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May 08, 2009

Magnetic Monopoles Found in Spin Ice

A special material called spin ice, co-discovered in 1997 by Professor Steven Bramwell of the London Centre for Nanotechnology has come close to revealing a secret of the universe.

Most computer memories store information magnetically, and if there was the ability to use magnetic rather than electric charges to read and write bits to and from those stores could have great advantages in speed and flexibility. What's more, the three-dimensional configuration of spin ice might allow for memories of much higher density than is currently possible.

Magnetic Monopoles are described at wikipedia

Spin Ice is described at wikipedia



Tiny magnetic poles that can move around a lump of matter are of interest for another reason: they might be exploited in futuristic memory elements for computers.

A magnet has two poles - north and south - but cut a magnet in half and each half has its own north and south pole. However much you divide a magnet, you can never isolate a pole on its own. You can’t say carry a north pole in your pocket and leave the south pole at home. Yet if such isolated poles or “monopoles” were to exist they would explain a lot about the universe, as first realised by the Paul Dirac, the pioneer of quantum mechanics, in the 1930s. Physicists have hunted for these monopoles for years because of the key role they play in the theory of the universe - but without success.

Ordinary water ice (H2O). has the strange property that the hydrogen (H) atoms remain disordered even at the absolute zero temperature. Spin ice is a substance composed of atoms like tiny magnets which show exactly the same pattern of disorder as ice’s hydrogens (the name ‘spin ice’ arises because the atoms’ magnetism is caused by spinning electrons). It turns out that if you lived inside a lump of spin ice - if spin ice was your universe - then you WOULD be able to isolate magnetic monopoles, as they really exist within the material. This was recently proved by scientists from Oxford, Dresden and Princeton. However, these monopoles cannot escape the material so are not exactly the elementary monopoles from the dawn of time dreamed of by particle physicists
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New Scientist Magazine has extensive coverage

It seems the elusive monopoles have been pinned down at last. But Blas Cabrera, who looked for monopoles in cosmic rays passing through his laboratory at Stanford University in the 1980s, sounds a note of caution. The monopoles discovered in spin ice are rather different beasts from those he and others were looking for. For a start, they are some 8000 times less magnetic and are free to move only within the spin ice, not to roam the wider universe. So they are not really analogous to electric charges, and it doesn't look as if they are going to solve the dark matter problem.







Do they count at all? Quite possibly. When Dirac dreamed up his cosmic monopoles, he imagined a vacuum as the lowest possible energy state that free space could assume. Monopoles then represented a higher-energy "excitation" of a vacuum, in much the same way that the low-energy two-in, two-out spin-ice state is excited to create monopoles. The new research even borrows elements of Dirac's description of free-space monopoles - such as the invisible "strings" he envisaged between pairs of poles that have separated. The similarities mean that the interactions of spin-ice monopoles could provide a way to learn about cosmic monopoles by proxy - for example, how they might have interacted in the early universe.


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