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If I had a large cylindrical neodimium magnet or stack of same, with a coil wound around it. In an unpowered state the magnet would have a certain attraction. With one polarity of current, I would imagine the coil would amplify the permanent magnet field/attraction. With the reverse it would decrease the attraction, potentially neutralise or invert it. Is this feasible or likely to damage the permanent magnet?

I would like a magnet that I could selectively disconnect, but whose force returns when power is removed.

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  • $\begingroup$ So you want to build an electromagnet that has a permanent magnet assist: it's been done successfully, in the past but I don't have the details at hand. it was possibly with a ceramic magnet instead of neodymium. $\endgroup$
    – Jasen
    Commented May 12, 2021 at 10:51
  • $\begingroup$ Sounds like the opposite is wanted - a permanent magnet with an electromagnetic reducer. $\endgroup$
    – Guy Inchbald
    Commented May 12, 2021 at 13:07

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When a magnet is first manufactured, it has no overall magnetism. To make a permanent magnet it has to be magnetized. This is done by putting a coil round it and applying a large pulse of DC current.

Generally speaking, it will remain magnetized until a strong enough pulse is applied in the opposite direction.

If you apply enough reverse current to cancel out the permanent field, it will be similar in magnitude to the current used to create the magnet and will flip the magnet across too.

The usual way to demagnetize or "degauss" an object is to apply an alternating current and slowly decrease its intensity to zero so that each peak magnetizes the material less and less. Another way can be to temporarily heat the material above its Curie point. But either way, you would have to re-magnetize it before switching off.

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  • $\begingroup$ Does this suggest a neodimium magnet could be used and it essentially be permanently switched with a pulse of field inducing current to either switch it permanently on or off? This seems to present a very interesting opportunity if true. I was under the impression the metal needed to be heated for the field to change, either align or misalign. $\endgroup$
    – J Collins
    Commented May 13, 2021 at 16:16
  • $\begingroup$ @JCollins "On-off" is a bad way to talk about magnets, they are essentially tri-state: one direction, demagnetized, or the other direction. As I said, to demagnetize one you need a gently decaying train of alternating pulses. I am not sure how you got "a pulse" from that. Heating to the Curie temperature will destroy any existing magnetism, cooling in the presence of an external magnetic field will re-magnetize it that way; it's how rocks get magnetized. But I assumed you had no such oven-baked solution in mind. $\endgroup$
    – Guy Inchbald
    Commented May 13, 2021 at 17:04
  • $\begingroup$ Pulse comes from your first paragraph, and makes the assumption that a well quantified single pulse could have the same effect on a known magnet as a degaussing wave on a less well defined magnet. Would you say that the magnetic field required to demagnetise a cold magnet would be similar in magnitude to that required to re-magnetise the same cold magnet? Correct, I am not thinking of a heating system. Regarding tri-state, I am looking to use two of the three states, magnetised in one direction or demagnetised. $\endgroup$
    – J Collins
    Commented May 13, 2021 at 17:13
  • $\begingroup$ @JCollins No, it doesn't work like that. This is the third time of saying, how many more must I say it; you need the long pulse train to demagnetize. You should be asking why it is that way, not suggesting it is wrong. It is in fact because of something called magnetic hysteresis, see en.wikipedia.org/wiki/Magnetic_hysteresis . I think you really need to understand the physics a bit better. $\endgroup$
    – Guy Inchbald
    Commented May 13, 2021 at 18:44
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If I understand correctly this is your question

"I would like a magnet that I could selectively disconnect, but whose force returns when power is removed."

There are a few video's on You Tube demonstrating this very effectively, without damage to the permanent magnet. Some applications use power as suggested others use a magnetic switch to turn the Neodymium magnet on and of as required.

This how to do it electronically using current

https://www.youtube.com/watch?v=4xZa1JGP2oc&list=PLdIAF1FoMAMG_MR0SwZooLLujjpiVcfRt&index=4

This is how to do it with a mechanical switching device uses other perm magnets

https://www.youtube.com/watch?v=MptNafdF_q8&list=PLdIAF1FoMAMG_MR0SwZooLLujjpiVcfRt&index=1

Good luck

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A simple answer to my question that I won't necessarily accept, but adds information gathered elsewhere: Neodymium magnets in particular are terrible at carrying magnetic flux, unlike iron, so could not reasonably be used as the core of a switchable magnet.

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