Magnetic monopoles - the magnetic version of a charged particle like electrons or protons - have recently been shown to exist in a special material called "spin ice". In the last two years Prof. Steve Bramwell (LCN) and his collaborators have demonstrated the existence of these monopoles and have observed their currents in a magnetic version of electricity, or "magnetricity".
Prof. Bramwell's team has now linked up with several North American laboratories to create a new spin ice material that contains a particularly high concentration of magnetic monopoles. The monopoles are so concentrated in this material that a significant fraction of them pair up to form dimers.
Prof. Bramwell and Mr. Jonathan Bloxsom of the LCN collaborated with Dr. Haidong Zhou (NHFL, Florida), Dr. Jason Gardner (NIST, Washington DC), Prof. Chris Wiebe (Winnipeg) and their co-workers.
Spin ice materials are simple transparent crystals that include atoms of "rare earth" elements. For a material to be spin ice it must have a particular crystal structure, but not all rare earth materials have this structure. The researchers therefore applied high pressure and high temperature to a non-spin ice rare earth material, dysprosium germanate, to turn it into spin ice. As the pressure was released, the new material remained a dense form of spin ice. The extra density allowed a greater concentration of magnetic monopoles to form when the spin ice was cooled to very low temperatures. By adding small amounts of heat to the sample the workers were able to show that about half of the magnetic monopoles had formed into pairs of opposite charge, so as to minimise the interaction energy, which is stronger in the denser material.
"This shows how we can control monopole properties through materials science" said Professor Bramwell. "Our collaborators in North America created a new spin ice material under extreme conditions, and we have showed how the monopoles in this new material interact particularly strongly. To relate monopole properties to material structure in this way is a prerequisite for any application of magnetic monopoles in spin ice."
The work is published this week in Nature Communications . It is the latest development following Prof. Bramwell's discovery of spin ice in 1997 and his discovery of magnetic monopoles and magnetricity in spin ice in 2009.
 High Pressure Route to Generating Magnetic Monopole Dimers in Spin Ice, Zhou, Bramwell et al., Nature Comm. (2011): DOI 10.1038/ncomms1483. http://dx.doi.org/
Figure: A spin ice created at high pressure and high temperature has more paired monopoles than an ordinary spin ice.