Motors, generators, transformers, audio systems, industrial machines, furniture, toys, levitating trains, recycling devices, etc., all have one thing in common. They all use magnetism in some form or another.
We are all familiar with the basics of magnets and their behavior. Magnets have two poles: the north pole and the south pole. While opposites attract, the two of the same poles repel each other. Certain materials like those that contain Iron can be magnets and other materials like Cobalt and Nickel are attracted to magnets even though they aren’t one themselves.
In this article, we explain the basics of magnetism in depth. We start with a brief history of magnetism and go on to cover Faraday’s and Lenz’s laws to understand what causes magnetism. We explain the classification, types, properties, and some examples of magnetic substances. In the later sections, we explain the working of electromagnets and the earth’s magnetosphere.
The modern name magnet is derived from Magnesia, a region in Greece where Thales of Miletus first found lodestones back in 600 BCE. Lodestones are naturally magnetized pieces of magnetite that can attract iron.
The next mentions of lodestones and their magnetic forces come from Chinese classic texts around 400 BCE. Chinese were the first to use magnetic lodestone compasses for navigation by the 12th century.
In the 1600s, William Gilbert, an English physicist, investigated the behavior of magnets scientifically. Carl Friedrich Gauss further explored the work. The accidental discovery of electromagnetism by Hans Christian Oersted in 1819 brought a new era of advancements in the field.
Andre Marie Ampere, Michael Faraday, and James Clerk Maxwell laid the foundations of electromagnetism in the mid-1800s. Werner Heisenberg refined the understanding of magnetism with quantum electrodynamics.
Magnetism is a property of moving electric charges or specific materials that lets them exert an attractive or repulsive force on objects. Much like how subatomic particles like electrons possess intrinsic properties of mass and charge, they also have a property called intrinsic magnetic moment.
Before discussing what causes magnetism, it is essential to understand how electromagnetic induction occurs. Here is a theoretical explanation of two important laws of electromagnetic induction:
Faraday’s Law: Faraday’s law states that whenever a conductor is placed in a varying magnetic field, an electromotive force (emf) is induced.
Lenz’s Law: Lenz’s law states that the direction of emf induced due to a magnetic field always opposes the cause due to which it is produced. If a change in the magnetic field is the reason for the induction of emf in a conductor, its direction will be such that the magnetic field produced due to the induced emf tries to oppose the one due to which it was created.
The intensity of the magnetic field is measured in Teslas (T). It is the value of magnetic field intensity that can put one-newton force per ampere of current per meter of conductor. Tesla is a large unit; hence Gauss (G) is commonly used to measure magnetic field strengths. For reference, the strength of the earth’s magnetic field is 0.0000305 T or 0.3 G.
Since many electrons revolve around an atom while spinning on their own axis, these particles behave as current loops to generate a magnetic field. Hence, particles with electric charge also happen to be magnetic.
These fields are called orbital magnetic fields but don’t necessarily contribute to the overall magnetic effects of an atom. This is due to the fact that in a filled shell, electrons exist in pairs that spin in the opposite direction to cancel out their effects to produce a net zero magnetic field.
Atoms of elements lying at the sides of s/p/d blocks in the periodic table have a full or nearly full outer shell; hence, they are not magnetic. Atoms of elements in the middle of blocks, such as Manganese, Iron, Cobalt, and Nickel, are examples of magnetic material. These are also the most common magnetic metals.
When a group of atoms forms a crystal, they have an option to align the magnetic fields with each other or in opposite directions. Just like all other entities in the universe, atoms, too, seek stability and will align in the configuration that requires lesser energy.
Consider Chromium, for example; though it has highly magnetic atoms, the alignment of the internal magnetic fields makes it a non-magnetic material. Hence, it is not included in the examples of magnetic substances.
We’ve already seen that atoms tend to have magnetic moments produced by electrons forming current loops. The magnetic moments due to paired electrons get canceled out because of their opposite spin. Unpaired electrons, on the other hand, generate small magnetic moments.
So what might happen when a magnet is brought close to an atom with only paired electrons?
The seemingly obvious answer is that it would not experience any force. It would neither be repelled nor attracted to the magnet. But that’s not right. There’s another phenomenon that comes into effect in this situation: electromagnetic induction.
When a magnet is brought close to an atom with only paired electrons, the strength of one current loop (due to one electron from the pair) gets intensified, while the strength of the other current loop formed due to the electron spinning in the opposite direction is diminished.
The currents can no longer cancel each other, and the magnet exerts an attractive force on one electron and a repulsive force on the other. The strength of the repulsive force is higher than the attractive force; hence, the atom tends to repel the magnet weakly. As a result of this interaction, a weak magnetic moment is formed that repels the magnet, giving rise to diamagnetism.
