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Magnetic Domains

Teacher/Instructor Jonathan Osbourne
Jonathan Osbourne

PhD., University of Maryland
Published author

Jonathan is a published author and recently completed a book on physics and applied mathematics.

Magnetic domains are collections of magnetic fields in the same direction. They are often found in ferromagnetic materials because their atoms align with magnetic fields in a process called a ferromagnetic phase transition. At high temperatures the atoms in ferromagnetic materials have random orientations but at low temperatures their orientations naturally naturally. Hard ferromagnetic materials' orientations stay after the removal of a magnetic field. Soft ferromagnetic materials' orientations are naturally random and are affected by magnetic fields.

So let's talk about magnetic domains, alright now we need to start off with a very important word that tells us how important magnetic fields are to a certain substance and that word is ferromagnetic.

If you have a material that is ferromagnetic, then what that means is that its atoms have fairly strong magnetic fields associated with them. Alright, now what that means is that magnetic forces are going to tend to try to push those magnetic moments the little magnetic fields that are associated with each of the atoms or molecules of the substance its going to try to align them all together so what we usually have in a ferromagnetic material at ordinary temperature is something that looks like this. The atoms that are right next to each other will tend to align in a certain direction and we'll get these magnetic domains that are associated with magnetic fields that are essentially all in the same direction.

Now notice that we've got neighboring domains that don't have magnetic fields in the same direction and that's because the temperature is too high to allow everything to fully, fully align. If I want a permanent magnet though, I need everybody fully aligned so what I'll do is I'll apply an external magnetic field, like this, and what that will do is it'll just kind of switch the orientations so that they align more with this external magnetic field and that's the idea that's how we magnetize a permanent magnet, nice external magnetic field switch it so that the magnetic domains become larger and larger and larger and then we get our nice permanent magnet that's called the ferromagnetic phase transition.

Alright, now let's look a little bit more carefully into the way that this takes place, so at high temperatures we've got lots of randomness going around; the atoms have too much energy to really listen to what the magnetic fields want them to do so we're going to get a random orientation and lots of these very very tiny magnetic domains. Alright at very low temperature on the other hand, the atoms and molecules of the material don't have enough energy to fight with the magnetic field so they're just going to align and they will even do that on their own even if I don't impose an external magnetic field, they will just align with themselves as long as its cold enough. The temperature that distinguishes between these two things is called the curie point temperature and this phase transition that takes us from a permanent magnet to a non permanent magnet we just kind of random orientations of these magnetic moments is called the curie transition.

Alright, so let's just talk little quickly about the two major types of ferromagnetic materials. We've got a hard ferromagnetic material in which the alignment will stay. If a material is a hard Ferro magnet then if I impose this magnetic field or this external magnetic field, then all of that magnetic domains will go inside and then if I take that magnetic field up I'll still have a magnetic field and that's this permanent magnet alright? So these are like these ceramic magnets, refrigerator magnets all those things are hard ferromagnetic materials. Now what about our soft ma- ferromagnetic material, it's still ferromagnetic so that means that it's still going to interact largely with the magnetic field but it's not going to be as loyal to any specific direction of the magnetic field so this is like iron you know like paper clips, steel, refrigerator doors right? Why does a magnet stick to a refrigerator door? I mean if I take two magnets and put them next to each other sometimes they'll stick and sometimes they'll repel but if I take a permanent magnet put it on to a refrigerator door, then take it off and turn it around, put it back on the refrigerator door, there's no repulsion that's because the refrigerator door will just do whatever the external magnetic field tells it to do. It's a soft ferromagnetic material so that means that it's still this kind of random orientation the magnetic domains will align if I have an external magnetic field, but if I take that external magnetic field away, they don't care anymore maybe they'll stay aligned for a little while and you've probably seen this on a screwdriver right you've got those little screwdrivers where it will hold a screw but it's not real strong, it's a really really really kind of weak magnetic field and that's because that screwdriver base is made of iron which is a soft Ferro magnet you can make it a little bit of the magnet alright and it'll stay just kind of a tiny bead but if you try to take another magnet and put it there in position to what it's magnetic field was, it will just drop it what it was doing and listen to the new magnetic field so that's the difference. Soft Ferro magnet hard Ferro magnet, permanent magnets these are just magnetic materials like refrigerator doors.

And that's magnetic domains.