Magnetic Materials

Magnetic Materials Definition

When we think about magnets, we are typically inclined to picture the typical bar magnet, made up of a north and south pole. However, a lot of materials that surround us in our everyday lives have the potential to become a magnet, as it just depends on their internal structure. We can define a magnetic material as the following.

But what makes magnetic materials so special that they are the only ones that are able to experience the influences of an external magnetic field? To understand this, let's take a deeper look at the structure of a solid magnetic material.

Fig. 2 - Due to the orbiting electron around a nucleus, an atom exhibits magnetic dipole-like behavior.

As we have covered before, atoms are the building blocks of all the materials we see around us. The internal structure of these atoms consists of a nucleus containing both protons and neutrons, whilst a superposition of electrons orbit the nucleus. As discovered by James Maxwell in 1865, the movement of charged particles, such as electrons, generates a surrounding magnetic field. Therefore, the precession of electrons around the nucleus similarly creates a mini magnetic field local to the nucleus. This is what we call the atomic dipole.

From the figure above, we can get a better picture of the internal structure of the atomic dipole. Using the right-hand grip rule, where we curl our fingers in the direction of the current and our thumb points in the direction of the north pole, we can see that the resultant north pole of this atomic dipole is in the upwards direction, to the top of the page indicated by the blue arrow.

Zooming out from the structure of the atomic dipole, magnetic materials are also made up of something called magnetic domains. These are structures consisting of neighboring atomic dipoles that are all orientated in the same direction. These magnetic domains have their own internal magnetic field, forming a continuous loop.

Fig. 3 - If a magnetic material has unaligned magnetic domains, the material remains unmagnetized.

The figure above has outlined the magnetic material as a rectangular shape (blue) whilst the internal magnetic domains are indicated by the arrows (pink). As we can see, the domains are largely unaligned as the arrows are mainly pointing in different directions, indicating to us that the material would not be exhibiting a cohesive magnetic field.

Magnetic Materials Classification

Now that we have established the internal structures of magnetic materials, let's get closer to how we can split up everyday materials into different categories.

Ferromagnetism

Materials classified as ferromagnets are ones that exhibit the strongest magnetic properties. They are able to exhibit this property due to the susceptibility of the magnetic domains making up the material, meaning that the domains are able to be aligned under the influence of an external magnetic field. The effect of the external field causes the domains to all align in the same direction, thus the combination of the individual magnetic fields from the domains generates a larger overall field that emerges from the material.

Furthermore, ferromagnetic materials also have the unique property of spontaneous magnetization. This occurs when the domains spontaneously align with one another despite the absence of an external field. Typically, this occurs when the material is placed in a very cold environment, cooling down the atoms in the material and eliminating large vibrations in the atoms. Each ferromagnetic material has a specific temperature called the Curie temperature, which indicates the temperature below which the material will exhibit spontaneous magnetization.

This is why ferromagnetic materials are the ideal ones to use when we are in need of strong magnets such as bar magnets or electromagnets; they retain their magnetic properties despite the absence of an external field and are easily magnetized.

Paramagnetism

In contrast to ferromagnetic materials, paramagnets exhibit a much weaker magnetic field when exposed to an external field, and exhibit an attractive force to the external field. This is due to an uneven number of electrons orbiting the atomic dipole. Furthermore, unlike ferromagnets, the internal atomic dipoles become unaligned when an external field is removed due to the vibrations of the atoms caused by the thermal energy of the substance.

Diamagnetism

Diamagnetism is a property that occurs in all materials and can be thought of as the opposite of paramagnetism; the internal structure causes the material to exhibit a repulsive force to the external field rather than an attractive force, like ferromagnetism and paramagnetism. Despite this, the repulsive force generated by diamagnetic materials is typically very weak such that we are not able to observe them in everyday life.

Magnetic Materials Properties

So how can we classify something as a magnet? Let's go over some properties that all magnets exhibit.

Magnetic Permeability

The magnetic permeability of a substance is a measure of how magnetized the substance becomes in response to an external field. In the context of magnetic domains, it indicates how easily these domains become aligned within the material when exposed to a field. Therefore, a high magnetic permeability reveals that a material can be extremely easily magnetized, an example of which is a ferromagnetic material. On the other hand,

Fig. 4 - The alignment of the magnetic domains results in the material exhibiting an external magnetic field.

The figure above is an example of how the internal magnetic domains of material look when aligned, indicating to us that the material is magnetized. However, it does not tell us about the magnetic permeability of the material as that depends on the internal electronic structure.

Magnetic Susceptibility

Magnetic susceptibility is similar to permeability in the sense that it explains how a material reacts under an external field, however, in this case, it showcases whether the internal magnetic field of the material is aligned against the external field or aligned with the external field. When materials such as ferromagnets and paramagnets are magnetized, they experience an attractive force, indicating that their field is aligned to the external field. Whereas materials such as diamagnets will experience a repulsive force, indicating that their field is aligned against the external field.

Examples of Magnetic Materials

Let's go back to a simple example of a magnet we regularly use in physics practicals, the bar magnet. We know that this magnet stays magnetized even in the absence of a magnetic field due to the fact that we can stick it to other magnets, like a magnet being stuck to a fridge door, or a magnet used to pick up paperclips. This reveals to us that these are made up of ferromagnetic materials; examples of these materials include iron, cobalt, nickel, and many others.

Fig. 5 - A compass needle is made up of the ferromagnetic material steel, an alloy of iron.

Another key example of where ferromagnets are used is in the needle of a compass! If you have ever tried using an old-school compass for yourself, you'll know that no matter which direction you face, the needle of the compass will always point to the true north. This is due to the fact that the Earth has its own intrinsic magnetic field, therefore the magnetized needle is always attracted to the north pole, making it a perfect device for lost travelers at sea.

Magnetic Shielding Material

So far in this article, we've talked about various types of magnetic materials and how they react under the influence of an external field. However, since all materials experience some sort of effect in the presence of magnetic fields, are there any that can stop the field lines from penetrating through?

Magnetic shielding is extremely important in households that have many different wires running concurrently to each other as the magnetic field generated by the current in one wire, could affect the others and distort the signal. Magnetic field lines could never be insulated due to the fact that magnets always occur in complete dipoles, meaning that the field lines traveling from the north pole will always reach the south pole of the magnet to form a complete loop. Therefore, in order to prevent the field lines from escaping, we instead attempt to redirect them. By surrounding these wires in a material with high magnetic permeability, the external field lines will then be aligned with the field lines of the covering material, allowing them to form complete loops without escaping.