What do farmed cows and your iPod earphones have in common? Magnets. Magnets play an integral part in day to day life from being involved in electrical motors, trains, recycling, televisions, computers, cars, earphones and, yes, cows.
Ferromagnetism (named this as iron is the most common example of a ferromagnetic metal) is the strongest type of magnetism and is the only one that produces forces strong enough to be felt with out sensitive equipment. The mechanism behind ferromagnetism begins with the current atomic model. This model states that atoms are made up of three basic components: protons, these have a positive charge; neutrons, these have no charge; electrons, these have a negative charge. The diagram below illustrates how electrons orbit the nucleus of the atom (made up of protons and neutrons).
Electrons are arranged in shells, where each shell has a maximum number of electrons it can accommodate. For example, the shell closest to the atoms nucleus can only hold up to two electrons, the second shell out can hold up to eight electrons and the third shell can hold up to eighteen electrons. The diagram below is a simplified representation of an aluminium atom showing the electron shells.
Within each shell the electrons are arranged in orbitals. There are four different types of orbital, these are the s, p, d and f orbitals. An s orbital can contain a maximum of two electrons, a p orbital can contain a maximum of 6 electrons, a d orbital can contain a maximum of 10 electrons and an f orbital can contain a maximum of 14 electrons. In an electron shell, the s orbital is always filled first, followed by the p orbital, then the d orbital and finally the f orbital (there are a few exceptions to this rule). Inside the orbitals there are sub-orbitals. Each sub-orbital is made up of a maximum of two electrons where one electron is spinning up and one is spinning down.
It is thought that for a metal to be ferromagnetic it must contain an incomplete sub-orbital in the d orbital of the 3rd electron shell. As well as containing a negative charge, an electron also exhibits a magnetic dipole moment (it acts like a miniscule magnet). A complete sub-orbital contains no dipole* as the magnetic moments of the two electrons cancel each other out, however, in the incomplete sub-orbital the magnetic dipole of the electron is not cancelled out therefore the atom has a total dipole. Microscopic magnetic moments in neighbouring atoms are held equivalent by quantum mechanical forces, forming macroscopic polarised regions known as domains. Each domain is made up of approximately one quadrillion (1015) atoms and is essentially a miniscule permanent magnet, containing a north and south pole. If the many domains of the metal are not aligned, they may cancel out each other’s charges meaning the metal will not be magnetized. However, if the metal is subjected to an external magnetic field its domains can be aligned causing the metal to be permanently magnetized.
There are four metals that are said to be ferromagnetic, they are iron, cobalt, nickel and gadolinium (below 20oC). The strongest commercial magnets around today are alloys of ferromagnetic metals and rare earth metals. These are known as the rare earth magnets.
The strongest, at room temperature, of the rare earth magnets and thus the strongest commercial magnet in the world, is an alloy of neodymium, boron and iron (NdFeB). At room temperature a NdFeB magnet has a maximum strength of 52 megagauss oersteds, or, to put it in context, enough strength to sever a finger if it were to find its way between two of these magnets. NdFeB magnets are however, only effective up until 150oC, above which temperature the alloys domains begin to disarrange due to the metal’s atoms gaining energy, causing the domains to orientate hence meaning the alloy will lose magnetism. Other ferromagnetic metal alloys include an alloy of samarium and cobalt; an alloy of aluminium, nickel and cobalt, known as AlNiCo, and an alloy of an iron oxide and usually either barium or strontium, known as ceramic. None of these alloys have a magnetism as powerful at room temperature as that of the NdFeB alloy, however they do have other properties that make them more appropriate for use in some scenarios. For example, AlNiCo magnets can operate at a higher temperature than NdFeB magnets (up to 540oC), thus making them more apt for use in high temperature environments. Ceramic magnets are better suited to corrosive environments than the NdFeB magnets due to their higher corrosive resistance. The bar chart below shows a basic comparison of cost, maximum operating temperature, corrosive resistance and strength between the rare earth magnets/ ferromagnetic metal alloys.
Their immense strength means that the rare earth magnets have many uses both commercially and industrially. Due to their large ‘magnetism to size’ ratio NdFeB magnets are used as the permanent magnet in earphones. Moreover, permanent magnets are vital to the functionality of earphones as earphones contain small speakers and without the magnet the speakers would not be able to produce vibrations and thus sound. Another way rare earth magnets are encountered day to day is in cars. The majority of cars today are equipped with an ABS system (anti-lock braking system). This system increases the safety of a car by, during breaking, preventing the car’s wheels from locking up, reducing the risk of skidding and allowing the driver to keep control of steering. A NdFeB magnet is used in the systems wheel rotary speed sensor, if the sensor detects that the wheel is locking (the speed is decelerating too rapidly), a hydraulic system is activated to reduce the pressure of the breaks on the wheel.
So far the applications of the ferromagnetic metal alloys discussed have all been mechanisms in modern technological systems. So where do the cows come in? Cows can contract Hardware Disease by ingesting small metallic objects such as nails or wire segments (often originating from the fencing surrounding the field). These objects are usually too heavy to pass through the cow’s digestive system and end up lodged in the reticulum. The item must be removed as it may irritate and eventually tear the reticular lining, this is where the magnets come in. A magnet, generally AlNiCo, is attached to a tube and inserted orally into the cow to remove the metal fragment. Magnets, again usually AlNiCo, can also be used in a preventative procedure to this disease as well as in the treatment. To reduce the likelihood of a cow contracting Hardware Disease an alloy pellet of aluminium, nickel and cobalt is fed to the cow. The pellet lodges itself at the bottom of the cow’s reticulum, because of its smooth, rounded shape, it causes no damage to the reticular lining. Post consumption of the metallic pellet, if the cow ingests any further metal fragments, instead of lodging themselves in folds of the reticulum and irritating the lining, they will be attracted to the magnet, thus preventing Hardware disease. AlNiCo is the preferred magnet in these procedures as it is not toxic or corrosive.
Rare earth magnets are some of the strongest magnets in our world today and can be encountered in a broad array of commercial products and industrial practices from agriculture, to motoring, to electronics. The vast number of applications are ever growing confirming that as our understanding of the quantum mechanics behind magnetism continues to expand, so does the number of applications. The future of rare earth magnets appears to be a prosperous one and it is exciting to wonder what innovative technology these alloys will make possible in the future.
*A dipole is a pair of equal and oppositely charged/ magnetized poles separated by a distance.
By Ollie Gleeson
Imperial College London