21 September 2009 08:07 Age: 170 days
Magnetic monopoles: 70 years from prediction to observation
An essential feature of a magnet – for example a compass needle – is two poles, commonly called ‘North’ and ‘South’, after the directions they point to on Earth. If such a magnet is broken into smaller pieces, this combination of two poles (a dipole), inextricably linked to each other, remains in the fragments, and continues with further division all the way down to an atomic level.
However, back in 1931, the theorist Paul Dirac predicted that magnetic poles could exist as separate entities called monopoles. Since then the search for this species has ranged from outer space to high-energy particle colliders without finding any evidence for their existence.
ILL scientists, working with teams from University College London and Oxford University, report in the journal Science (September 3rd 2009) the first observation of this elusive entity.
Their work focused on Ho2Ti2O7 a member of a family of materials known as ‘spin ices’, in which recent theoretical work had shown magnetic monopoles could exist [cite Nature 451, 42-45 (3 January 2008)]. The magnetic configuration of the dipole moments surrounding the atoms in spin ice arrange themselves in a similar way to the proton arrangement in water ice – hence the name, with two spin pointing in and two spin pointing out of the tetrahedra within the spin ice structure. A monopole is obtained by flipping an individual spin thus enabling the movement of individual magnetic fluxes, monopoles.
The magnetic correlations in Ho2Ti2O7, were mapped out with the polarised neutron spectrometer D7 at ILL, which provides a unique tool to explore the correlations in poorly ordered magnetic materials and enables weak features to be isolated from the large nuclear scattering. D7 was recently upgraded through the ILL’s Millennium Programme, increasing its detection rate by a factor of at least 70, enabling yet more subtle features to be detected and explored. This enabled small disturbances to be seen in the arrangement of the magnetic dipoles in the spin ice that point to the presence of magnetic monopoles.
The rules that constrain the magnetic structure of spin-ices are analogous to those for Coulomb phases in which the interaction energy between particles follow a r-3 dependence. It is therefore expected that excitations that break the constraints create effective monopoles. The measurements on D7 confirmed the presence of such a magnetic Coulomb phase through the observation of characteristic ‘pinch points’ (Figure 1). In the ground state, where there are no monopoles, the pinch points are very sharp. However, on warming to 1K, the formation of monopoles is revealed through a particular disturbance of the magnetic structure, seen in the neutron data as a broadening of the pinch-points. The precise width of the pinch point temperature dependence was observed on the cold-neutron three-axis spectrometer IN12, which is operated by the Forschungszentrum Jülich in cooperation with the CEA Grenoble as a German/French CRG instrument at the ILL (Figure 2). IN12 is dedicated for high resolution studies at low energies allowing a detailed investigation of selected areas in energy and momentum space.
Figure 1. Top: Spin flip scattering at 1.7 K with pinch points at (0 0 2), (1, 1, 1), (2, 2, 2) etc. in the spin-ice Ho2Ti2O7 . Bottom: Spin-flip scattering predicted from Monte Carlo simulation and scaled to fit the experimental data.
Figure 2. Top: spin-flip (SF) and non-spin-flip (NSF) scattering in the region of the pinch-point at (0,0,2. Bottom: empty areas between pinch points are increasingly filled with rising temperature independent of wavevector. The broadening of the pinch points indicates thermal exciation of the monopoles creating finite Dirac-strings
Apart from solving a 70-year magnetic mystery the discovery could have long-term consequences in technology – rather as the discovery of how to manipulate electron spin led to the field of spintronics and the sort of advanced memory technology found in Apple’s iPod.
Professor Andrew Boothroyd a member of the Oxford team and co-author said: ‘It’s perhaps not too far-fetched to imagine that, rather as we can build electrical circuits using positive and negative electrical charge, we might find a way to build magnetic circuits using magnetic monopoles. It might one day lead to new types of memory elements, electrical circuits or sensor devices.’
