Do Permanent Magnets Dissipate?

Thanks for the quick reply! In response to your answer, maybe I should rephrase my question then: Are there any permenent magnets that do not lose their magnetism over time? Such as neodymium-iron-boron magnets, or maybe something else? The strength of magnetism does not necessarily matter; I just need to know if permenent magnets that don't lose their magnetism exist.

temperature is a statical sort of thing. Even at temperature well below the curie temperature, there is a non zero chance that a given domain will momentarily get hit with an effective thermal energy that approaches what it would see at the curie temperature. So there is not a perfect magnetic stability, even below the curie temperature. For some materials, like hard steel, magnetism decays with a half life short enough for the effect to be directly observable. With high coercivity materials, samarium cobalt or neodymium, the decay rate is very much slower. I think the term that describes this effect is magnetic disaccomodation.

There is an analogous effect for dielectrics that have been polarized with an electric field (electrets and piezo transducers).

AlNiCos certainly do

Graham

You meant "statistical"? Yes.

The general idea is that magnetic domains don't want to be in the long-range order that we call "magnetized" because it's a high-energy situation for them relative to the unmagnetized, disordered state we usually find magnetizable materials in. When we magnetize something we add energy to it by ordering the domains.

To get a feel for it build an analogy. Take two small bar magnets and let them join naturally, all poles together like a four-way handshake. Notice their external field is greatly reduced; the magnets are analogs of the domains in a magnetizable material that isn't magnetized. Imagine stacking more magnets to the sides, building up a "magnetic crystal" so that each pair of magnets attracts each nearby magnet. It doesn't quite work in threespace with real magnets but you can get pretty close.

Now pull the first two magnets apart and stack them north-south so that one pole of each is free. Their external fields add just like ordered domains. Now stack more to the sides so they _don't_ attract each nearby magnet and notice that the field gets stronger and stronger.

(Of course this is very hard to do without using say duct tape; the analogy doesn't account for the fact that the magnetic forces between domains are much weaker than the chemical bonds making up the lattices that support the domains).

This is basically what happens when a piece of magnetizable material is magnetized; the domains are rotated so that the fields add, but it's an unstable state, and incoming energy (like heat energy shaking the lattices) can allow some of the domains to "fall" back into the old, familiar, comfortable four-way handshake. Notice that all four of Juno's examples do just that; excite the lattices so that the domains have some freedom to rotate. John, you're right; even at temperatures well below the Curie point all magnets will slowly demagnetize due to the random local domain rotations caused by low-energy phonons exciting the lattices.

Now for the last question; the only true "permanent" magnet I know of offhand would be a superconducting loop; it only has one domain in a manner of speaking.

Mark L. Fergerson

It is called entropy and the answer is yes.

In a practical sense (depending on what time span you have in mind, of course), the answer is "yes," meaning only that there are "permanent" magnets whose magnetization will not appreciably decrease over the expected useful life of whatever product they're used in. Over the long term, of course, the answer is "no" - eventually, any "permanent" magnet will slowly lose magnetization unless there is some mechanism provided for refreshing it. Of course, "long term" can mean some pretty lengthy periods.

Bob M.

(snip)

Damn spell checker.

(snip) To quote Doctor Memory (from Firesign's "I think We're All Bozos on This Bus"), "The system is less energetic if domains of opposite polarity alternate." And then he excuses himself for a nanosecond to flush the toilets and set off the fireworks.

Ceramic magnets are pretty permanent I believe.

Graham

Handy explanation of the processes involved, and why permenet magnets eventually will lose their polarity. I suppose even at cryogenic tempuratures, it is possible for magnets to lose their strength? Interesting comment you made at the end of your reply though:

Now this might be more what I'm looking for. I'm designing a theoretical machine that requires parts capable of remaining (truly) permanently magnetized. As you say, these superconducting loops only have one domain; in that case, would it be possible to arrange them in a way to function identically as their traditional magnet counterparts?

Sure, just much slower than at shirtsleeve temperatures.

I don't see why not (as long as the temperature is kept low enough that they don't quench). A clearer answer depends on a clearer question; IOW what do you want them to do?

