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The actuator of a hard drive head consists of two very strong neodymium magnets, with an electromagnetic coil between them. If you take that apart, the magnets attract each other very strongly. There's no doubt the field between them is very strong.

But if you flip them back to back, there is no repulsion force - there is pretty much nothing. While the magnets are very strong on one side, they seem completely inert on the other.

I understand how a U-shape of a shaped magnet allows it to focus field near the two exposed poles, but how does a flat rectangular plate manage to keep the field over one flat, wide surface and nearly none near the other?

[sorry about the heavy edit but it seems the question got totally muddled with irrelevant discussion about unipolar magnets and possibility or impossibility to remove magnetic field from one side. Only heavy rephrasing may help.]

Edit: some photos:

1: The magnets stuck together. They hold really hard, I'd be unable to just pull them apart but I can slide one against the other to separate them. 1

The magnets in "inert position" - they don't act on each other at all, just like two inert pieces of metal. 2

The magnets seem to have two poles located on the surface off-center. Here, a normal magnet placed against the hard disk magnet, centering itself on one side, then flipped - on another. 3 4 5

The metal shield seems to act like completely unmagnetized ferromagnetic. I can stick the "normal magnet" any way I like to it, and it doesn't act on another piece of ferromagnetic (a needle) at all. 67

When I apply a small magnet to it, it becomes magnetic like any normal "soft" ferromagnetic - attracts the needle weakly. It behaves as if the (very powerful) neodymium magnet glued to the other side wasn't there at all. 8

Unfortunately the neodymium magnets are glued very well and so fragile I was unable to separate any without snapping it, and then the "special properties" seem to be gone.

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I'm curious. What happens when you rotate the magnets with respect to each other? Do they have a preferred configuration to "snap" into? –  santa claus Mar 27 '13 at 8:00
    
Since I don't have this in front of me: Are you certain that the whole thing is actually a magnet? Is it possible that they are just backed with a thick layer of some similar-looking substance that prevents them from getting close together and exerting a strong repulsive force? –  Colin Fredericks Mar 27 '13 at 15:30
    
@ColinFredericks: It seems there's some 2mm thick magnet glued on surface of some 3mm ferromagnetic sheet metal larger than the magnet itself. The metal appears to work like a shield, reacts to normal magnets like normal ferromagnetic metal (as if the neodymium magnet wasn't there at all). If there is any repulsive force, it feels to completely cancel out with attraction - both magnets seem not to affect the metal pieces at all - normally sticking a magnet to a ferromagnetic will make it act like a magnet as long as the contact is held, not in this case, the alloy "doesn't see" the neodymium. –  SF. Mar 27 '13 at 17:58
    
@Alex: When I stick them the "working sides" together it appears there are some zones of attraction and repulsion - I can't rotate them freely nor slide against each other. But the backsides seem completely inert, no attraction/repulsion, reacting like plain ferromagnetic metal to other magnets or if sticking the two neutral-to-active side. –  SF. Mar 27 '13 at 17:59
    
@SF. This is what I suspected. If they were truly "one-sided" (I'm not saying this is what you said, this is just for the sake of clarification) then they would snap together willy nilly. This behaviour shows that they behave a lot more like U-magnets, which means that both poles are on the same side -- hence the strong field on one side and the weak field on the other. –  santa claus Mar 27 '13 at 22:05
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4 Answers

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The metal plates the magnets are glued to are an Iron and Nickel alloy that has a very high magnetic permeability called a Mu-metal.

I don't understand all of the details of magnetism or how Mu-metal works but that should get you started.

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It appears this is the right track at least - the mysterious metal the magnet is glued to acts as a shield - I didn't know shielding magnetic field like this was possible at all... –  SF. Mar 28 '13 at 23:03
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After reading all of the relevant articles, be best layman's description of what is happening is not that it's blocking the field, it's channeling it. Magnetic field lines can not be broken or stopped. They will start at one poll and go to the other. By placing a metal with an exceptionally high permeability (think extremely low "resistance" to magnetic fields) the metal basically "short circuits" the field. There is almost no field on the backside of the metal plate because all of the field on that side of the magnet is traveling through the plate rather than through the air. –  Brandon Enright Mar 29 '13 at 0:43
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I think that you are mistaking "one-sided" magnets and "unipolar" magnets.

