Monopoles apart
Four research papers, two of which were published in the journal Science recently, and two submitted to the physics preprint archive, suggest that a long-sought icon of fundamental physics has finally been discovered - the magnetic monopole. This fundamental research could have enormous potential in materials research, nanotechnology, and eventually instrumentation.
We are all familiar with separated charges - electrons are negative, protons are positive, but the magnetic equivalent - separated particles with just a "north" and "south" pole have always remained elusive entities. After all, chopping a bar magnet in half, again and again simply results in aller and aller N-S bar magnets. The limit would be a north particle with no south pole, but scientists have never managed to "chop up" a magnet to reach this ultimate state.
For the last 70 years, however, physicists have been searching for magnetic monopoles with some degree of anticipation. Their quest hinged on the work of various scientists in particular British physicist Paul Dirac who postulated in 1931 that magnetic monopoles could exist at the end of tube-like Dirac strings.
Rather than existing throughout the universe, however, such magnetic monopoles would have to exist within a special type of material, a spin ice. In this state one can picture free-floating north and south poles. To a hypothetical observer in the material, they would appear as magnetic monopoles.
Tom Fennell and colleagues Pascale Deen, Andrew Wildes, and K. Schmalzl of the Institute Laue Langevin, in Grenoble, France, worked with Dharmalingan Prabhakaran and Andrew Boothroyd of the Clarendon Laboratory Oxford Physics and Bob Aldus, Des McMorrow, and Steve Bramwell of the LCN, to carry out neutron scattering experiments to image the world inhabited by such monopoles in the material holmium titanate.
"Spin ice materials are magnetic substances in which the spin directions map onto hydrogen positions in water ice," the researchers explain, "Their low-temperature magnetic state has been predicted to be a phase that obeys a Gauss' Law and supports magnetic monopole excitations: in short, a Coulomb phase."
In a second paper, Bramwell and his team working with colleagues at ISIS in Oxford produced subatomic particles called muons that were then used as a probe for the monopoles in dysprosium titanate spin ice. In this experiment the charge of the monopole was directly measured and found to be equal to that predicted by theory.
In independent work, Jonathan Morris, Alan Tennant and colleagues at the Helmholtz-Zentrum Berlin für Materialien und Energie have, in cooperation with colleagues from Dresden, St. Andrews, La Plata, Argentina, and Oxford , for the first time observed magnetic monopoles and how they emerge in a real material. They too used neutron scattering to investigate a single crystal of dysprosium titanate. This material crystallises with a remarkable geometry, the so-called pyrochlore lattice.
The team saw that the magnetic moments within this material had reorganised into a "spin-spaghetti" of contorted dipole strings. "These recent papers provide overwhelming proof of the existence of magnetic monopoles in spin ice," explains Bramwell, "in particular we have measured the monopole charge and observed monopole currents ogous to electricity. We have also used neutrons to measure the length of the so-called Dirac strings that run between North and South monopoles.
Bramwell adds that the results demonstrate how real materials can form particles within that resemble the basic building blocks of the universe. "The amazing thing about spin ice monopoles," he says, "is how perfect they are: they really do look just like those monopoles expected to exist somewhere in the universe."
In the Morris work, the existence of magnetic monopoles is described as an emergent state of matter. They form from special arrangements of dipoles and are completely different from the constituents of the material within which they form. "We are writing about new, fundamental properties of matter," says Morris, "These properties are generally valid for materials with the same topology, that is for magnetic moments on the pyrochlore lattice. For the development of new technologies this can have big implications. Above all it signifies the first time fractionalisation in three dimensions is observed."
Bramwell also sees incredible implications in the discovery of magnetic monopoles. "Why nature should reproduce a mini-universe within a material, we do not yet know," he says. "As well as having implications for fundamental physics, the monopoles could be harnessed in the way that electrical charges are, for technology. A magnetic version of electricity is a long way off," he empahsises, "but these results are an important first step".
