Magnets always have a north pole and a south pole. Physicists have managed to separate them in unusual materials called spin ices, enabling each pole to move freely. Image: Cordelia Molloy Photo Researchers, Inc.
Editor's note: The original online version of this story was previously posted.
Magnets are remarkable exemplars of fairness—every north pole is invariably accompanied by a counterbalancing south pole. Split a magnet in two, and the result is a pair of magnets, each with its own north and south. For decades researchers have sought the exception—namely, the monopole, magnetism’s answer to the electron, which carries electric charge. It would be a free-floating carrier of either magnetic north or magnetic south—a yin unbound from its yang.
Two research groups—one led by Tom Fennell of the Laue-Langevin Institute in Grenoble, France, and the other by Jonathan Morris of the Helmholtz Center Berlin for Materials and Energy—have offered experimental evidence that such monopoles do in fact exist, albeit not as electronlike elementary particles. Rather they exist as unbound components inside so-called spin ices. These man-made materials take their name from their similarity to water ice in terms of their magnetic nature. The French-led team experimented with holmium titanate and the Germany-based group, dysprosium titanate.
Claudio Castelnovo, a physicist at the University of Oxford on the Morris team, explains that the compounds offer a peculiar combination of order and freedom that facilitates the dissociation of the poles. Internally, the tiny magnetic components in spin ices 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 then the physicists gave a little kick to the system by increasing the temperature. The rise excited the components and introduced defects in these chains, Castelnovo explains—in the bar-magnet analogue, one of the mag-nets is flipped, breaking the head-to-tail continuity.
On either side of that defect, then, are two norths at one end and two souths at the other. Those concentrations of charge can float free along the string, acting as—voilà—magnetic monopoles, which the teams conclude they saw based on the way neutrons scattered off the spin ices. “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 points out. “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 each other.”
But Kimball Milton, a University of Oklahoma physicist who reviewed the status of monopole searches in 2006, is not convinced. A genuine magnetic monopole “implies to me it’s a point particle, and it’s not” in the studies, Milton says. “It’s an effective excitation that at some level looks like a monopole, but it’s not really fundamentally a monopole.”
He also asserts that it is “completely wrong” to describe, as the researchers do, the chain of magnetism within spin ices as a Dirac string, a hypothetical invisible tether with a monopole at its end that was envisioned in the 1930s by English physicist Paul Dirac. The magnetic strings in the spin ice do not fit the Dirac definition, Kimball feels, because they are, in fact, observable and merely carry flux between two opposing so-called monopoles. “Real monopoles, if they existed, would be isolated, and the string would run off to infinity,” he insists.