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I opened up my hard drive and pulled out (among other things) the top and bottom braces for the reading arm.

Each bracket contains a really powerful neodymium affixed to its underside.

Hard drive magent from Wikipedia

When the two undersides come within a few inches of eachother, they accelerate toward each other and are seemingly unstoppable.

However, when they are placed close together but back to back (i.e. the magnets on the underside do not face eachother) there is no attraction whatsoever!

Somehow the material the bracket is made of is completely shutting down the magnetic field. This is strange because I tried placing both magnets on different sides of metal boards, wood, plastic etc and they are always attracted. But back to back, even up close, there is no attraction.

How does this work? Why is this happening?

Neodymium back to back

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    $\begingroup$ You probably read this on wikipedia, but just in case somebody else has not... The greater force exerted by rare-earth magnets creates hazards that are not seen with other types of magnet. Neodymium magnets larger than a few cubic centimeters are strong enough to cause injuries to body parts pinched between two magnets, or a magnet and a metal surface, even causing broken bones. $\endgroup$
    – user81619
    Aug 12, 2015 at 20:37
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    $\begingroup$ I doubt the bracket is blocking the magnetic field. But if those are typical hard-drive magnets then they are quadrupole magnets. This means that while they may have very large surface fields, their field strengths drop off very fast with distance. Also, you will find that it takes some figuring out to know which orientations result in them attracting and which result in them repelling. If they are beside each other and attracting and you just flip one over then they will repel. You need to then rotate about a perpendicular axis to get them back to attracting. $\endgroup$
    – gleedadswell
    Aug 12, 2015 at 23:23
  • $\begingroup$ @gleedadswell yes they are exactly as you describe. Yet still when back to back touching, they literally do not attract or repel at all. It's quite strange. I've tested with a thick wood desktop so I know they can certainly attract from an inch away $\endgroup$
    – CodyBugstein
    Aug 13, 2015 at 1:41
  • $\begingroup$ Well, you can't block magnetic fields (unless you discover the mythical magnetic monopoles...). So my guess is that the brackets are just every-day steel and their magnetic permeability is high enough that the fields are routed through the brackets, so the field outside the brackets is very small. I'm surprised there isn't enough stray field outside the brackets to result in an observable magnetic force, but I can't say I have a great intuition for this. Someone who designs electrical transformers for a living could probably comment more intelligently on stray fields. $\endgroup$
    – gleedadswell
    Aug 13, 2015 at 23:58

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Those are nickel/steel brackets that have high magnetic permeability and saturation. Due to the magnets shallow thickness the magnetic fields are completely diverted into the bracket preventing any mag fields from penetrating through it. Notice how the magnet is strongly attracted to the bracket due to magnetic attraction, if you unglue the magnet from the bracket and attempt to block two magnets from eachother you will notice that one magnet will (strongly) stick to one side and if you attempt to attach a magnet on the other side it will push the other magnet off the other side. This happens due to oversaturation and the magnetic fields begin competing with eachother.

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