Why Is Iron The Best Core For An Electromagnet?

Iron is widely regarded as the best core for an electromagnet, but why? It isn't the only magnetic material, and there are plenty of alloys such as steel that you might expect to be used more in the modern age. Understanding why you're more likely to see an iron core electromagnet than one using another material gives you a brief introduction to many key points about the science of electromagnetism, as well as a structured approach to explaining which materials are mostly used for making electromagnets. The answer, in short, comes down to the material's "permeability" to magnetic fields.

The origin of magnetism in materials is a little more complex than you might think. While most people know that things like bar magnets have "north" and "south" poles, and that opposite poles attract and matching poles repel, the origin of the force isn't as widely understood. Magnetism ultimately stems from the motion of charged particles.

Electrons "orbit" the nucleus of the host atom a little like how planets orbit the Sun, and electrons carry a negative electric charge. The motion of the charged particle – you can think of it as a circular loop although it's not really quite that simple – leads to the creation of a magnetic field. This field is only generated by an electron – a tiny particle with a mass of about a billionth of a billionth of a billionth of a gram – so it shouldn't surprise you that the field from a single electron isn't that big. However, it does influence electrons in neighboring atoms and leads to their fields aligning with the original one. Then the field from these influence other electrons, they in turn influences others and so on. The end result is the creation of a little "domain" of electrons where all of the magnetic fields produced by them are aligned.

Any macroscopic bit of material – in other words, a sample big enough for you to see and interact with – has plenty of room for a lot of domains. The direction of the field in each one is effectively random, so the various domains tend to cancel each other out. The macroscopic sample of material, therefore, won't have a net magnetic field. However, if you expose the material to another magnetic field, this causes all of the domains to align with it, and so they will all also be aligned with each other. When this has happened, the macroscopic sample of the material will have a magnetic field, because all of the little fields are "working together," so to speak.

The extent to which a material maintains this alignment of domains after the external field is removed determines which materials you can call "magnetic." Ferromagnetic materials are ones that maintain this alignment after the external field has been removed. As you may have worked out if you know your periodic table, this name is taken from iron (Fe), and iron is the best-known ferromagnetic material.

The description above emphasizes that moving electric charges produce magnetic fields. This link between the two forces is crucial for understanding electromagnets. In the same way as the movement of an electron around the nucleus of an atom produces a magnetic field, the movement of electrons as part of an electric current also produces a magnetic field. This was discovered by Hans Christian Oersted in 1820, when he noticed that the needle of a compass was deflected by the current flowing through a nearby wire. For a straight length of wire, the magnetic field lines form concentric circles surrounding the wire.

Electromagnets exploit this phenomenon by using a coil of wire. As the current flows through the coil, the magnetic field generated by each loop adds to the field generated by the other loops, producing a definitive "north" and "south" (or positive and negative) end. This is the basic principle that underpins electromagnets.

This alone would be enough to produce magnetism, but electromagnets are improved with the addition of a "core." This is a material that the wire is wrapped around, and if it's a magnetic material, its properties will contribute to the field produced by the coil of wire. The field produced by the coil aligns the magnetic domains in the material, so both the coil and the physical magnetic core work together to produce a stronger field than either could alone.

The question of which metal is suitable for electromagnet cores is answered by the "relative permeability" of the material. In the context of electromagnetism, the permeability of the material describes the ability of the material to form magnetic fields. If a material has a higher permeability, then it will magnetize more strongly in response to an external magnetic field.

The "relative" in the term sets a standard for comparison of the permeability of different materials. The permeability of free space is given the symbol μ₀ and is used in many equations dealing with magnetism. It is a constant with the value μ₀ = 4π × 10⁻⁷ henries per meter. The relative permeability (μ_r) of a material is defined by:

_μ__r = μ / μ₀

Where μ is the permeability of the substance in question. The relative permeability has no units; it's just a pure number. So if something doesn't respond at all to a magnetic field, it has a relative permeability of one, which means it responds in the same way as a complete vacuum, in other words, "free space." The higher the relative permeability, the greater the magnetic response of the material.

The best core for an electromagnet is therefore the material with the highest relative permeability. Any material with a relative permeability higher than one will increase the strength of an electromagnet when used as a core. Nickel is an example of a ferromagnetic material, and it has a relative permeability of between 100 and 600. If you used a nickel core for an electromagnet, then the strength of the field produced would be drastically improved.

However, iron has a relative permeability of 5,000 when it is 99.8 percent pure, and the relative permeability of soft iron with 99.95 percent purity is a massive 200,000. This huge relative permeability is why iron is the best core for an electromagnet. There are many considerations when choosing a material for an electromagnet core, including the likelihood of wastage resulting from eddy currents, but generally speaking, iron is cheap and effective, so it is either somehow incorporated into the core material or the core is made from pure iron.

Many materials can work as electromagnet cores, but some common ones are iron, amorphous steel, ferrous ceramics (ceramic compounds that are made with iron oxide), silicon steel and iron-based amorphous tape. In principle, any material with a high relative permeability can be used as an electromagnet core. There are some materials that have been made specifically to serve as cores for electromagnets, including permalloy, which has a relative permeability of 8,000. Another example is the iron-based Nanoperm, which has a relative permeability of 80,000.

These numbers are impressive (and both exceed the permeability of slightly impure iron), but the key to the dominance of iron cores is really a mixture of their permeability and their affordability.