Questions about magnetism

[usexpat97]

I know we have a couple hard-core (high-energy?) physicists on this board from time-to-time. I had a couple questions about magnetism that I need help with. The physics boards are a bit too hard-core, whereas I need more of a "physics for dummies" view.


1) Where do we stand with magnetic monopoles now?
https://phys.org/news/2019-08-breakthrough-magnetic-monopoles-technologies.html

What do we know? What is the potential impact on technology? This article seems to imply that monopoles are an observed phenomenon now, in that they have observed monopoles hopping from one lattice site to the neighbor. Yet if monopoles truly were observed, that would be big news, would it not (?).


2) What are geometrically frustrated magnets? In "physics for dummies" terms? Is this something we observe when we cryogenically cool an antiferromagnetic material to a near-zero-kelvin state? Is this something we seek to fabricate, or is it more of an undesired natural phenomenon in that conflicting magnetic forces cause materials to settle into an irregular, unpredictable low-energy state as we near 0 degrees?


Anyway, I hope I get lucky with this. And if I do, thanks!

 

[chilehed]

Anyway, I hope I get lucky with this

So do I.

 

[jacks]

I'm not a physicist and most likely know less about magnetism than you do, so I can't help with your question. However, I once invented perpetual motion with paper clips and two magnets. (Yes, at room temperature.) It really wasn't that hard.

 

[FrumiousBandersnatch]

I know we have a couple hard-core (high-energy?) physicists on this board from time-to-time. I had a couple questions about magnetism that I need help with. The physics boards are a bit too hard-core, whereas I need more of a "physics for dummies" view.


1) Where do we stand with magnetic monopoles now?
https://phys.org/news/2019-08-breakthrough-magnetic-monopoles-technologies.html

What do we know? What is the potential impact on technology? This article seems to imply that monopoles are an observed phenomenon now, in that they have observed monopoles hopping from one lattice site to the neighbor. Yet if monopoles truly were observed, that would be big news, would it not (?).


2) What are geometrically frustrated magnets? In "physics for dummies" terms? Is this something we observe when we cryogenically cool an antiferromagnetic material to a near-zero-kelvin state? Is this something we seek to fabricate, or is it more of an undesired natural phenomenon in that conflicting magnetic forces cause materials to settle into an irregular, unpredictable low-energy state as we near 0 degrees?


Anyway, I hope I get lucky with this. And if I do, thanks!

As I understand it, 'these are not the magnetic monopoles you're looking for...'

The theoretical, much-sought-after MMs that might be 'out there' are fundamental particles, whereas these MMs are 'quasi-particles' that exist in a magnetic ionic crystal called a spin-ice, although they appear to have the properties expected of MMs, they're magnetic defects in the crystal structure rather than particles.

 

[Ophiolite]

However, I once invented perpetual motion with paper clips and two magnets. (Yes, at room temperature.) It really wasn't that hard.

I suspect you may have rediscovered perpetual delusion, rather than inventing perpetual motion.

 

[essentialsaltes]

1) Where do we stand with magnetic monopoles now?

The Bandersnatch is correct. These 'monopoles' are not elementary or fundamental particles (which would be an amazing discovery), but more of a combined effect of multiple particles interacting in a way that 'looks like' a monopole.

2) What are geometrically frustrated magnets? In "physics for dummies" terms?

Not my area of expertise, but Wiki offers some help. Here's what I think I know...

As you probably know, many elementary particles have their own intrinsic magnetic properties (spin), and atoms and molecules may also have magnetic properties due to a complicated interaction of electrical properties and spin.

From playing with two bar magnets, you probably know that two magnets will snap together happily side by side in one orientation, but if you flip one magnet, it's hard to press them together. It takes energy to press them together. So there is a difference in energy between these two states. In nature, especially at low temperature, systems like to find themselves in the lowest energy state.

We know that opposites attract, so the low energy state is usually something like this with the North and South poles of the magnets

|N| |S|
|S| |N|

But if we have a system of three magnets in a triangle, how will our three little magnets find the lowest energy? The first two magnets can have opposite orientations, one with North up, and the other with North down, but the third one can't be opposite to both of them simultaneously. It has to choose one or the other. It is 'frustrated' in finding the best way to lower the energy with both its neighbors.

So it's all about the geometry of particular crystal lattice shapes that prevent (frustrate) some of the magnets from finding the best possible orientation simultaneously.

Is this something we seek to fabricate, or is it more of an undesired natural phenomenon in that conflicting magnetic forces cause materials to settle into an irregular, unpredictable low-energy state as we near 0 degrees?

That I don't know. It looks more to me like an active subject of interest, since it toys with our usual ideas of falling into a lowest energy state, so it has theoretical interest. From my brief skim, I can't say whether this is 'good' or 'bad' or 'useful' or 'not useful'.

 

[usexpat97]

As I understand it, 'these are not the magnetic monopoles you're looking for...'

The theoretical, much-sought-after MMs that might be 'out there' are fundamental particles, whereas these MMs are 'quasi-particles' that exist in a magnetic ionic crystal called a spin-ice, although they appear to have the properties expected of MMs, they're magnetic defects in the crystal structure rather than particles.

Hi,

Thanks. But another question: where can I better understand the expected behavior of an MM? Is it a violation of Maxwell's? I mean after all, if the sum of the poles does not add up to 0, then I do not expect the summation of H to add up to 0.

 

[usexpat97]

The Bandersnatch is correct. These 'monopoles' are not elementary or fundamental particles (which would be an amazing discovery), but more of a combined effect of multiple particles interacting in a way that 'looks like' a monopole.



Not my area of expertise, but Wiki offers some help. Here's what I think I know...