...strictly one-sided magnet (generating no magnetic field whatsoever on one side) is impossible...

No. There is no problem in having a magnet with a field on one side. The hdd head is the example (here is a picture).

What you cannot have is a magnet with only one pole -- only S or only N. So, to claim that you have an "impossible" or "nearly impossible" magnet, you need not just to see if it "attracts metallic objects on one side" -- that works even with basic U-shaped magnets (some pictures here). What you need to check is that you have only one magnetic pole all over the magnet.

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Nice explanation. Thanks for the link to the hdd head. However, I could argue, that there is field on the other side, as the divergence of $\vec B$ should be always zero. So in the picture you have, only the relevant field lines to the HDD technology were drawn. –  gns-ank Mar 24 '13 at 13:02
    
The picture you linked is an oversimplification showing only the strongest field. Search images of "U shaped magnet field" and you'll see see photos where layout of iron filings shows the layout of the field. Sure it's strongest between the poles but the lines run all around the magnet - and it attracts metal objects on all sides, although the "arc" side does so much weaker than the "open" side. I guess in this case the effect is similar but much, much stronger and with no shape advantage. But no, you can't -entirely- eliminate magnetic field, just vastly reduce it. –  SF. Mar 24 '13 at 13:08
    
Specifically, you did not answer the question. So if there is no problem creating a magnet with field on one side, how does one go about it? Specifically, not a horseshoe shape but a very flat bar that has magnetic field on side of one wide flat surface and no field at all on the other flat side. And no, I did not confuse magnetic monopoles with zones of magnetic void. Zones of magnetic void are just as impossible. –  SF. Mar 24 '13 at 13:19
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@SF Look, maybe you was not confusing unipolar magnets with one-sided magnets. But the way you phrased your question (before your edit) unambiguously states exactly what I did quote. Not only you completely changed your question to hide the fact -- it seems that you've also downvoted me, did you? I'm sorry, but I consider such a behavior to be extremely dishonest. –  Tziolkovski Mar 24 '13 at 14:43
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@SF And here we have similar dishonest thing again. In your original question you've said exactly that: " ...one side of the magnet is extremely strong, the other is barely detectable... " U-shaped magnet was exactly the case in point. It actually answered your question. You just didn't like it to be so obvious. Thus changing the question. Thus downvote. –  Tziolkovski Mar 24 '13 at 16:45
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A Halbach array? Basically the magnetic domains are oriented in a pattern that produces the appearance of a strong magnetic field on one side and a significantly reduced magnetic field on the other side.

http://en.wikipedia.org/wiki/Halbach_array

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I would also guess it's something like this. This is similar to a regular U-magnet, but squashed flat. –  santa claus Mar 27 '13 at 8:02
    
I wonder too - but it doesn't look like the case here. I tried to stick a normal magnet to it - one side - it has two poles, located on the "active surface" symetrically off-center. –  SF. Mar 27 '13 at 18:15
    
@SF. That was exactly what I expected would happen. It's just like a U-magnet! Just imagine someone squashing the U-shaped part until it's almost completely flat. You get exactly the behaviour in the pictures! –  santa claus Mar 27 '13 at 22:03
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The field appears to be unipolar but this is because both poles are on the same side of the magnet. This is the reason that the test magnets snap into a special configuration -- opposite poles attract, and the test magnet also has a "north" and "south" end.

I am only repeating what the other commenters have said, that is, that this is like a U-magnet or a (very) primitive Halbach array.

If you hold two U-magnets "butt-to-butt" they will not attract. This is because a U-magnet is just a bar magnet bent into a "U" shape, and the field lines for a bar magnet go from pole-to-pole. Therefore it makes sense why the field lines would be strongest near the poles, and not at the "butt."

When you squash the "butt" part flat, you get the weird magnet with a strong field on one side.

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If I hold the U-shaped magnets tip-to-tip with tips as far as they are when you put the two butt-to-butt they won't visibly attract each other either. The force is normally inversely proportional to distance from the pole but here 3mm from the pole it's near zero. –  SF. Mar 28 '13 at 22:59
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