strong but temporary magnets. If the magnets used in these devices were to retain their magnetism for a longer period, the devices would not function properly.
Steel is slow to be magnetized but retains the acquired magnetism for a long time. Steel is said to have low susceptibility but high retentivity. Steel is used for making magnets.
Materials respond differently to the force of a magnetic field. There are three main classifications of magnetic materials. A magnet will strongly attract ferromagnetic materials, weakly attract paramagnetic materials and weakly repel diamagnetic materials.
The orientation of the spin of the electrons in an atom, the orientation of the atoms in a molecule or alloy, and the ability of domains of atoms or molecules to line up are the factors that determine how a material responds to a magnetic field. Ferromagnetic materials have the most magnetic uses. Diamagnetic materials are mainly used in magnetic levitation.
Questions you may have include:
- What are ferromagnetic materials?
- What are paramagnetic materials?
- What are diamagnetic materials?
This lesson will answer those questions. Useful tool: Units Conversion
Ferromagnetic materials are strongly attracted by a magnetic force. The elements iron (Fe), nickel (Ni), cobalt (Co) and gadolinium (Gd) are such materials.
(See the Periodic Table in the Chemistry section for more information on the elements.)
The reasons these metals are strongly attracted are because their individual atoms have a slightly higher degree of magnetism due to their configuration of electrons, their atoms readily line up in the same magnetic direction and the magnetic domains or groups of atoms line up more readily.
(See Factors Determining Magnetic Properties for more information.)
Iron and steel
Iron is the most common element associated with being attracted to to a magnet. Steel is also a ferromagnetic material. It is an alloy or combination of iron and several other metals, giving it greater hardness than iron, as well as other specialized properties. Because of its hardness, steel retains magnetism longer than iron.
Alloys of iron, nickel, cobalt, gadolinium and certain ceramic materials can become "permanent" magnets, such that they retain their magnetism for a long time.
Strongly magnetic ferromagnetic materials like nickel or steel lose all their magnetic properties if they are heated to a high enough temperature. The atoms become too excited by the heat to remain pointing in one direction for long.
The temperature at which a metal loses its magnetism is called the Curie temperature, and it is different for every metal. The Curie temperature for nickel, for example, is about 350°C.
Paramagnetic materials are metals that are weakly attracted to magnets. Aluminum and copper are such metals. These materials can become very weak magnets, but their attractive force can only be measured with sensitive instruments.
Temperature can affect the magnetic properties of a material. Paramagnetic materials like aluminum, uranium and platinum become more magnetic when they are very cold.
The force of a ferromagnetic magnet is about a million times that of a magnet made with a paramagnetic material. Since the attractive force is so small, paramagnetic materials are typically considered nonmagnetic.
Certain materials are diamagnetic, which means that when they are exposed to a strong magnetic field, they induce a weak magnetic field in the opposite direction. In other words, they weakly repel a strong magnet. Some have been used in simple levitation demonstrations.
Bismuth and carbon graphite are the strongest diamagnetic materials. They are about eight times stronger than mercury and silver. Other weaker diamagnetic materials include water, diamonds, wood and living tissue. Note that the last three items are carbon-based.
The electrons in a diamagnetic material rearrange their orbits slightly creating small persistent currents, which oppose the external magnetic field.
Although the forces created by diamagnetism are extremely weak—millions of times smaller than the forces between magnets and ferromagnetic materials like iron, there are some interesting uses of those materials.
A thin layer of water laying on the top surface of a very power magnet will be slightly repelled by the magnetic field. This can be verified by viewing the reflection off the water surface and seeing a slight dimple on the surface.
Used in levitation
The most popular application of diamagnetic materials is magnetic levitation, where an object will be made to float in are above a strong magnet. Although most experiments use inert objects, researchers as the University of Nijmegen in the Netherlands demonstrated levitating a small frog in a powerful magnetic field.
Magnets will strongly attract ferromagnetic materials, weakly attract paramagnetic materials, and weakly repel diamagnetic materials. Ferromagnetic materials have the most magnetic uses. The main practical use for diamagnetic materials is in magnetic levitation.
Always do your best
Resources and references
Diamagnetic, Paramagnetic, and Ferromagnetic Materials - From Non-Destructive Testing (NDT) Resource Center
Diamagnetic levitation - - Study from UCLA
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Classifications of Magnetic Materials