About magnetic monopoles...........

According to various media outlets ran news of the discovery of magnetic monopoles in spin ice.

Man, a little witness news in physics, it a cursory reading can be confusing. It seemed useful to clarify that in fact it was made, and to emphasize about this news a few things.

What was found?

First of all, we are here of course not talking about the opening of the magnetic monopole as a single elementary particle .Such particles have been, and remain hypothetical objects, whether they exist in nature or not - is unknown.

What is observed experimentally in several papers - is the collective electronic excitations in a special magnetic media, looking mesoscopic (ie, at distances much greater than atomic) if magnetic monopoles.

In principle, the media reports it is also written, but for some reason it notes begin with the words of these Dirac monopoles and sometimes about their quest, and then move to new jobs. This is confusing.

Where have the monopoly?

Now it's just a picture that helps to visualize yourself these monopoles.

What is a magnetic monopole? This is a "magnetic charge", iemagnetic analog of an isolated electric charge. Ie such a facility from which the radiating magnetic field lines, and where the magnetic field decreases with the distance in Coulomb's law 1 / r ^ 2.

A distant analog of a magnetic monopole is well known to those who know even a little about magnetism - are domain walls separating domains in ferromagnets, and even better - in a ferromagnetic wire. In it is also on both sides stick the magnetic field lines. Only two-dimensional domain wall and monopole - a point.

As such monopoles can I create? Imagine a chain of atoms with nonzero spin (a spin of the electron shells of atoms is exactly generate magnetism). You can arrange the interaction between the spins, so that in the normal state all the spins in the chain is looking at each other in the head:

.... O → o → o → o → o → o → o → o. ...

This chain generates a more or less uniform (along the chain), the magnetic field, it looks like a thin and long solenoid. If now deployed back in some areas, we get this structure:

.... O → o → (-) ← o ← o ← (+) → o → o. ...

Ie atoms, which is a turn directions, you would like to compare the positive and negative magnetic charges - ie, plot of "incorrect" orientation of the spins is restricted to druh sides "magnetic charges".

This is, in principle, normally, in such a situation, yet no one talks about watching monopoles - in fact we have a chain, and there it ends. The situation changes if we have the material, which (with a magnetic point of view), consists entirely of weave such chains, the magnetic field from which the average offset. Then, if some of the chains to create an "incorrect" area, then it ends - the magnetic charges - will be seen, but the chain itself - no. Ie it would seem that in the matter at a distance from each other are two opposite magnetic charge.

And now the final step - the interaction between the electrons in this material should be arranged so that the chain is not restrained, these magnetic charges that it allowed them to easily be removed at any distance. And if you want to jump to another chain. Ie that the "pole of one magnet" ceased to be associated with each other (although the two poles, of course, somewhere present in the sample). That's when you can already speak about separate trips through the crystal positive and negative magnetic charges. It is these individual "magnetic defects of the crystal and were detected in the experiment.

Various comments

What's wrong with that trivial? Non-trivial to find such a substance in which there are lined up and bound to each other spin chains. At the same substance should not be spontaneously magnetized. Moreover - in the substance of the spins must be some "freedom of the coup, if the neighboring spin flipped. Finally the interaction between the electrons should not be too harsh, it must allow the spins to flip, and a magnetic charge removed from each other.

All this could be realized in the material, known as spin ice . On the one hand, such a material (at low temperatures, of course) has a completely clear crystal lattice. But on the other hand, there are four equal-free spin on each site - two look inside, two outside. This configuration allows several different orientations, which is very similar to the orientation of water molecules in ordinary ice. It is this diversity more energetically equivalent orientations of the spins and gives freedom to the coup.

This type of phenomenon, incidentally, is called in physics a beautiful word frustration , and materials themselves -frustrated .

By the way, the necessary condition here is the presence of a sufficiently large number of free spins in the outer electron shell of atoms. This explains why, in the compounds include f-elements such as dysprosium in Dy ₂ Ti ₂ O ₇ .