Available online 29 March 2011.
Physicists from the UK and France have developed a new approach to inducing magnetic charges to flow in tiny crystals using short magnetic-field pulses, in a similar way to electrical charges flowing through batteries and biological systems. They have created an environment where long-lived currents of magnetic monopoles can be created and manipulated through applying magnetic fields in spin ice, a crystalline material, in what is termed ‘magnetricity’.
The study, published in Nature Physics [Giblin et al. Nature Physics (2011) doi: 10.1038/nphys1896], revealed that the magnetic molecules in spin ice are unlike those in normal magnets, which always consist of dipoles. In spin ice they are arranged in triangular pyramids that stop them from being able to line up easily with all their poles pointing in the same direction. The researchers showed that a molecule will occasionally squirm and flip over, creating two separate poles, with the actual molecule remaining where it was. The new poles are able to move about independently of each other while continuing chain reactions from the flipping molecules move them on from one pyramid to another.
Once the poles have moved far enough away from the original molecule, they start to lose all memory of each other, and the dipole becomes split in two, creating two monopoles. Although monopoles are more commonly associated with magnetic charges inside a vacuum, in spin ice the poles behave similarly, with each having a magnetic charge that interacts with each other in the same way as electric charges.
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Currents of magnetricity are born when north poles and south poles split up and move around independently of each other. Figure courtesy of Steven Bramwell.
The breakthrough came years after the discovery of spin ice as a new form of ferromagnetism, and much more research into its properties, with Steven Bramwell in particular focusing on the behavior of magnetic excitations in spin ice. In 2009 the research team obtained the first evidence of the magnetic currents of these charges using muon spin rotation, and the new study originated from a desire to confirm this through the more straightforward method of magnetometry and the development of a capacitor effect for the magnetic charges. Achieving this meant they could identify the microscopic properties of monopoles.
The team now hope to develop films of spin ice and examine couplings with electric and electromagnetic fields, and also to examine the properties of artificial spin ices: arrays of micromagnets designed to mimic spin ice.
The study shows that magnetic currents acting just as electric currents can exist in real ferromagnet-like materials, so magnetronic circuitry and devices may be possible in the future. However, as the currents are only exhibited in crystals kept near to absolute zero, this will not be easy, so they are looking to expand the temperature range where the currents occur, and to investigate links with other phenomena, such as light or electrical impulses.