October 16, 2009

Magnetricity Charge and Current of Monopoles in Spin Ice

Magnetic Wien effect, and the detection of magnetic charge by implanted muons.

‘Magnetricity’ observed and measured for the first time

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 in Nature, 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.

BBC News coverage of magnetricity

But by engineering different spin ice materials to modify the ways monopoles move through them, the materials might in future be used in "magnetic memory" storage devices or in spintronics - a field which could boost future computing power.

Popular Science coverage

This new discovery opens up the possibility that magnetic monopoles could be used for computer storage. If magnetic polar identity can flow through crystals of spin ice, then the current of identity could replace positive and negative charges with positive and negative monopoles as the information storage medium. And since controlling the magnetic identity of electrons underlies quantum computing, this ability to alter that identity with a current positions spin ice as the new, leading candidate for quantum computing chips.

New Scientist coverage

The result could lead to the development of "magnetronics", including nano-scale computer memory.

In September, two teams of physicists fired neutrons at spin ices made of titanium-containing compounds chilled close to absolute zero. The behaviour of the neutrons suggested that monopoles were present in the material.

To get more detailed information on the monopoles than had previously been possible, Bramwell's team injected muons – short-lived cousins of electrons – into the spin ice. When the muons decayed, they emitted positrons in directions influenced by the magnetic field inside the spin ice.

This revealed that the monopoles were not only present but were moving, producing a magnetic current.

It also allowed the team to measure the amount of magnetic charge on the monopoles. It turned out to be about a 5 in the obscure units of Bohr magnetons per angstrom, in close agreement with theory, which predicted 4.6. Unlike the electric charge on electrons, which is fixed, the magnetic charge on monopoles varies with the temperature and pressure of the spin ice.

From Nature:

Measurement of the charge and current of magnetic monopoles in spin ice

The transport of electrically charged quasiparticles (based on electrons or ions) plays a pivotal role in modern technology as well as in determining the essential functions of biological organisms. In contrast, the transport of magnetic charges has barely been explored experimentally, mainly because magnetic charges, in contrast to electric ones, are generally considered at best to be convenient macroscopic parameters rather than well-defined quasiparticles. However, it was recently proposed that magnetic charges can exist in certain materials in the form of emergent excitations that manifest like point charges, or magnetic monopoles3. Here we address the question of whether such magnetic charges and their associated currents—'magnetricity'—can be measured directly in experiment, without recourse to any material-specific theory. By mapping the problem onto Onsager's theory of electrolytes, we show that this is indeed possible, and devise an appropriate method for the measurement of magnetic charges and their dynamics. Using muon spin rotation as a suitable local probe, we apply the method to a real material, the 'spin ice' Dy2Ti2O7 (refs 5–8). Our experimental measurements prove that magnetic charges exist in this material, interact via a Coulomb potential, and have measurable currents. We further characterize deviations from Ohm's law, and determine the elementary unit of magnetic charge to be 5 B Å-1, which is equal to that recently predicted using the microscopic theory of spin ice. Our measurement of magnetic charge and magnetic current establishes an instance of a perfect symmetry between electricity and magnetism.

Condensed-matter physics: Wien route to monopoles

Determining the magnetic charge of monopoles in a crystalline host seemed a mountain too high for physicists to climb. An experiment based on Wien's theory of electrolytes has now measured its value.

The exotic class of crystalline solids known as 'spin ices' has proved, perhaps surprisingly, to be a repository of some elegant physical phenomena. Spin ices are rare, three-dimensional systems in which the magnetic moments (spins) of the ions remain disordered even at the lowest temperatures available.

Magnetic Charge Transport (19 page pdf)

Bramwell papers on arxiv

Zero-point entropy of the spinel spin glasses CuGa_2O_4 and CuAl_2O_4

Experimental Proof of a Magnetic Coulomb Phase

Pinch Points and Kasteleyn Transitions: How Spin Ice Changes its Entropy

Title: Dy2Ti2O7 Spin Ice: a Test Case for Emergent Clusters in a Frustrated Magnet

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