Why are some magnets stronger than others? What are the practical limits to making a super strong magnet?
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Domains are made up of something even smaller, called magnetic dipole moments, which are the spin of individual electrons within the material. Recall that Ampere’s law suggests that a moving (or spinning) charge produces a magnetic field. These moments are grouped into local domains within the crystal structure of the material. Normally, all of these domains are oriented in random directions, so the bulk net field produced by them is 0. Locally they do create a field, but it’s cancelled out by the sum of their neighbors.
When an external magnetic field is applied to any ferromagnetic material (iron, cobalt, nickel, etc) then the individual moments and domains begin to align themselves with the external field. If the field is weak, only a few line up. If the field is strong, many line up. The amount of moments or domains that line up is related to the permeability or susceptibility of a material. This means the material is now creating its own net magnetic field. When all domains have been aligned you get magnetic saturation, and you can no longer increase the material’s magnetic field. When you take the magnetic field away, the moments begin to un-align. When there is no external field, the domains stay a little bit aligned, this is called hysteresis. The reverse field necessary to completely un-align all the moments, thus creating 0 net field, is called the coercivity. This is why an electromagnet works with iron, even though iron isn’t magnetic by itself.
There are also permanent magnets where the moments and domains are not movable, they are fixed and aligned in place. Sometimes a very very strong magnet can still move them, but then they’re fixed in place after you remove the magnet. What makes a magnet stronger than others is the number of, and nature of the domains.
The strongest permanent magnets are called neodymium. They have a specific crystal structure that naturally holds domains aligned in only a single direction, they have very high coercivity. Also, neodymium magnetic dipoles are made from 4 spinning electrons per atom, while iron only has 3.
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