Freezing magnetic monopoles: How dipoles become monopoles and vice versa

Aug 09, 2012
The unit cell for "spin ice" materials consists of two tetrahedrons. The arrows show the orientation of the magnetic atoms within the material Credit: Stephen Powell

Magnetic monopoles, entities with isolated north or south magnetic poles, weren't supposed to exist. If you try to saw a bar magnet in half, all you succeed in getting are two magnets, each with a south and north pole. In recent years, however, the existence of monopoles, at least in the form of "quasiparticles" consisting of collective excitations among many atoms, has been predicted and demonstrated in the lab. Now Stephen Powell, a scientist at the Joint Quantum Institute (JQI) and the University of Maryland, has sharpened the theoretical framework under which monopoles can operate.

"Steady flows of magnetic monopoles are apparently impossible," Powell said, "but transient currents have been demonstrated, and one could imagine creating an alternating current, the magnetic equivalent of AC electricity." This so-called 'magnetricity' might be exploited for designing new kinds of high- storage.

The laws of electromagnetism predict a great symmetry between electric and . This equality does not extend, however, to isolated magnetic "charges." Isolated electric charges, in the form of electrons, are of course quite common. Such charges attract or repel each other with a force inversely proportional to the square of the distance between the charges. A positive charge and a negative charge can team up to form a neutral . The situation in magnetism seems different: dipoles yes, monopoles no.

But new ideas and new experiments have changed the conventional thinking. First, experiments with cold electrons flowing in a two-dimensional sheet could, under the action of powerful magnetic fields, be coaxed into moving in circular orbits. These orbits in turn seem to interact to produce quasiparticles which have a charge equal to a fraction of the conventional . This was called the effect. Could there be an analog for magnetic dipoles? Could circumstances allow the existence of isolated (or fractional) ?

Recent experiments and Germany and France point to this possibility in so called "spin ice," a solid material made of the elements dysprosium (Dy), titanium (Ti), and oxygen (O). The basic building block of these materials is a pair of tetrahedral groupings, with (typically) two Dy atoms (each of which acts like a tiny dipole magnet of its own) pointing out of each tetrahedron and two pointing in. This is analogous to the orientation of hydrogen atoms in water ice, hence the name "spin ice."

Normally all magnetic poles should be confined within two-pole couplets---the traditional magnetic dipole. However, at a low enough temperature, around 5 K, "frustration" among the magnetic atoms---they want to align with each other but can't because of the inherent geometry of the material---leads to a disordered state with strong, synchronized fluctuations. Unpaired magnetic poles can form amid this tumult. That is, particles (quasiparticle excitations, to be exact) in spin ice with a net magnetic "charge" can exist and move about. A gas of electric charges is called a "plasma," so some scientists refer to the analogous tenuous cloud of magnetic charges as a "monopole plasma."

Stephen Powell's paper, published presently in the journal Physical Review Letters, explores what happens when the fluctuations are frozen by, for example, still-colder temperatures or a high-strength magnetic field. He shows how the monopoles are confined into magnetically neutral dipoles again. He is the first to prescribe the phase transition from the monopole phase (also called the Coulomb phase since the monopoles feel the same inverse-square force effect as electric charges) into the pole-confined phase.

Going to those lower temperatures, and observing how monopoles freeze into dipoles, will be difficult to achieve in the lab since it is hard to coax the magnetic atoms into interacting strongly enough. But Powell thinks it can be done. Furthermore, if this transition were like other phase transitions, then it should be subject to a body of laws called "universality," which typify many such phenomena---water turning into ice is a favorite example. Powell is the first to address how universality pertains to the freezing process, when monopoles in spin ice lapse back into dipoles at super-low temperatures.

"These kinds of magnetic monopoles are not just mathematical abstractions," said Powell. "They really appear. They can move around, at least a little bit. Scientists need to understand how monopoles behave, even at the lowest temperatures where they get locked back into dipoles." Powell's framework for monopoles includes testable predictions about how to observe the transition from monopoles into confined poles.

Explore further: Thermoelectric power plants could offer economically competitive renewable energy

More information: "Universal monopole scaling near transitions from the Coulomb phase," Physical Review Letters 109, 065701 (2012)

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User comments : 15

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NeutronicallyRepulsive
5 / 5 (2) Aug 09, 2012
SatanLover
1 / 5 (5) Aug 09, 2012
is this a study in response to the shifting magnetic poles on earth?
baudrunner
3.7 / 5 (6) Aug 09, 2012
No, it's a study in response to theories concerning the puzzling method of propulsion used by that UFO being studied (still, after all these years) in Area 51.
vacuum-mechanics
1 / 5 (2) Aug 10, 2012
The laws of electromagnetism predict a great symmetry between electric and magnetic forces. .. A positive charge and a negative charge can team up to form a neutral electric dipole. The situation in magnetism seems different: dipoles yes, monopoles no.

