What are magnetic domains and where is a good site to read up on them?
A piece of ferromagnetic substance, for example, iron, nickel, cobalt, has different macroscopic regions, each region has magnetic spins or molecular magnets oriented in the same direction and is called a Magnetic Domain. The directions of magnetic spins of different domains are arbitrary, so that the given piece is not magnetised overall and does nor produce any magnetic effects in its neighbourhood. The process of magnetisation of this piece (transforming the piece into a magnet) consists in aligning the directions of the spins of various domains gradually. The saturation of the process of magnetisation occurs, when the spins of all the domains are aligned and now this magnet has the maximum moment
Magnetic domain come up mostly in ferromagnetic materials with permanent magnetic dipoles. Usually, in all solids, permanent dipoles tend to go in all directions, creating a large number of domains all canceling each other. In ferromagnetic solids, for more advanced reasons, the dipoles tend to line up with each other. Sadly for them, that takes up a lot of energy. So, they will divide in domains to minimize the magnetic energy. In ferromagnetics, if they kept splitting in smaller and smaller domains like in other materials, it would also take more energy. I'm sorry i can't make it clearer, it's all very mathematical stuff. Here's a decent site about it. hyperphysics.phy-astr.gsu.edu/hba.
