Kip holds a PhD in Engineering from The University of Texas at Austin and was an occasional substitute lecturer in engineering classes at that institution.
Everyone loves to play with magnets. It seems almost like magic to feel them push and pull on one another, even though they're not touching. It did seem like magic to ancient people, but today we understand those forces and where they come from, so we know it's just science. Magnetic fields arise from moving electric charge (current), and we use this effect all the time when we make motors, generators, and so on. But, we've also learned that some materials can cause a magnetic field all by themselves. Pieces of such materials might not start out making a field, but, if we place them in a separately generated field and then remove them, we find that they 'remember' that and thereafter create a magnetic field of their own. Such materials are called ferromagnetic.
All magnetic fields arise from electric charges in motion. In magnetic materials, the moving charges are the electrons that revolve around the atoms or molecules of the material. You might ask why, then, are not all materials magnetic, since all materials are made of atoms or molecules, and all atoms and molecules have electrons in motion associated with them?
That's a great question! But it has a great answer too. First of all, some atoms and molecules have electron distributions so that the motions of the various electrons cancel out as far as producing a magnetic field goes. Even in materials that do have 'uncancelled' electrons, sometimes the atoms and molecules themselves have random arrangements, so that the tiny fields produced by each one wind up canceling each other out. So in both cases, we wind up with no large overall field.
In ferromagnetic materials, the arrangement of the atoms or molecules is easy to change - when we apply an external magnetic field to such a material, the arrangement will shift so that the fields produced within the material 'line up' with the external field. Then, when we remove the external field, the internal fields help to hold one another aligned, so the material continues to produce a significant externally-measurable field.
Magnetic Domains & Ferromagnetism
The changes to alignment within the material don't usually go all the way to the atomic or molecular scale. It's common to find that a piece of the material is made up of regions within which the atomic/molecular fields are already aligned. But, when the material is in a non-magnetized state these regions produce fields in random directions, and they all cancel out. An external field can align the fields of entire regions. Scientists call these regions magnetic domains. It's important to understand that the physical regions themselves do not rotate during the realignment process. Rather, the region's magnetic field can be realigned within the region.
Ferromagnetic materials are materials in which the domains will tend to keep one another magnetically aligned (once they become aligned) so that most or all of the domains produce a field in the same direction. There are other classes of magnetic materials, such as ferrimagnetic (which follow the same principal, except only a fraction of the domains become permanently aligned and hence have a weaker self-generated field) and antiferromagnetic (which have domains that take on an alternating pattern of alignment and hence make very little field of their own). Of these, ferromagnetic materials have the strongest and most noticeable magnetic fields.
The most common ferromagnetic materials are iron, nickel, and cobalt (and most alloys formed of these elements). If you check a periodic table of elements, you'll find that these three elements are next-door neighbors. This makes sense, since location in the periodic table has to do with an element's electron structure, and that structure also controls the extent to which atoms of the element can produce magnetic fields.
When an object is heated, the atoms or molecules it's made of vibrate more strongly. Temperature is, in fact, just an 'averaging' of the vibrations of many atoms or molecules. If you heat a permanent magnet to a high enough temperature, this vibrational energy becomes strong enough to overcome the tendency of the domains to remain aligned. The alignment (and the magnet's magnetic field) will be lost. The magnet could be re-magnetized after it cools, but it will not recover its field on its own.
A permanent magnet can have its direction of magnetization reversed if a suitably strong external field is applied in the reverse direction. The external field has to force all of the domains to reverse their alignment. This process causes the generation of some heat within the object, so there is an energy loss. This gives rise to a phenomenon called hysteresis in magnetic materials. In most AC motors and generators, the magnetic materials are alternately forward and reverse magnetized, so hysteresis is a source of inefficiency in these devices.
Ferromagnetism is the ability of a material to produce a magnetic field all by itself. In order to do this, the atoms or molecules of the material must have a suitable electron structure, and the individual atoms and molecules have to have an orientation with respect to one another that allows the fields to add up. Most ferromagnetic materials contain regions called domains, within which the individual atomic/molecular fields are already aligned, but it usually requires exposure to an external magnetic field to line up the fields of the domains in one direction. In ferromagnetic materials, once this is done, the field created by the (now aligned) domains will maintain the alignment, unless the object is heated to a high enough temperature. Reversing the direction of magnetization in a ferromagnetic material usually causes some heat losses, giving rise to the effect known as hysteresis.
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