Symmetry seems arisen from mathematic rather than physics. May be understanding the mechanism of electromagnetism such as what electric charge is, or how magnetic force work etc. (as below) then we would found that magnetic monopole is just a dream.
http://www.vacuum...id=21=en
Satene
not rated yet Aug 10, 2012
Symmetry seems arisen from mathematic rather than physics.
Symmetry is geometric effect and the geometry indeed does follow from physics, the geometry of inertial particle packing in particular. The math is abstract construct of humans about it and it didn't exist before few thousands of years at all.
MrVibrating
not rated yet Aug 10, 2012
Magnetic fields are dipolar because magnetic fields are a flux vector - hence the prospect of monopoles seems oxymoronic. I understand they fall out of the Dirac equation, but can anyone explain this apparent logical inconsistency?
Eoprime
1 / 5 (1) Aug 10, 2012
No, it's a study in response to theories concerning the puzzling method of propulsion used by that UFO being studied (still, after all these years) in Area 51.


You should try to use /sarcasm tags ;)
antialias_physorg
3 / 5 (2) Aug 10, 2012
Magnetic fields are dipolar because magnetic fields are a flux vector - hence the prospect of monopoles seems oxymoronic. I understand they fall out of the Dirac equation, but can anyone explain this apparent logical inconsistency?

It does seem odd - especially since all magnetic fields are somehow related to the motion of electric charges (electric currents within domains, electron/proton spins or even the asymmetry of charged quarks within neutrons)

Principally you always have an axis of rotation with any rot component in the Maxwell equation - so you always get a north and south pole.

How this works with particles that don't have integer spin (fermions) I have no idea. Maybe there you can have a rotation axis with a non-even number of ends which would create a monopole?
MrVibrating
not rated yet Aug 10, 2012
As far as i understand it your initial point is correct - a magnetic interaction is essentially an energy exchange between charges in relative motion, per SR.

However the actual agents of this exchange eluded Einstein. The modern explanation is virtual photons; they're essentially rectified or corralled from the quantum vacuum, and the energy they convey is quantised as momentum, of either positive or negative sign (depending on attraction or repulsion) in units of h/bar (ie. basically, spin). Thus it's a boson actually mediating the force, between our fermions. Albeit a 'virtual' one.

Thus a monopole, according to this model, would only exchange virtual photons of one sign, regardless of direction or orientation. This in itself seems no harder to accept that virtual photon exchanges in the ordinary dipole case, yet it all breaks down for me when i try to understand it classically, per Maxwell. Evidently a monopole moment isn't a flux vector, or at least within normal 3-space...
SatanLover
not rated yet Aug 10, 2012
No, it's a study in response to theories concerning the puzzling method of propulsion used by that UFO being studied (still, after all these years) in Area 51.

You do realize nasa has articles regarding the shifting magnetic poles?

no need to demonize me, nice try.
nathj72
not rated yet Aug 12, 2012
For those of you trying to interpret how this work via maxwells equations, well it is like trying to explain the electron from maxwells equations. There is a term in the equations for magnetic current and a term for electric current. The equations say nothing on how a magnetic or electric conductor can exist. The equations only tell you about photons and how they interact with electric conductors, magnetic conductors, other dielectric or magnetic materials.

Now if someone figures out how to easily produce magnetic conductors then a whole slew of new applications are possible because it gives us another way to handle the photon. For those of you that know of artificial magnetic conductors, well they are narrowband and just not good enough.
MrVibrating
not rated yet Aug 13, 2012
The term for magnetic flux density (not 'current') is B, and the flux in question is always a vector, having direction and magnitude. This directionality - the term's very vectorness, even if that isn't a word - is what forms the dipolarity. Maxwell's 'molecular vortices' are easily interpreted as orbital or spin moments and the system seems very consistent with itself and experimental reality.

Monopoles thus just don't seem to fit....
nathj72
not rated yet Aug 14, 2012
@MrVibrating If you look at the full form of maxwells equations that is used in all electromagnetic simulators there is a term M for magnetic current density. Also a plane wave solution to maxwells equations yields no H-field that closes on itself. It seems you are misinterpreting what maxwells equations explain, which is the photons interaction with matter.
MrVibrating
not rated yet Aug 15, 2012
FWIW M is net flux density as a function of B & H, but perhaps you're thinking of the photoelectric effect (per Einstein)? Maxwell deals with the relationships between charge or current and magnetism, and his equation for Gauss's law for magnetism explicitly precludes magnetic monopoles.

My inability to understand monopoles boils down to the vector paradox. I get the impression what i really need is an idiot's guide to Dirac...
nathj72
not rated yet Aug 18, 2012
@MrVibrating There is a term added to maxwells equations for magnetic conductors (M). The reason this term exists is for simplification purposes when solving maxwells equations. If magnetic monopoles are found then gauss's law will be modified to include the monopole. Gauss's law does not state magnetic monopoles do not exist, it states they have not been observed. You are correct you need to understand it via dirac, cause maxwell does not explain and cannot explain anything about it. Just how to use it.

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