Susceptibility: The magnetic field inside the material results from the external magnetic field that is applied to the material and the one that is set up due to the specific nature of the material (influenced by external factors). This influence is characterized by magnetic susceptibility, which is a dimensionless quantity correlating the external magnetic field and the field set up due to the nature of the material. It also helps to understand how materials respond to external magnetic fields. It is small and negative for materials, which are termed diamagnetic.
Diamagnetic materials: The materials that experience a weak repulsion when placed in a magnetic field are called diamagnetic materials. When an external magnetic field is applied to such materials, they develop a weak magnetic field in the opposite direction of the external field and hence get repelled.
As soon as the external magnetic field is removed, the paired electrons cancel out the magnetic moments, and the material loses magnetization as a whole. Hence, diamagnetism is a temporary phenomenon.
Examples of diamagnetic materials include Bismuth, Water, Copper, Diamond, Gold, Lead, Mercury, Hydrogen, Nitrogen, Silver, Silicon, Phosphorus, Antimony, and more.
Diamagnetic materials are weakly repelled by the magnetic field.
When placed in a magnetic field, diamagnetic substances try to align themselves in a perpendicular direction from the field.
Diamagnetism does not depend on temperature.
All materials with paired electrons exhibit some degree of diamagnetism.
Continuing the previous discussion, let’s consider a material having atoms with unpaired electrons. Each atom would have its own permanent magnetic moment due to the current loop formed by the unpaired electron. When no external magnetic field is present, the magnetic moments of the atoms will be arranged randomly such that they cancel each other out and have a net zero magnetization.
When this material is placed in a magnetic field (of, let’s say, a bar magnet), the tiny dipoles of individual atoms change their orientation to get somewhat aligned in the direction of the applied magnetic field. This causes them to be weakly attracted to magnets. The thermal energy of the element doesn’t let all the magnetic moments align perfectly in the direction of the applied magnetic field, and hence the magnetization is weak. The strength of magnetization reduces with an increase in the temperature (thermal agitation).
The value of susceptibility is small and positive for paramagnetic materials.
Paramagnetic materials: The materials that experience a weak attraction when placed in a magnetic field are called paramagnetic materials. When an external magnetic field is applied to such materials, they develop a weak magnetic field in the same direction as that of the external field and hence get attracted.
When the external magnetic field is removed, the magnetic moments return to their random alignment, and the material loses magnetization as a whole. Hence, paramagnetism is a temporary phenomenon.
An interesting thing to note here is that not all materials with unpaired electrons exhibit paramagnetism. On the contrary, diamagnetism is a universal phenomenon. Most elements in nature have paired electrons and hence are somewhat diamagnetic. Only materials which have an unpaired electron and a dominant paramagnetism are classified as paramagnets. For other elements with unpaired electrons like copper, silver, and gold, diamagnetism dominates; hence, they are repelled by the magnetic field.
Examples of paramagnetic materials include Oxygen, Calcium, Aluminum, Chromium, Lithium, Magnesium, Platinum, Tungsten, Niobium, Samarium (at room temperature), and more.
Paramagnetic materials are weakly attracted by the magnetic field.
When placed in a magnetic field, paramagnetic substances try to align themselves in the direction of the field.
Paramagnetism reduces with increases in temperature.
Only those elements with unpaired electrons and weak diamagnetism exhibit paramagnetism.
We’ve seen how most elements, even the ones with unpaired electrons, don’t get attracted or repelled by magnets. However, there are four elements in the periodic table that showcase strong attraction towards magnets. These elements do have atoms with unpaired electrons, but there’s a difference in their crystal structure that makes them different from paramagnetic materials.
Ferromagnetic materials contain groups of atoms with magnetic moments in the same direction. These groups are called magnetic domains. When ferromagnetic materials are placed near a magnet, their magnetic domains undergo reorientation. They align themselves in the direction of the magnetic field to form a giant domain where all magnetic moments are aligned in the same direction. As a result of this interaction, the material shows extremely strong magnetization.
The strength of magnetization increases until the ferromagnetic material reaches a saturation point, beyond which increasing external magnetic field no longer increases the magnetization of the material.
Ferromagnetic materials: The materials that experience a strong attraction when placed in a magnetic field are called ferromagnetic materials. When an external magnetic field is applied to such materials, they develop a strong magnetic field in the same direction as that of the external field and hence get attracted.
What happens to ferromagnetic materials after removing the external magnetizing field depends on their retentivity.
Retentivity: It is defined as the ability of a material to retain magnetization after the removal of an external magnetizing field.
Soft ferromagnetic materials lose their magnetic properties, whereas hard ferromagnetic materials keep them. Hard ferromagnetic materials are used to make permanent and strong magnets. Soft ferromagnetic materials are used in applications where we would not want the magnetization to stay, for example, to make cores for electrical machines (motors, transformers, etc.).
