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B How do magnets really work?

  1. Aug 2, 2016 #1
    Hello PF!

    I am writing a scientific report about magnets, but have really no clue how magnets actually work? Research has yielded a few ideas, specifically from here and here.

    However, sometimes (in iron, nickel, and cobalt for example) you’ll have one or more un-paired electrons. The magnetic fields of these electrons aren’t canceled out by another, oppositely-oriented, electron. As such they lend an overall magnetic field to the atom they inhabit.​

    and the other

    In most materials almost all the electrons form pairs, with the magnetism from the two paired electrons exactly canceling. The result ultimately is due to something called the Pauli exclusion principle, which says that no more than one electron can exist in any particular quantum state. So if there's some nice low-energy state waveform for an electron to sit in in some molecule, it tends to get two electrons for the two possible quantum states with that form: one spin up, the other spin down. ​

    I would take this as said, but these are relatively old explanations and I was just curious if up until this year, 2016, there have been any developments in the study of magnetism to explain this better? Say, two neodymium magnets, why do they attract, and how are they so powerful!?
     
  2. jcsd
  3. Aug 2, 2016 #2

    Charles Link

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    The best explanation I have for the magnetism in permanent magnets is that it is due to bound surface currents that result from the edge/surface effect of having electron spins in the same direction throughout the material.

    The reason the magnetism is so strong for the best permanent magnets is nearly all of the electrons (i.e. nearly one per atom) have their unpaired spin aligned in the same direction. The magnetic fields that result from the surface currents(that come from the magnetization and aligned spins) actually are what creates the magnetic field (both inside and outside the magnet) and this magnetic field (from the surface currents) is also what maintains the spins in the same direction (along with the quantum exchange effect that says that adjacent spins that point in the same direction are lower in energy than if they pointed opposite. )

    The direction of the spin is upward (perpendicular to the plane of any circular motion) for one direction (e.g. clockwise) and points downward for counterclockwise. There are calculations (done by doing computations originating with the surface currents) that show that a "pole" model that gives a north and south pole at the endfaces of the magnet can also be used to compute the magnetic field from a magnet.

    The "pole" model gives the exact same answer for the magnetic field, but the "pole" model doesn't explain the underlying physics which is explained by the magnetic surface currents. With the best permanent magnets, it often takes very high temperatures, i.e. 700 degrees Centigrade or higher, (a temperature known as the Curie temperature), to disrupt the permanent magnetization. It also requires magnetic fields as strong or stronger than the field inside the magnet to reverse its direction and make the magnetization point the other way. For these reasons, high quality permanent magnets are extremely stable.
     
    Last edited by a moderator: Aug 2, 2016
  4. Aug 2, 2016 #3

    Charles Link

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    One other item I can add to the above: The question might arise, why is iron sometimes a permanent magnet and other times a non-permanent magnet? In the non- permanent magnet case the iron can become temporarily magnetized and greatly enhance the magnetic field of a current carrying solenoid when placed inside of it. It can also be temporarily magnetized when the magnetic field from another nearby permanent magnet is present. When the applied magnetic field is removed, the iron of the non-permanent magnet returns largely to its original state. ## \\ ## And the reason for the two different types of iron, the best I know it, is that in the case of the permanent magnet, the iron is nearly enough of a single crystal form that there can exist considerable magnetic order in the material. Once it becomes a permanent magnet, it is likely to remain that way. (see also post #2.) On the other hand, in the case of the non-permanent magnet type iron, the material is very much polycrystalline so that all kinds of small magnetic domains form in the material with the magnetization (each domain is a small permanent magnet) pointing in very random directions. When it encounters an external magnetic field, some of these domains will increase in size and/or change their orientation of their magnetization to align with the applied field. The result is a temporary iron magnet that largely returns to the more random state upon removal of the applied magnetic field.
     
    Last edited: Aug 2, 2016
  5. Aug 4, 2016 #4

    Charles Link

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    I thought this was a very good question by the OP, and I would enjoy seeing if anyone else has anything to add, so by commenting I'm pushing it to the top one more time in the queue.
     
  6. Aug 4, 2016 #5
    Well, there are a few things in your posts that may need some amendments.
    - The number on unpaired electrons in ferromagnetic materials does not need to be "almost" one.
    It can be more than one. In iron for example, there are 4. However the average magnetic moment per atom at saturation is about 2 Bohr magnetons so it is like about two electrons per atom contribute to magnetism.
    - Both soft and hard magnetic materials have a domain structure. And this is not directly related to being or not single crystal. A single crystal can split into domains too.
    The softness (or hardness) depends on how easy is for the domains walls to move during magnetization-demagnetization. This is controlled by anisotropy of the material (shape anisotropy may be also used) and impurities in the sample (they can "pin" the domain walls).
    So, for example, pure and low carbon iron is "soft" whereas high carbon steel (or steel with foreign atoms - an alloy) may be "hard".
     
  7. Aug 4, 2016 #6

    Charles Link

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    Thank you. This is what I was looking for. The materials science is complex enough that I kind of thought I might be missing on a couple of the finer details.
     
  8. Aug 10, 2016 #7
    Thanks for the replies!
    In an effort to gather credible sources to reference, I came across this website, and was wondering what you think about its credibility?
     
  9. Aug 10, 2016 #8

    DrClaude

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    It's ok for its intended level.
     
  10. Aug 10, 2016 #9

    Charles Link

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    For a good, credible reference, the textbook Introduction to Electrodynamics by David J. Griffiths covers the bound magnetic surface currents in detail. It is very mathematical and at the level of the upper level undergraduate physics major, but it is a well-known text that is used by many of the universities in their physics programs.
     
  11. Aug 15, 2016 #10
    Hi@trontor

    Have a quick look at this. You really have posed one off the most interesting questions possible, the answer has so many layers to it!

     
  12. Aug 15, 2016 #11

    Charles Link

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    An interesting video. Considering R.P. Feynman says it's very difficult to give any kind of nearly complete explanation for the magnetism, (and he knew plenty about electromagnetic theory and even won the Nobel Prize for his Quantum Electrodynamics (QED)), I think we did reasonably well in the above postings by at least presenting the magnetic surface currents concept.
     
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