A magnetic charge can behave and interact just like an electric charge in some materials, according to new research led by the London Centre for Nanotechnology (LCN) which could lead to a reassessment of current magnetism theories, as well as significant technological advances.
The research, published today in Nature (1), proves the existence of atom-sized magnetic charges called ‘magnetic monopoles’ that behave and interact just like more familiar electric charges. It also demonstrates a perfect symmetry between electricity and magnetism – a phenomenon dubbed ‘magnetricity' by the authors from the LCN and STFC’s ISIS Neutron and Muon Source .
In order to prove experimentally the existence of magnetic current for the first time, the team mapped Onsager's 1934 theory of the movement of ions in water onto magnetic currents in a material called spin ice. They then tested the theory by applying a magnetic field to a spin ice sample at a very low temperature and observing the process using muon relaxation at ISIS, a technique which acts as a super microscope allowing researchers to understand the world around us at the atomic level.
The experiment allowed the team to detect magnetic charges in the spin ice (Dy2Ti2O7), to measure their currents, and to determine the elementary unit of the magnetic charge in the material. The monopoles they observed arise as disturbances of the magnetic state of the spin ice, and can exist only inside the material.
Professor Steve Bramwell, LCN co-author of the paper, said: “Magnetic monopoles were first predicted to exist in 1931, but despite many searches, they have never yet been observed as freely roaming elementary particles. These monopoles do at least exist within the spin ice sample, but not outside.
"It is not often in the field of physics you get the chance to ask 'How do you measure something?' and then go on to prove a theory unequivocally. This is a very important step to establish that magnetic charge can flow like electric charge. It is in the early stages, but who knows what the applications of magnetricity could be in 100 years time."
Dr Sean Giblin, instrument scientist at ISIS and co-author of the paper, added: “The results were astounding, using muons at ISIS we are finally able to confirm that magnetic charge really is conducted through certain materials at certain temperatures – just like the way ions conduct electricity in water.”
Figure 1: Magnetic Wien effect, and the detection of magnetic charge by implanted muons.
a, In zero field, magnetic charges occur as bound pairs, but some dissociate to give a fluctuating magnetic moment (green arrow).
b, The field energy -QBrz competes with the Coulomb potential -micro0Q2/4pir to lower the activation barrier to dissociation.
c, The application of a transverse field causes dissociation as charges are accelerated by the field.
d, In the applied field, these charges remain dissociated while more bound pairs form to restore equilibrium. Magnetic moment fluctuations due to free charges produce local fields that are detected by implanted muons (micro+).
(1) Measurement of the charge and current of magnetic monopoles in spin ice
S. T. Bramwell, S. R. Giblin, S. Calder, R. Aldus, D. Prabhakaran & T. Fennell
Nature 461, 956-959(15 October 2009)