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Factors Determining Magnetic Properties

by Ron Kurtus (revised 15 February 2016)

There are three main factors that determine the magnetic properties of a material.

The most basic factor is the configuration of the electrons in the material's atoms. At the next level, the ability of the atoms or molecules in the material to align magnetically is important in determining whether the material responds to a magnetic field. A final factor is the alignment of domains or sections in a solid object.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion



Electron orientation

Electrons can behave as tiny magnets, each with north (N) and south (S) poles. When an atom's electrons are lined up in the same orientation, with most having their N pole facing one direction, the atom becomes like a magnet, with N and S poles. It is also possible for the electrons to be in various directions, making the atom not magnetic.

Moving electrons create magnetic field

The reason that electrons can behave like tiny magnets is the fact that when electrons move, they create a magnetic field. Placing a compass near a wire carrying DC electrical current can show that a magnetic field is created due to the electrons moving through the wire.

A magnetic field is also created when electrons rotate around a nucleus and when they spin while in orbit.

(Note that modern theories of the atom no longer accept the Bohr or solar system model. In the new theories, electrons are thought of as clouds or strings. You should be aware that there are new explanations, but for the sake of understanding we will still follow the Bohr model of spinning electrons rotating around a nucleus, similar to planets rotating around the Sun.)

Spinning electrons

Electrons have a property called spin. This spinning creates a magnetic field with N and S poles, just as the spinning Earth has magnetic poles. Note that the N pole on an electron is really a North-seeking pole, just as in a magnet.

If electrons in the shells of an atom spin in the same direction, the atom will exhibit a magnetic field and will respond to the forces of a magnet. If half of the electrons spin one way and the rest spin the other way, they will neutralize each other and the material will not be affected by a magnetic field

This atom is barely magnetic because all its electrons are not aligned

This atom is barely magnetic because all its electrons are not aligned

Strong and weak electron alignments

Atoms such as iron have most of their electrons aligned in the same direction. Thus, iron or nickel would be attracted to a magnet. Aluminum only has a few electrons aligned, and thus it is only weakly magnetic. An element with half of its electrons oriented one way would not be attracted to a magnet.

Atomic and molecular alignment

Although some atoms may be highly magnetic, they really need to be aligned to make a material magnetic. If magnetic atoms are facing different directions, their fields will cancel out each other.

Solids and fluids

Since the atoms or molecules in a solid are fixed in place, most magnetic materials are solids. This is because once the atoms or molecules become aligned, they tend to stay in place. An example is seen when you magnetize a piece of iron.

As a material becomes heated or when it is in its liquid or gaseous state, the atoms or molecules are in rapid motion and are not aligned. Thus, fluids are seldom magnetic.

An exception is when a magnetic material such as iron is in its liquid state and is continually rotating around an axis. In such a situation, the atoms can be aligned in one direction, even though they are in rapid motion.

For example, the core of the Earth is made of liquid iron. Since the Earth rotates on its axis, the liquid iron is rotating, thus creating the Earth's magnetic field. Also, the Sun rotates on its axis, and the material in its plasma state creates the Sun's magnetic field.

Molecules

If two or more elements are chemically combined to form a molecule, it is quite possible that the compound is not very magnetic because the orientations of the atoms in the molecule work against each other.

A good example of this is to compare the magnetic properties of iron as compared to its compounds if iron oxide (rust) and iron sulfide. A piece of iron is highly magnetic, but a hunk of rusty iron is not.

Alloys

Metals of different elements can be mixed when they are in the molten or liquid state to form alloys. These combinations result in materials with slightly different physical and chemical properties than the elements by themselves.

If the metals typically respond well to a magnetic field--such as iron and nickel--then their alloy has even a stronger reaction to magnetism. On the other hand, there are some alloys of iron--such as forms of stainless steel--that do not respond well at all to a magnet.

Domains

The final factor in a material being magnetic concern the orientation of its domains in a solid. A group of atoms in a metal may become aligned, but the various groups may be misaligned. These groups are called domains.

It is necessary to line up many of the domains in a material like iron in order for it to become a magnet.

Magnetic material with domains misaligned

Magnetic material with domains misaligned

Aligned domains makes material highly magnetic

Aligned domains makes material highly magnetic

Summary

Alignment of electrons, atoms and domains are important in determining the magnetic response of a material and whether it is a magnet. Since the atoms or molecules need to be aligned, gases and liquids are typically not magnetic, and most magnets are solid metals. An exception is in the rotating liquid iron core of the Earth and the rotating plasmas of the Sun.


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Resources and references

Ron Kurtus' Credentials

Websites

Magnet Madness - Basics of magnetism

Types of Magnetism

Magnetic domains - From Non-Destructive Testing (NDT) Resource Center

Magnetism Resources

Books

Top-rated books on Magnetism

Top-rated books on Physics


Questions and comments

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.


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