It’s a current! It’s a magnet! It’s . . . magnetricity? Is there even such a thing?
In fact, there is such a thing as “magnetricity,” according to a new study published in Nature Physics by scientists working in the U.K. and in France.
Specifically, the team’s findings have built upon the previously demonstrated phenomenon of “magnetricity” by showing that long-lived currents of magnetic charges can be induced to flow through special substances like the so-called spin ice.
Spin ice’s structure is key to researchers’ ability to observe “magnetricity”: its chemical formula can be either Ho2Ti2O7 (holmium titanate) or Dy2Ti2O7 (dysprosium titanate), but in either compound, the resulting chemical structure is peculiar, resembling the crystal structure of water molecules in ice.
As the individual molecules of spin ice come together, they do so in triangular pyramids, with adjacent pyramids sharing apices.
In spin ice, this strange configuration requires that two points of the pyramid have inward pointing magnetic spins while the other two point out. All the molecules in the array are thus satisfied.
According to Oleg Tchernyshyov, a physicist at Hopkins who is unrelated with the current findings, “spin ice has some resemblance to water ice in that both have some loose dipoles. In water ice, these are electric dipoles (water molecules are polar), whereas in spin ice these are magnetic dipoles.”
However, if something comes along and disrupts the carefully maintained magnetic balancing act, the spin ice is unhappy, and what results is called an excitation of the system. Interestingly, this excitation can lead to an even stranger phenomenon: magnetic monopoles.
In order to restore balance to the excited spin ice configuration, the “defective” magnetic spins are separated.
Sometimes this separation becomes large enough that the two magnetic “charges” act independently — and thus “magnetic monopoles” are born.
These magnetic charges are in a sense free of their original tetrahedral molecular arrangements and of each other. It is their freedom which then allows them to be manipulated by application of magnetic fields into moving as a “magnetric” current.
The amazing thing, however, is that these induced currents can be relatively long-lived — up to minutes, even, according to the study just published in Nature Physics.
While the finding that “magnetricity” exists is not a new one (magnetic charges/monopoles were first theorized by physicist Paul Dirac in the 1930s and magnetic currents were first observed in 2009 by Bramwell and his team in Britain), the ability to better characterize and study the magnetic monopoles responsible for the phenomenon is rather new — and rather exciting, too.
The behavior of “magnetricity” within spin ice in this arrangement is very much analogous to the behavior of electricity.
The magnetic “charges” even act according to predictions made by Coulomb’s laws, which are usually applied to electric circuits.
Furthermore, the behavior of these magnetic “monopoles” in spin ice is highly reminiscent of the behavior of electricity in a well-known electric component: a capacitor. The spin ice seems to store the magnetic “charges” and release them over time — a handy trick if ever “magnetronic” devices should be invented in the future!
Electricity and magnetism have long been known and have shown to be related (think of the electromagnetic light spectrum), but they aren’t quite equal.
As Tchernyshyov wrote, “There is a profound asymmetry in Maxwell’s equation[s]: they include electric charges, but not magnetic ones. Dirac’s idea was to restore a complete symmetry and in the process explain the quantization of electric charge.”
Despite the new findings showing that magnetic counterparts to electric charges can exist, magnetic currents and electric currents are different.
In particular, the movement of the charges through their respective conductors is quite disparate: whereas electrons can flow through a material to create a current, magnetic charges only move locally and instead induce a “current” by inciting their neighboring dipoles to flip.
“Collective motions of these [magnetic] dipoles can be seen as the propagation of a single monopole over a long distance,” Tchernyshyov wrote.
In other words, this domino effect of long lines of dipoles flipping is what is observed as “magnetricity.”
One monopole’s effective path through a conductor inherently blocks the way for subsequent monopoles; this means “magnetricity” will never be able to achieve direct currents (DC), though it is possible that alternating current (AC) based “magnetronic” devices could.
String theory buffs out there, don’t get too excited just yet: monopoles observed in spin ice aren’t quite the true magnetic monopoles of Dirac’s predictions (monopoles which would prove to be the holy grail for string theorists).
However, they are important models for how such charges might act and react empirically.
That’s not to say that magnetic monopoles can’t exist out of the stringent conditions found in spin ice (which must be kept in a super-condensed state at nearly absolute zero).
In fact, other teams of researchers have already shown that microarrays of nanowires can exhibit the same “magnetric” properties as spin ice — so there is still hope for finding those elusive monopoles.
