Large-scale cousin of elusive ‘magnetic monopoles’ found at NIST

By | October 7, 2009

Image: Magnetic monopoles are created (top) when the spin of an ion in one corner of a spin ice crystal is knocked askew, creating a monopole (red sphere) and adjacent antimonopole…

Any child can tell you that a magnet has a “north” and a “south” pole, and that if you break it into two pieces, you invariably get two smaller magnets with two poles of their own. But scientists have spent the better part of the last eight decades trying to find, in essence, a magnet with only one pole. A team working at the National Institute of Standards and Technology (NIST) has found one.*

In 1931, Paul Dirac, one of the rock stars of the physics world, made the somewhat startling prediction that “magnetic monopoles,” or particles possessing only a single pole—either north or south—should exist. His conclusion stemmed from examining a famous set of equations that explains the relationship between electricity and magnetism. Maxwell’s equations apply to long-known electric monopole particles, such as negatively charged electrons and positively charged protons; but despite Dirac’s prediction, no one has found magnetic monopole particles.

Now, a research team working at NIST’s Center for Neutron Research (NCNR), led by Hiroaki Kadowaki of Tokyo Metropolitan University, has found the next best thing. By creating a compound that under certain conditions forms large, molecule-sized monopoles that behave exactly as the predicted particles should, the team has found a way to explore magnetic monopoles in the laboratory, not just on the chalkboard. (Another research team, working simultaneously, published similar findings in Science last month.**)

“These are not the monopole particles Dirac predicted—ours are huge in comparison—but they behave like them in every way,” says Jeff Lynn, a NIST physicist. “Their properties will allow us to test how theoretical monopole particles should behave and interact.”

The team created their monopoles in a compound made of oxygen, titanium and dysprosium that, when cooled to nearly absolute zero, forms what scientists call “spin ice.” The material freezes into four-sided crystals (a pyramid with a triangular base) and the magnetic orientation, or “spin,” of the ions at each of the four tips align so that their spins are balanced—two spins point inward and two outward. But using neutron beams at the NCNR, the team found they could knock one of the spins askew so that instead three point in, one out … “creating a monopole, or at least its mathematical equivalent,” Lynn said.

Because every crystal pyramid shares its four tips with adjacent pyramids, flipping the spin of one tip creates an “anti-monopole” in the next pyramid over. The team has created monopole-antimonopole pairs repeatedly in a relatively large chunk of the spin ice, allowing them to confirm the monopoles’ existence through advanced imaging techniques such as neutron scattering.

While the findings will not tell the team where in the universe to search for Dirac’s still-elusive magnetic monopole particles, Lynn says that examining the spin ice will permit scientists to test certain predictions about monopoles. “Maxwell’s equations indicate that monopoles should obey Coulomb’s Law, which indicates their interaction should weaken as distance between them increases,” he says. “Using the spin ice crystals, we can test ideas like this.”

via Large-scale cousin of elusive ‘magnetic monopoles’ found at NIST.

Yeah, so what? What’s the big deal about a magnet with only one pole? I’ve read some interesting claims like this one from June 2004:  High Energy Magnetic Monopole Sequestered by U.S. Government .


… The monopole is thought of as electric charge’s magnetic cousin, but unlike positive or negative charges, north or south poles always occur together in what’s called a dipole. A lone north or south pole simply doesn’t show up in the real world. Even if you take a bar magnet and cut it in half down the middle, you won’t get a separate north and south pole, but two new dipole magnets instead. For symmetry-minded theorists, however, it’s natural that there should be a magnetic equivalent of charge. String theories and grand unified theories rely on its existence, and its absence undermines the mathematical feng-shui of the otherwise elegant Maxwell’s equations that govern the behavior of electricity and magnetism. What’s more, the existence of a magnetic monopole would explain another mystery of physics: why charge is quantized; that is, why it only seems to come in tidy packets of about 1.602×10–19 coulombs, the charge of an electron or proton.

For decades, scientists have kept their eyes peeled for the elusive monopole, but perhaps they were looking in the wrong place. – symmetrymag

As I understand it from this Scientific American article, north and south still exist, but in pairs that cancel eachother out. Macroscopically you have a monopole acting material made up, on a small scale, of frustrated partially caged north and south poles which are unable to align.

… A pair of papers published online this week in Science offer experimental evidence that such monopoles do in fact exist, albeit not as electron-like elementary particles, a caveat that one self-professed purist says disqualifies them from genuine monopole status.

Both studies examine the magnetic behavior of a family of rare-earth materials known as spin ices—one group using holmium titanate and the other dysprosium titanate. The man-made spin ices take their name from their similarity to water ice—at the molecular level their internal magnetic structure is analogous to the arrangement of protons in ice.

Claudio Castelnovo, a postdoctoral physicist at the University of Oxford who co-authored one of the Science papers and also co-wrote a paper in Nature last year describing how monopoles might be realized in spin ices, explains that the compounds offer a peculiar combination of order and freedom that facilitates the dissociation of the poles.

At low temperatures, there is still some magnetic wiggle room in the spin ice’s lattice structure, but not a lot—the magnetic freedom of the system is frustrated, so to speak. “As a result, this is a substance that has degrees of freedom that look the same, microscopically, as you would see in a fridge magnet,” Castelnovo says. “But a fridge magnet is able to order so as to act as a fridge magnet and stick to metals, while this one is not able to achieve this level of ordering in spite of having this magnetic structure inside, because of this frustration.”

Internally, the tiny magnetic components arrange themselves head to tail in strings, like chains of bar magnets stretching across a table in different directions. In a very cold, clean sample, those strings form closed loops. But excitation induced by a rise in temperature can introduce tiny defects in these chains, Castelnovo says—in the bar-magnet analogue, one of the magnets is flipped, breaking the head-to-tail continuity. “You have your path that is north–south–north–south, and at a certain point one of the needles actually twists 180 degrees and points the wrong way,” he explains.

On either side of that defect, all of a sudden, is a concentration of magnetic charge—two norths at one end, two souths at the other. Those concentrations can float free along the string, acting as—voilà—magnetic monopoles.

“The beauty of spin ice is that the remaining degree of disorder in this low-temperature phase makes these two points independent of each other, apart from the fact that they attract each other from a magnetic point of view because one is a north and one is a south,” Castelnovo says. “But they are otherwise free to move around.”

Of course, this method of synthesizing monopoles cannot bring a north into existence without also generating a south—the key is their dissociation. “They always have to come in pairs,” Castelnovo says, “but they don’t have to be anywhere specifically in relation to one another.”s

One thought on “Large-scale cousin of elusive ‘magnetic monopoles’ found at NIST

  1. Peter Martel

    The magnetic monopole “verified” by neutron scattering isn’t quite as nimble as the electron monopole; however, who am I to quibble. Since the results do not seem to defy Dirac’s theory, let’s accept the experimental results for now.
    At the very least, the neutron results are incontrovertible in themselves.
    Peter Martel

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