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Dia, para & ferromagnetic materials

  1. Jun 5, 2012 #1
    From another thread I think I understand that ferromagnetic material can exist any time, but dia & para magnetic materials exist only when there is an external field.

    My overview is
    Macroscopically ferro materials have to align their domains & can remain aligned when field is removed, leaving permanent dipoles.
    Para materials have relatively few permanent dipoles & are less likely to remain permanent.
    Diamagnetism only exists to oppose any external field in accordance with Lenz's law.

    Could someone confirm or clarify this set of statements, please.
  2. jcsd
  3. Jun 7, 2012 #2
    Well, materials can be in ferro-, dia- or paramagnetic states.

    Of these, only ferromagnets will carry a macroscopic magnetic moment.

    Paramagnets have lots of microscopic (atomic) magnetic moments, but they are completely disordered, each moment only reacts to an external magnetic field.
    Therefore the material is only magnetized in the presence of an external field.

    Diamagnets are similar, only that the net magnetic moment per atom is zero, and the moments appear only in the presence of an external field.

    Ferromagnets have lots of atomic magnetic moments like paramagnets. But unlike paramagnets, these moments interact with each other. They "talk to each other", aligning all in the same direction within a certain region (=domain). So each domain has magnetization. From one domain to another, they tend to be aligned in different directions, tominimize the magnetic field they generate (because that costs energy). But with an external field these domains can all be pulled into the same orientation.
    Hard ferromagnets will then keep that orientation, whereas the domains in soft ferromagnets will re-align randomly when the external field is removed.

    On top of these 3 types you can also get antiferromagnets where the atomic magnetic moments are aligned - but instead of being all parallel they form patterns like up-down where the sum of the microscopic moments is zero, so that there is no macroscopic magnetization.
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