Mark L. Fergerson

Well, essentially I'm trying to create a levitating rod inside of a drum, and have the rod rotate inside in mid-air, levitated by magnets located on the inside of the drum. It would look something like this:

_________________________________ | | | ************************************* | | ************************************* | |________________________________|

I know that there already exist devices like that, but they all depend on magnets, which as we've discussed previously, dissipate over time. I'm trying to create a version that won't lose it's magnetic properties, and be able to keep rod properly levitated. Remember, this is an entirely theoretical machine, but I'm just trying to figure out if it is at least physically possible. With your proposal of using a superconducting coil as a replacement for the magnets, that would solve the problem of the drum and rod losing magnetic properties. By replacing the exterior surface of the rod and interior surface of the drum with superconducting coils, one would be able to levitate the rod without any eventual magnetic loss.

If temperature is a concern for superconducting coils, then perhaps placing the whole apparatus in a cryogenic chamber would keep everything at a proper temperature. Obviously, as no room-temperature superconducting material yet exists, I suppose I will need to modify my plans to include a cryogenic encasement. In your opinion, do you think the use of superconducting coils in a cold enough environment be able to levitate the rod without any eventual loss of magnetic ability?

That looks like the best solution at the moment. As I can't use a normal magnet(since it will dissipate over time, which it what I'm trying to avoid), superconducting coils seem to be the best option for

*truly* permanent magnetism. Your suggestion, "a superconducting tube-magnet inside a superconducting vessel" is exaclty what I plan to use. But, as I am not an expert on superconducting materials and their properties, I've got a question. Would a superconducting coil require a current of electricity to operate as a magnet, or can it perform this function without the application of electricty?

superconductors repel magnets.

if you want levitation put a normal magnet in a superconducting bowl...

or in your case possibly a superconducting tube-magnet inside a superconducting vessel.

Bye. Jasen

Once you start a supercurrent (what a superconductor, er, superconducts) going in a closed loop, it keeps going "indefinitely".

That's in quotes to indicate that it depends on certain conditions not changing; the temp stays low enough, the loop isn't broken, no huge magnetic field is imposed, etc.

IOW it doesn't need any more power to keep going because it has no way to dissipate the power it took to start the supercurrent, as opposed to ordinary resistive circuits which do, you see.

BTW it sounds like you're trying to reinvent the magnetic bearing (for which Google).

Mark L. Fergerson

yes they are "charged" by beeing cooled to superconducting temperature in the presence of a magnetic field (often from an electromagnet)

after they are superconducting the external magnetic field is removed and this induces the current that turns them into superconducting magnets.

Bye. Jasen

Thanks for explaining that for me. The conditions required for a properly charged superconducting coil to have a singular domain and remain that way seem to be pretty easy to take care of, save for one: The temperature. Although a superconductor has very little resistance, wouldn't there be a non-zero chance that at any temperature above absolute zero, there would be resistance, and that the resistance would slowly eat away at the energy of the closed loop? So that, over a very long period of time, the electrical charge running through the loop would be released as heat energy, then the superconducting coil would fail to have a singular domain, then the levitating rod would fail because the magnetic properties that kept it suspended have failed? If I am missing something, or if you're simply referring to temperatures going beyond the Curie temps, then perhaps I am wrong, and the coil will retain it's singular domain properties forever, which is what I'm trying to achieve. I'm not trying to reinvent magnetic bearings, although they do bear a close resemblance to some of what my project is concerned with. Thanks for your help.

_Zero_ resistance.

No. This is a quantum-mechanical effect; the resistance curve has a singularity (goes to zero) at Tc.

The short answer is that since this is a quantum-mechanical effect, the statistics say that while there indeed is a non-zero possibility for a phonon (quantum of heat) to quench a superconductor while it's well below Tc, it's about as likely as all the neutrons in it decaying at once.

The long answer gets very mathematically messy, but the bottom line is that as far as failure modes go, put that one way down on the list.

Mark L. Fergerson

Nope, it's exactly zero:

"...Superconductors are also able to maintain a current with no applied voltage whatsoever, a property exploited in superconducting electromagnets such as those found in MRI machines. Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a current lifetime of at least 100,000 years, and theoretical estimates for the lifetime of persistent current exceed the lifetime of the universe...."

Cheers! Rich

I saw a way cool thing on some science show - they took a disk of superconductor and put it in the bottom of a beaker, dropped a little magnet on it, and poured in some liquid helium. When the superconductor got down to temp, the magnet simply rose. It was almost spooky!

Cheers! Rich