Magnetism: A Very Short Introduction
By Stephen Blundell
Do you own any magnets? Most people, when asked this question, say no. Then they remember the plastic letters sticking to their refrigerator door, or the holiday souvenir that keeps takeaway menus pinned to a steel surface in their kitchen. Maybe I do own a few, they say.
A bit more thought reveals that many more magnets are lurking elsewhere in their home. They are installed in anything containing an electric motor, from the washing machine to the hair dryer, from the kitchen blender to any appliance containing a cooling fan. Most loudspeakers and microphones contain magnets. And if you own a modern car, it will almost certainly be packed full of magnets, inside sensors measuring liquid levels, monitoring wheel speed and seat-belt status, running motors for the wipers, windows, sun-roof and starter, the pump for windscreen washer, not to mention applications in the dashboard instrumentation and loudspeaker system.
Though some magnetic applications have had their day, such as old-fashioned cathode ray tube televisions (which contained electromagnets for guiding the electron beam to the screen) and audio and video cassette tape, new ones appear. The computer hard disk is a vast array of billions of tiny magnets, each one storing the precious ones and zeros that together assemble into your digital data. If your home contains at least one computer, then you are a magnetobillionaire.
Magnetism is also one of the oldest technologies to have transferred from pure science toy to a world-changing application. That application was, of course, the magnetic compass and its introduction into Europe from China at the end of the twelfth century had a dramatic effect on the development of European exploration and colonization.
Today, as we look for alternative sources of power, magnets come to our aid. Though magnets are located inside the generators and turbines inside oil, gas and nuclear power stations, they are also in those powered by wind and tide. Even at the heart of the experimental fusion reactor being constructed in Cadarache in France (ITER, the International Thermonuclear Experimental Reactor) there are enormous superconducting magnets being constructed to confine the fiery plasma in its doughnut-shaped orbit.
Since humanity finds magnets so useful we spend a lot of effort trying to improve them. The first magnets were lodestones, the mineral magnetite (an oxide of iron) that was simply dug out of the ground. Today we have been able to fabricate incredibly powerful magnets and one of the best ones is a compound based on the element neodymium. This is one of the so-called rare-earth elements, a group of rather obscure metals that inhabit a region towards the bottom of the periodic table.
In the early nineteenth century, the element cerium was isolated from its oxide, and named after the dwarf planet Ceres. Later, cerium oxide was found also to contain another element, lanthanum, whose name appropriately enough derives from the Greek ‘to lie hidden’’. By the 1840s it was realized that, mixed in with lanthanum, was another very similar element, didymium (the name connotes “twin’’). In 1885, the Austrian chemist Carl Auer von Welsbach found that didymium itself was composed of two elements, praseodymium (green twin) and neodymium (new twin).
It took a further century for people to work out how to incorporate neodymium into a particular compound (also containing iron and boron) and make incredibly strong permanent magnets. Because neodymium magnets are extraordinarily powerful, they can be used sparingly in applications. In a wind-turbine application, the use of neodymium magnets makes the turbine considerably lighter and more compact than would be the case using ferrite magnets. This greatly improves their efficiency, which is what wind turbines are all about. You find neodymium magnets in iPhones (both for driving the vibrate feature and producing the sound in the ultra-light headphones) and in the motors of modern cars such as the Prius (there is about a kilogramme of the element in each car).
The bad news is that neodymium is not very common and mining it is not very environmentally friendly. As the world’s appetite for high-end consumer electronic products increases, so does its demand for neodymium. China is the dominant provider and it has been capping exports, choking off the supply and raising prices. For a largely unknown and rather obscure element, neodymium is rapidly becoming a highly valuable commodity.
Human beings have studied magnetism for three thousand years. We’ve been using magnets for many centuries. We certainly need magnets for an amazing number of applications and to produce the power that sustains our civilization. But we can’t let up on the quest to continually discover new magnetic materials with better properties and more environmentally sustainable constituents.
Stephen Blundell is Professor of Physics at the University of Oxford and a Fellow of Mansfield College. His research is focused on magnetism and superconductivity and he is the author of Superconductivity: A Very Short Introduction (OUP, 2009) and most recently Magnetism: A Very Short Introduction (OUP, 2012).