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Ferromagnetic materials

  1. Sep 25, 2014 #1


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    Hello everyone. I'm not entirely sure if this is the right section to post this question in but if it isn't, then I apologize.

    I'm studying for a test in E&M and I've come across a topic that had me thinking and I'm not quite sure if the explanation I'm thinking of is entirely true.

    Please note that I don't know quantum physics yet, so explaining the bigger picture of things is much preferable.

    Say, I got a piece of ferromagnetic material. Now from what I've read and comprehended, it's basically made of a lot of domains and each domain contains billions of magnetic dipole moments that are aligned in different directions in each domain.
    If I apply external magnetic field in some direction, then the domain that contains the magnetic dipole moments which are parallel to the magnetic field starts enlarging itself and its "neighbours" start changing their direction in the same one as the magnetic field so at the end of the proccess all of the magnetic dipole moments face in the same direction.
    When turning off this external magnetic field, they stay "frozen" in that direction.

    Now a couple of questions:

    1. a) Is what I described above true (in the big picture)?
    b) There were also a couple of lines concerning the fact that there are "preferable" directions in which the spins would align. I'd appreciate it if someone could explain that.

    2. Is that how they pretty much make magnets? they take a piece of ferromagnetic material, they put it in an external magnetic field and then what happens is the process I described above, and it becomes a magnet.
    Also, is the strength of each magnetic determined by how "good" the magnetic dipoles are aligned?
    For example, a really strong magnet has most of its magnetic dipoles in the same direction and weaker magnets have more "messy" ones?

    3. What happens if I applied no external magnetic field? What properties does the material have then?

    Thank you!
    Last edited: Sep 25, 2014
  2. jcsd
  3. Sep 26, 2014 #2
    1a) yes, that is more or less what is going on.
    b) Quantum mechanics (crystal field in particular) simply is such that some moment directions have lower energy than others. The main reason is that the electrons that carry the magnetic moment also have an anisotropic (=not round, but with some kind of 3D shape) charge distribution. If the charges of neighboring atoms are closer in some directions than in others, then the asymmetric charge cloud of the electrons will try to orient itself such that the energy is minimized.

    2) Yes. Usually you take a powder of a ferromagnetic base material and then sinter it or more mix it with some epoxy to make the shape you want. Then you put the whole thing into a big electromagnet, energize the magnet and thus polarize your ferromagnet.

    The strength of the ferromagnets is determine by how well it is polarized, but also by the properties of the base material. That is why Nd2Fe14B magnets (Neodymium magnets) are much stronger than ferrite magnets. For anisotropic ferromagnets (e.g. those that have an "easy" direction) the magnetization depends also on how well the powder grains and their easy direction are aligned with the field direction you want after processing and magnetization.

    The magnetization of the base material also depends on the temperature.

    3) The material is still magnetic, the magnetism is just "hidden".
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