Ferromagnetism & curie temperature

we've already seen that atoms that have unpaired electrons behave like tiny magnets they have magnetic moment however not all materials that have in fact most materials that have atoms like these they hardly behave like magnets in fact they only show very slight attraction to magnets and we called this phenomena paramagnetism however there are very few elements in fact four on the periodic table iron cobalt nickel and gadolinium which show incredibly strong attraction to magnets and they do have atoms that have unpaired electrons so what's so special about these elements by the way this phenomena where they show incredibly strong attraction is what we call ferromagnetism it's named after ion ferum the first one where we saw this effect so that's the big question what's so special about these elements all right so let's first look at a paramagnet say we could look into a piece of aluminium aluminium is a paramagnet it contains atoms that have magnetic moments that behave like tiny magnets so the first question i have is why doesn't a piece of aluminum itself behave like a magnet well for that we need to look at all the atoms and what i want to do is for simplicity to show the magnetic moment to show the direction of the magnet i'm just going to draw an arrow mark over here just for simplicity all right so if you could peek inside you'd find that all these atoms are randomly oriented these tiny magnets are randomly oriented so all of their magnetic moments cancel out and you don't get anything but now let's look at a piece of ferromagnet let's say we peaked into an iron imagine this is a piece of iron what would we find over here well here you would find something like this oh do you notice the difference well first of all you here also you see groups of atoms can be randomly oriented and as a result the net magnetic moment can still be zero not all pieces of iron need to be magnet right but the speciality of ferromagnets is you can see these groups and let me help let me help you see that you can see groups of atoms and there can be billions of them that are all aligned in the same direction that is the speciality of ferromagnets what causes this well it's going to be a mystery for us because i looked at a lot of literature it turns out we need to learn quantum mechanics for that and we're not going to do that in this video so we're just going to accept it it's a quantum mechanical phenomena but what's important is that they're doing it spontaneously you don't need a magnet you don't have to bring it close to a magnet they spontaneously arrange themselves in groups that are all aligned together and these groups which have all aligned magnetic moments we give a name to that we call it magnetic domains so that is the big difference between paramagnets and ferromagnets although at an atomic level they look the same they both have magnetic moments at a macro level you have magnetic domains inside these ferromagnets okay so what do these magnetic domains do well to answer that now let's look at their behaviors in presence of an external magnetic field again let's start with the paramagnet what happens if i were to bring a giant bar magnet or if i were to somehow create a strong magnetic field outside and just to make sure that you can see the atoms carefully properly i'm just going to make the magnetic field a little dim all right so you have a strong magnetic field to the right what's going to happen to these atoms well we've seen before the field tends to turn these atoms and align it in the direction of the field something like this and how much it is able to align depends upon how strong the field is and it also depends upon the temperature if you have very high temperature they are vibrating a lot it becomes harder to align but what you end up getting is you get some kind of a weak alignment it's not very random now but it's not completely aligned but you get some kind of a weak alignment and as a result of that you can kind you now see this alien bar of aluminium is slightly slightly magnetized this side being north this side being south and as a result this will now get slightly attracted by the magnet okay what happens over here well here when you bring a giant bar magnet or if you have an external magnetic field the effect is the same it tends to turn these atoms and align them but now it's not the atoms alone that turn it's the entire domains that turn so due to the field maybe this entire domain will turn right and get aligned and so now in in effect you see that this this domain got bigger and if you make that field stronger more domains get get aligned completely and eventually it's possible that all the domains get aligned in the same direction and you now have one giant magnetic domain where all the atoms are aligned in the same direction as a result this piece of iron is incredibly incredibly strongly magnetized and as a result of that it shows very very strong attraction so the big difference is here the alignment is super super weak because you don't get the domains you don't get that you know quantum mechanical effect but here due to that quantum mechanical effect all of these are aligning themselves together almost perfect alignment and that makes it an incredibly strong incredibly strongly magnetized okay what happens when i get rid of that magnet again let's come back over here if i get rid of that magnet we've seen that there's no reason for these atoms to stay in that way they will quickly go back to this random orientation and they would quickly lose whatever slight magnetism it had achieved so paramagnetism is very temporary weak and temporary you can imagine these atoms to be extremely undisciplined if there is no supervisor around they just go back to being random and doing whatever they want to do what happens if you get rid of the magnet over here here it really depends upon the crystal you can have a piece of iron in which even after getting rid of that magnet all of these atoms stay aligned the entire magnetic domains stays at it as it is and now we have a permanent magnet these crystals are called hard magnets because they get very strongly magnetized and it's very hard to break their magnetism even without an external magnet they stay aligned but you know what you can take iron itself and you can heat treat it and change its properties and when you do that what you find is after removal of the magnet most of the domains would just go and flip back and you would find in those crystals you pretty much lose their magnetism such ferromagnets are called soft magnets meaning they're not soft in the sense like a pillow they're still hard but they're soft in the sense you know if you get rid of the external magnetic field they immediately lose their magnetism so hard magnets are used in building permanent magnets because they can retain their magnetism on the other hand soft magnets are useful as cores of electromagnet here it's only when a current is running you want that thing to get magnetized and produce very strong magnetic fields right but when the current stops you don't want it to create magnetic field so you don't want the magnet to stay a magnet right so you want the magnetism to disappear and so we use soft soft ferromagnets over here the final question is does temperature affect the ferromagnetism and it turns out it does if you were to heat this piece of iron and you heat it say above the temperature of 770 degrees celsius this is for iron then it turns out that above this temperature iron loses all of its magnetic which means iron now has become paramagnet it loses all of its ferromagnetic property it becomes a paramagnet and this temperature above which a ferromagnet loses its magnetic domains and becomes a paramagnet we give a name to that temperature we call it the curie temperature and i'll let you you know go and research who is this named after is it mericury or husband pericuri okay it's for you to do that and here are the query temperatures of the four ferromagnetic elements you can see if iron and cobalt have a very high value and nickel and galilean gadolinium look at that it has a very low value your textbook shows uh you know i think about 40 degrees or something but a couple of literatures are checked they show about 20 degrees which means even at room temperature gadolinium loses its ferromagnetic properties but here's the thing they will only lose their magnetism above the curie temperature but what happens if you cool them back below the curie temperature if you go if you cool iron back below 770 degrees celsius then all of its magnetic domains are again spontaneously formed and it's back being a ferromagnet so it might lose whatever magnetization it already previously had but now it's ready to be magnetized again so about query temperatures they behave like a paramagnet but you cool them below they'll come back spontaneously aligned many domains are formed they behave like a ferromagnet