Isolated Magnetic Charges - The Search for Magnetic Monopoles

We all learned about isolated electric charges in high school. These are the reason why balloons stick to walls after rubbing them on a course surface such as your jumper - the electric charges are manipulated inside the balloon and a difference of charge (hence, attraction) is built up between the wall and the balloon. First predicted by Dirac in the 1930's, isolated magnetic "charges", or monopoles have proved elusive. The search for these has led scientists to investigate an exotic class of materials called spin ice. 

Magnetic monpoles have been found at low and intermediate temperatures as quasi-particle excitations in the spin ice materials, which are an unconventional type of magnet. Usually we think of magnets with a north and south pole. If you cut a bar magnet in half, you get two smaller magnets with respective north and south poles. The existence of monopoles would provide a perfect duality between electricity and magnetism, are required by theories such as string theory and other grand unification theories, and provide an explaination for the quantised nature of electric charge. 

Neutron scattering experiments on spin ice were able to reveal signatures of these monopole excitations, usually at intermediate temperatures. Recently, a study in Nature Materials used diffuse neutron scattering to look at a certain plane in spin ice. This plane (hk7) contains a large concentration of oxygen atoms. It was found that monopoles are pinned to oxygen defects and this has a big effect on the dynamics of the monopoles. 

A Nature Physics paper reported on an upturn in the specific heat of spin ice at long-timescales (more than a million seconds or so - much longer than "standard" measurements ~ 600 s). If correct, this suggests previously unseen order in spin ice at low temperatures, which is not understood. Understanding the oxygen defects with diffuse neutron scattering is the first step towards finding out.