Spin Ice: The Monopole Would-Be

Orlando Soto-Cáceres

For decades, physicists have searched for a magnetic monopole. Nevertheless, after so many years of research on has yet to be found. It is common to find magnets that have a north (N) and south (S) pole, but these have never been reported to be separate. This might seem paradoxical, because we can readily find isolated electrical charges such as electrons or ions. In recent years, however, researchers have reported the behavior magnetic monopoles in crystals called spin ice. So are they monopoles or not?

Spin ice crystals are shiny, honey-colored crystals that at first glance, appear to have no particular properties. Put these crystals at really cold temperatures, though, and unusual its unusual characteristics might surprise you. One of these properties is that when cooled nearly to absolute zero, the atomic moments remain in a disordered state, as opposed to the majority of magnetic materials that acquire magnetic order when cooled.

How does this happen? Every atom in a given structure carries with it a magnetic moment, which can be compared to a really small bar magnet. These atomic moments are constantly interacting with each other through their magnetic fields until they line up in order, resulting in the formation of a crystalline structure. In the case of spin ice, the atoms do not acquire the order, but still form the crystalline structure. When these disoriented atoms are in sufficient numbers, they affect the polarization of the crystal, and there is no longer a north or south pole in the structure, but rather in the atoms isolated in space within the crystalline lattice.

Do these “magnetic monopoles” have a charge? In October 2009, Bramwell et al, reported in Nature (journal of scientific research) measuring the force experienced by spin ice’s particles in the presence of a magnetic field. He reported it being approximately 4.6 µB Å-1, a thousand times smaller than that of a “cosmic monopole” - a theoretical monopole whose charge has been calculated by physicists- and that they interact by Coloumb’s law. However, it is important to know that these crystals do not conduct electricity like most metals would, but instead like water. This makes sense, because water is a polar molecule. Therefore, there is no “one charge” to spin ice, but millions of isolated charges entrapped in the crystal.

If it is not a monopole, then what is it? It turns out that although spin ice is not a monopole in itself, due to its inherent quality of containing disoriented atomic moments within it, it is certainly a special kind of material. So special, it can reproduce the effects of a monopole by changes in the interactions and arrangement of atoms. This is particularly interesting because it can give scientists ideas as to how monopoles might originate naturally or of the potential applications of these materials. This means that the search for a real monopole is still on. This makes one wonder: How many more monopolitic materials will be found before encountering the real thing?