Magnets on the Atomic Level
According to Faraday’s law, magnetic fields are created by electrical currents. But if this is the only way that magnets can exist, then how do permanent magnets, like the one keeping your shopping list stuck to the fridge, have magnetic fields?
A fridge magnet might seem simple, but there are actually some complicated quantum mechanics at work. Only a handful of metals, like iron, nickel, and rare earth elements can be used to make a permanent magnet. To find out what makes them special, we must explore the material on the atomic scale. We must first understand is that current is nothing more than charges in motion. The negatively charged electrons orbiting the atom’s nucleus create a current as they swirl around. That means every electron is a tiny magnet all on its own. In addition to this, all electrons intrinsically carry their own magnetism that arises from a quantum mechanical property called “spin”. Everything all around you is full of little magnets.
Everything is made of atoms with moving electrons, but only a few special materials are magnetic. That’s because the electrons in many materials occur in even numbers. They pair off and cancel out each other’s magnetic fields. Magnetic materials are special on the atomic level because they have unpaired electrons. But that’s not the end of the story. Even if a material is full of magnetic moments, they are not necessarily well organized because thermal effects cause the individual magnetic moments to point in random directions. In this situation, the material is not magnetically ordered, and thus has little magnetic field strength. The key is that if the material is cooled sufficiently, the disordering effect weakens and the magnetic moments can enter an ordered state at what is called the transition temperature. For some materials, this occurs above room temperature, but for many others it is well below room temperature. In many cases, this distinguishes materials that are useful for applications and those that aren’t.
In iron and other magnetic materials, after ordering those tiny magnetic fields can build upon each other to make a stronger field. An important example is seen for ferromagnets, where the individual magnetic moments become co-aligned below the transition temperature. But even this is not usually simple, because the atoms form little regions within the piece of iron called domains. The atoms inside of a domain have magnetic fields that point in the same direction, but the domains organize randomly with respect to each other, meaning their magnetic fields can point in a lot of different directions. The result is a piece of metal that has little to no magnetic field strength because the different directions are canceling each other out. You would have a hard time sticking something to your fridge with just any old piece of iron. But if you put that piece of iron in a strong external magnetic field from a different magnet, the fields within the domains will start pointing in the same direction. Once those fields are aligned you finally have a permanent magnet.