Coercivity: It is the ability of a material to resist a change in magnetization
Examples of ferromagnetic materials include Iron, Cobalt, Nickel, and Gadolinium.
Ferromagnetic materials are strongly attracted by the magnetic field.
When placed in a magnetic field, ferromagnetic substances strongly try to align themselves in the direction of the field.
Ferromagnetism reduces with increases in temperature. Ferromagnetic materials lose their magnetic moments above Curie temperature and behave as paramagnetic materials.
Only those elements with unpaired electrons, weak diamagnetism, and magnetic domains exhibit paramagnetism.
When the adjacent magnetic moments of material are arranged in an antiparallel fashion, it becomes antiferromagnetic. The magnitude of all the moments effectively cancels each other out, and the net magnetism is zero.
Ferrites/Ferrimagnetic materials have an anti-parallel alignment of magnetic moments just like antiferromagnetic materials. However, the magnitude of these moments is different, making the material magnetized.
Examples of antiferromagnetic materials include Manganese Oxide, Manganese Sulfide, Chromium Oxide, Nichrome, and more.
Examples of ferrimagnetic substances include metal oxides like magnetite.
Based on the duration of magnetic effects, magnets are classified into the following categories:
Materials that retain their magnetic properties for a long period of time are called permanent magnets. Ferromagnetic materials, including iron, nickel, cobalt, and some rare earth metals alloys like the ones that use Neodymium are used to make permanent magnets.
AlNiCo magnets are made up of aluminum, nickel, and cobalt and remain one of the most widely used permanent magnets. They are not as strong as magnets made with rare-earth elements but are more readily available.
Materials that simply act as magnets under the influence of a magnetic field, and lose their magnetic properties as soon as the external field is removed, are called temporary magnets. Any current carrying conductor can behave as a temporary magnet.
Electricity and magnetism are just like the two sides of the same coin, much like how Mass-Energy and Time-Space are.
When an electric current is made to flow through a conductor, a magnetic field is set up around it, the direction of which can be determined by the right-hand thumb rule. This effect is intensified by wrapping a wire over a soft ferromagnetic core and the resulting setup becomes an electromagnet.
Electromagnets are among the most popular temporary magnets used in a number of applications, including electric motors, radios, and several other household devices and industrial systems. The strength of the electromagnet can be accurately adjusted by changing the number of turns of the electromagnet coil, the dimensions of the electromagnet, or the core material.
Check out the Wevolver Student Researcher Program 2021 submission that explains the application of magnets in a hyperloop: Developing A Scalable Hyperloop
The source or causes of the earth’s magnetic field are not completely understood yet. However, it is believed that the earth’s magnetism is due to its metal core. The outer core contains liquid metal, and the inner core is under such a high pressure that the metal solidifies.
There is a continuous flow of electricity in the earth’s core which sets up a magnetic field around the planet. The earth, as a whole, behaves as a giant bar magnet. The magnetic field at the poles points vertically up/down at the north/south poles of the planet. The earth's magnetic poles lie slightly away from geographic poles and gradually keep changing their position over time.
Earth’s magnetosphere shields the planet from the solar wind. The solar wind is composed of streaming charged particles like protons and electrons from the sun. Some of these particles have high radiation energies that can cause damage to life on earth. But the presence of the earth’s magnetic field deflects most of the particles in the solar wind to protect the planet.
A compilation of beautiful aurora lights as visible from some Nordic countries.
When the solar wind comes in contact with the earth’s magnetosphere, it creates a magnetic storm that stretches the magnetosphere while bringing some charged particles towards the earth. When these particles come in contact with neutral oxygen and nitrogen atoms in the atmosphere, they emit light. This phenomenon gives rise to the aurora borealis or aurora australis (northern/southern lights).
Without the earth’s magnetosphere, the earth’s atmosphere and life as we know it wouldn’t exist.
Magnetic poles always exist in pairs. Examples of magnetic substances include iron, nickel, cobalt, stainless steel, and many rare earth metals.
Diamagnetic materials like copper and gold are weakly repelled by a magnetic field.
Paramagnetic materials like calcium and aluminum are weakly attracted by a magnetic field.
Ferromagnetic materials like iron, cobalt, and nickel are strongly attracted by a magnetic field.
Permanent magnets don’t lose magnetism with time, whereas temporary magnets do. Both have their own set of applications.
Electromagnets are temporary magnets that can be formed by wrapping a current carrying conductor over a ferromagnetic core.
Earth, too, behaves like a giant magnet. The earth’s magnetosphere protects the atmosphere from the solar wind.
 Magnetism: History of the Magnet, MPI magnet, [Online], Available from: https://mpimagnet.com/education-center/magnetism-history-of-the-magnet
 MAGNETS: How Do They Work?, minutephysics - YouTube, [Online], Available from: https://www.youtube.com/watch?v=hFAOXdXZ5TM&ab_channel=minutephysics