UK scientists have directly measured magnetic charge moving in a solid, and proved that the movement exactly parallels the flow of electric charge in ionic solutions.
The team came from the London Centre for Nanotechnology (LCN) and the Science and Technology Facility Council's, ISIS Neutron and Muon Source at the Rutherford Appleton Laboratory in Oxfordshire.
See also: Photo Story - Magnetic monopole flow measured
"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," said Professor Steve Bramwell of LCN. "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 could be in 100 years time."
Magnetic monopoles were first predicted to exist in 1931, but have never been observed as freely roaming elementary particles.
This said, they were recently been shown to exist within a material called 'spin ice' (crystalline Dysprosium titanate Dy2Ti2O7 close to absolute zero where it acts as a 'frustrated magnet' (see below).
The spin ice was held at 0.35K, a temperature where there is just enough energy to flip some spins and create monopole-antimonopole pairs.
A magnetic field applied across the sample pulled apart the monopole pairs, effectively moving magnetic charge and creating a magnetic current.
The experiment was done inside the ISIS particle accelerator where a beam of muons could be used to detect this flow.
According to Giblin, muons implanted into the spin ice decay into positrons whose travel direction depends on the spin of the muon - which is susceptible to atomic magnetic environment where the muon landed.
In effect, the positrons carry out of the sample information about the atomic magnetic environment .
Positrons leaving the sample were detected in two different sensors, and the ratio of the positrons hitting the two targets was used to deduce the flow of magnetic current, dubbed magnetricity by the team.
The movement of magnetic charge was found to map onto Onsager's 1934 theory of the movement of ions in water.
"There is perfect symmetry between electric behaviour and magnetic behaviour," Giblin told EW. "Magnetic charges behave exactly like ions in water."
A magnetic monopole?
Monopoles have not been found alone, however they can exist in a similar way that holes exist in semiconductors.
A sea of bar magnets would align themselves with each other - north (N) to south (S), and S to N - to form one large magnet with, on the small scale, an even field close to the surface.
If a single one of those magnets were reversed, the others would stay still and there would now be a net N magnetic charge - a monopole - where its N end butts against the N end of the next magnet along, and a net S at the other end.
Reversing the next magnet along will move the two charges apart.
Magnetic materials are similar to this sea of magnets, with atoms acting like bar magnets whose polarity is set by spin.
According to Dr Sean Giblin of the Rutherford Appleton Laboratory, strong attraction within conventional ferromagnetic materials makes it close to impossible to switch the spin of one atom as it would immediately flip back.
However, there are 'frustrated' magnetic materials which have atomic dipoles that are trapped in such a way that they can never all be aligned to form any sort of macroscopic magnet.
In frustrated magnets, little energy is required to flip a dipole to form a monopole pair, and little additional energy is required to flip subsequent dipoles to move the charges around.
Such a frustrated magnet, when held close to absolute zero, is Dy2Ti2O7, also known as 'spin ice'.
The ice part of the name is derived from a comparison with frozen water where the proton pair of one molecule is displaced away from proton pair of the next. In spin ice, two spins point one way in the crystal lattice, then the next two point the other way.