You'll have to clarify what you mean by "opposite demagnetised".  That's a contradiction in terms as far as magnetics are concerned.

There are electromagnets that operate on 12v input.  Check McMaster Carr.

But not electromagnets?
Sure, there are assemblies that use permanent magent materials with low resistance to demagnetization so that they can be easily magnetized or reversed with a DC pulse.  To get them really demagnetized though takes an AC pulse.  They are not very stong magnets.

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MagMike (Materials) 5 Jul 06 15:08  
You'll have to clarify what you mean by "opposite demagnetised".  That's a contradiction in terms as far as magnetics are concerned.
 
Thanks for the help MagMike, sorry I see I have made a contradiction. It should have read ... the opposite - magnitised when the 12v currant goes off

Hello HappyDayz,

I am not aware of a standard product that would do that.  

As EdStainless mentioned there are assemblies that can be easily magnetized North or South with a DC pulse.  But it sounds like you are asking to have the polarity flip just because the current was turned off.  It's possible to construct something that would do that, but it would be quite complicated.

What is your application?

Thanks for your help

You can use a permanent magnet to create a magnetic field.  The field can be cancelled by applying power to an electromagnet that bucks the permanent magnet field.