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Thinking about magnetic coercive field

  1. Aug 26, 2012 #1
    I am trying to better understand some concepts in magnetism and I hope that someone here can help clarify a few questions that I have.

    I am trying to interpret the meaning of the specific coercive fields of magnetic objects. For the sake of discussion, let’s use neodymium (rare-earth) magnets as an example. These magnets have a very high coercive field. Again, for the sake of discussion let’s say the magnet has a coercive field of 12,000 Oe (~955 kA/m),
    This would mean that in order to remove magnetization (take it to the M = 0 line on the hysteresis curve) of the magnet one would need to apply this minimum magnetic field (H-field) strength.

    Does it make sense to think about the field outside the magnet? What would the field in the air / vacuum be right outside the magnetic at this point?

    If we know that,
    H = (1/μ) * B
    Where mu is the magnetic permeability of the material, we could find the B-field if we know the H-field (from the coercive field). Of course, this is not value inside the magnet, since there is a magnetization that needs to be accounted for. But in the vacuum surrounding the magnet, there should be a field that we can find.

    In the above example, could we say that the magnetic induction field (B-field) need to zero the object’s magnetization would be,
    B = μ_0 * H_c
    B = (4pi E-7 N/A^2) * (955000 A/m) = about 1.2 Tesla
  2. jcsd
  3. Aug 26, 2012 #2
    Your conversion from A/m to T is about correct because Nb-Fe-B magnets have a permeability around 1. Other materials like AlNiCo have µr >> 1 .

    Several fields are defined in permanent magnets. 955kA/m for an Nd-Fe-B is rather the field it offers for the using circuit, not the demagnetizing field which uses to be even bigger and translates to >2T.

    This means that the induction needed to make or erase such a magnet can't be attained from a magnetic core and must be created without the help of permeability, by using a huge power for a short duration. The standard method is capacitor discharge, like 1m3 capacitors for 0.1dm3 magnets. In 10ms, it's loud and the cables must be hold firmly.

    I suggest you to find material data sheets on manufacturers' websites for the material you want. They give magnetization curves which are rather clear.
  4. Aug 27, 2012 #3
    In my example, I was not considering the field inside the magnetic (be it NdFeB, or whatever), but rather the air/vacuum immediately outside the magnet.

    I should clarify that I am not really interested I specific case, but rather a more general understanding of what is going on. I just used NdFeB as an example because I thought using real numbers might clarify the question I was asking.
  5. Aug 29, 2012 #4
    Can anyone else offer some insight?
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