 Ferromagnets come in two flavors, ones that can be permanently magnetized and these are used in our magnets. And ones that can be temporarily magnetized. These are the ones that we use as a core inside electromagnets like maybe a solenoid or inside a transformer. The idea is when you pass a current through it, only then it gets magnetized. It becomes like a strong magnet. But when you stop passing the current through it, it stops behaving like a magnet. But they're both ferromagnets. So there must be some difference in their properties, right? And that's what we're going to explore over here. What's the difference? And in practice, how do we figure out the difference using the hysteresis graph? To explore the difference in their properties, we need to look at what makes a ferromagnet ferromagnet. And we've talked about that. It's magnetic domains. We've seen that in ferromagnets, atoms behave like tiny magnets because they have unpaired electrons. But it's not just that. We've seen that there are groups of atoms that are all completely aligned, spontaneously aligned in the same direction. And you know that when you have atoms aligned in the same direction, these tiny magnets aligned in the same direction, the magnetism adds up. And so each domain produces a very strong magnetic field. But the domains like you can see over here can all be randomly aligned. So one domain can tend to cancel out another domain's magnetic field and so on. So if you have randomly aligned domains, like what you see over here, then their magnetic fields can all cancel out and the whole ferromagnet still does not behave like a magnet. But let's see what happens when I close the circuit and pass a current through it. So when I pass a current through it, there's an external magnetic field that is generated. This is the field that is generated by the solenoid. And we've seen before that magnetic domains have a tendency to get lined up in the direction of the field. So all these domains we'll find if the field is strong enough, we'll get lined up in the direction of the vacuum field generated by the solenoid. And now the ferromagnet is super strongly magnetized. It behaves like a strong magnet. But what happens when you switch off the current? When you switch off the current, the vacuum field, the field generated by the solenoid, disappears, be not disappears. And in these ferromagnets, what we'll find is that once that external field disappears, almost all of those domains which got aligned, they will go back to being random. It's like all these undisciplined kids. Once you get rid of the field, they'll all go back to being, you know, whatever they were doing earlier. And as a result, they stop behaving like a magnet. But what happens over here? Well, even here we can start by thinking about the domains. And let's say we currently have an un magnetized ferromagnet over here. But over here, what will happen is when you switch on an external magnetic field, just like before, you'll find all the domains get aligned in the direction of the magnetic field. But the difference is if you get rid of that external magnetic field, most of the domains stay, they don't turn back. Okay. Maybe some of the domains might turn back. So I'll just show one turning back, but most of the domain just stay as it is. And that's why these are permanently magnetized. So what's the big difference we find between these two? We see over here, this has low retention, meaning it can't retain its magnetization. This one has a very high retention. It can retain its magnetization. And so we write this has low, we call it retentivity, retentivity. And this other one has high retentivity, high retentivity. And such magnets which have very low retentivity such ferromagnets, they're called soft ferromagnet. It doesn't mean that they're soft. It's like a pillow. It basically means magnetically soft. Okay. And these are called hard ferromagnets. So steel is an example for a hard ferromagnet. And there's something called a soft iron, which is an example for software magnet. Soft iron is basically iron that has been heated to an extremely high temperature and then cooled back down. We call that as annealing. Okay. Now comes the question, how do we figure out whether something has high retentivity or low retentivity and maybe other properties practically? One of the best ways of doing that is by drawing or plotting a hysteresis graph. In previous videos, we've seen hysteresis graph is a graph of a vacuum field generated by say a solenoid versus the magnetic field inside the ferromagnet. Now my question to you is based on what you've just seen and based on your knowledge of hysteresis graphs that you've gotten probably from previous videos, I want you to make a prediction of what would be the difference in the hysteresis loops that you would find for a hard ferromagnet versus a soft ferromagnet. So can you pause the video and think about, make a guess of how the hysteresis graph would look like. All right. Here we go. If you were to experimentally plot hysteresis graphs, you would get something like this. This would be the big difference you would see. The hard ferromagnets would tend to have a fat hysteresis graph and the soft ferromagnets would have a very slim, very thin hysteresis graph. But let's say, how does it make any sense? Well, if I start from here, this is the point where we have a very strong magnetic field and we have all the domains completely aligned in the same direction. Now when I go back and I decrease the vacuum field, notice the magnetic field inside pretty much stays the same. This is the one that's representing that most of the domains stay aligned. And as a result, even when the external field is zero, look, the field inside the magnetic field is super strong. And so this represents your high retentivity. This point represents your retentivity. And we can say the same thing on the other direction as well. You can see when the magnetic field on the other direction also when we reduce it and make it zero, the magnetic field inside stays pretty high. So these two points represent this length, you can say in the graph represents retentivity and you can see high retentivity. What happens in this slim graph? Well, if you look carefully, you can see over here, again, if we start from this point, as I decrease the magnetic field, the vacuum field, you can see the field inside pretty quickly drops almost to zero, not exactly zero, because all ferromagnets have some retentivity, not zero retentivity. They will have some retentivity, but very, very low retentivity. Look at this, very low compared to this one. Again, the same, same is the case from the other direction. So again, this represents, this represents the retentivity. There is another point of interest for us. And that is this point. This is the point where the magnetic field inside has gone to zero, which means this magnet has the ferromagnet has been demagnetized. And how did we do that? By putting a magnetic field in the opposite direction. This is the point where some of the domains have aligned to the left and about half of them have aligned to the right and the magnetic field has canceled out. So the amount of magnetic field that is needed to be put in the opposite direction to demagnetize the ferromagnet, this, this thing is often what we call coercivity or coercive force, coercivity. And you can see that hard ferromagnet should have very high coercivity. Why? Because they should be hard to demagnetize, right? These, we want them to be permanent magnets. So if you keep them close to another magnet, we don't want it to, it's magnetism to change. And so we need this to be very large. But look at the coercivity of your temporary magnets. You can see it's very, very tiny, very, very, almost zero, which means it's, you need almost zero low coercivity. Let me write that. So you get low coercivity over here. So this means temporary magnets not only have very low retention of magnetization, they can be very easily demagnetized as well. Permanent magnets not only have very high retention, but they're also very hard to demagnetize as well. So that's the basic difference as we see. But can you see a similarity? One major similarity you see is in both of these cases, we want to make sure that when you have some external field, when you put an external field, we want the magnetic field inside to be much, much larger than the magnetic field outside. That's the defining feature, another defining feature of our ferromagnets. And we want that to be true in both of them. We want both of them to generate very strong magnets, you know, magnetic fields. And so that is often called permeability. So both of them, we want both of them to have high permeability, permeability. This basically means you want both of them to allow the magnetic field lines to easily pass through them. You want both of them to get magnetized to a very high value. That's what it means. In both cases, you want that property. So these are the major differences and similarities between the two.