As you probably know, many elementary particles have their own intrinsic magnetic properties (spin), and atoms and molecules may also have magnetic properties due to a complicated interaction of electrical properties and spin.

From playing with two bar magnets, you probably know that two magnets will snap together happily side by side in one orientation, but if you flip one magnet, it's hard to press them together. It takes energy to press them together. So there is a difference in energy between these two states. In nature, especially at low temperature, systems like to find themselves in the lowest energy state.

We know that opposites attract, so the low energy state is usually something like this with the North and South poles of the magnets

|N| |S|
|S| |N|

But if we have a system of three magnets in a triangle, how will our three little magnets find the lowest energy? The first two magnets can have opposite orientations, one with North up, and the other with North down, but the third one can't be opposite to both of them simultaneously. It has to choose one or the other. It is 'frustrated' in finding the best way to lower the energy with both its neighbors.

So it's all about the geometry of particular crystal lattice shapes that prevent (frustrate) some of the magnets from finding the best possible orientation simultaneously.
'.

That triangle, if accurate, sounds like an excellent "physics for dummies" description of geometric frustration. So is this just another manifestation of where you pass a 90-degree quantum particle (e.g. an electron with 90-degree spin) through a readout--and get a perfectly random distribution of 0-degree/180-degree spin out? (a phenomenon which probably has a name...)

 

[essentialsaltes]

So is this just another manifestation of where you pass a 90-degree quantum particle (e.g. an electron with 90-degree spin) through a readout--and get a perfectly random distribution of 0-degree/180-degree spin out? (a phenomenon which probably has a name...)

Probably not. The interactions with the other magnets would act as a 'measurement' that would force the atom or whatever into a particular state, rather than as a superposition of multiple quantum states.

 

[usexpat97]

Probably not. The interactions with the other magnets would act as a 'measurement' that would force the atom or whatever into a particular state, rather than as a superposition of multiple quantum states.

Let me see if I can paraphrase correctly:

A particle locomotes along a quantum wave function. So if you superimpose wave functions, that is similar to superimposing multiple RF signals at different frequencies in the air. i.e. Rock 97.9 FM and News 1300 AM travel in the same air, your antenna picks up this complicated RF signal, which you cannot make heads-or-tails of. At least, not without a mid-pass filter, tuned specifically to pick up 97.9 FM.

Whereas a magnetic spin is a spin--not a locomotion--and thus is not represented by a wave function. The behavior is more like the force of the abutting poles of the first two magnets each cancel out their force on the third, thus making other terms/forces dictate the behavior of the third (namely, the other poles of the first two magnets).

Out in left field? Close?

 

[FrumiousBandersnatch]

Hi,

Thanks. But another question: where can I better understand the expected behavior of an MM? Is it a violation of Maxwell's? I mean after all, if the sum of the poles does not add up to 0, then I do not expect the summation of H to add up to 0.

Sorry, but what I posted is pretty much all I know about MMs.

 

[essentialsaltes]

Whereas a magnetic spin is a spin--not a locomotion--and thus is not represented by a wave function.

Out in left field? Close?

out in left field I'm afraid. Spin certainly can and often is represented by a wave function and that wave function can be in a superposition of states. But those superposition states, where the electron can be both spin up or spin down before you measure them, generally have to be carefully prepared, and separated from any outside influences.

But a magnet in a lattice can't be separated from outside influences, because there are other magnets around.

 

[usexpat97]

Sorry, but what I posted is pretty much all I know about MMs.

Thanks. I'll take what I can get. I go home with a fried brain every day now. This must be how med school students feel. I'm being expected to build stuff based on physical laws I have not completely wrapped my head around.

I found this about Maxwell in Wikipedia: "Gauss's law for magnetism states that there are no "magnetic charges" (also called magnetic monopoles), analogous to electric charges."

...which of course adds to my confusion. Theoretical physicists are adamant that there in fact are monopoles now. Which by definition, means Gauss' Law is not entirely accurate, and thus Maxwell's equations would get broken by a monopole. (but I know how it is...Physics 101 says electrons repel each other. Except...not entirely true. Electrons form Cooper pairs. They attract one another)

 

[usexpat97]

out in left field I'm afraid. Spin certainly can and often is represented by a wave function and that wave function can be in a superposition of states. But those superposition states, where the electron can be both spin up or spin down before you measure them, generally have to be carefully prepared, and separated from any outside influences.

But a magnet in a lattice can't be separated from outside influences, because there are other magnets around.

I assume, though, spin superposition states could spontaneously exist in a high-energy environment. Could they not? And not be exceedingly rare in nature?

e.g. in plasma, everything is free radicals, electrons are probably crazy mixtures of all kinds of things, aren't they?

 

[sjastro]

Thanks. I'll take what I can get. I go home with a fried brain every day now. This must be how med school students feel. I'm being expected to build stuff based on physical laws I have not completely wrapped my head around.

I found this about Maxwell in Wikipedia: "Gauss's law for magnetism states that there are no "magnetic charges" (also called magnetic monopoles), analogous to electric charges."

...which of course adds to my confusion. Theoretical physicists are adamant that there in fact are monopoles now. Which by definition, means Gauss' Law is not entirely accurate, and thus Maxwell's equations would get broken by a monopole. (but I know how it is...Physics 101 says electrons repel each other. Except...not entirely true. Electrons form Cooper pairs. They attract one another)

It was Dirac (who proposed the existence of antimatter) who suggested the quantization of electric charge could be due to the existence of a magnetic monopole with magnetic charge gₘ = 0.5hcn/e ≈ 68.5en where h is Planck’s constant, c the speed of light, e the electric charge and n is an unspecified integer.
This would modify Maxwell’s equations rendering them symmetric with respect to both electric and magnetic source terms.

The trouble with magnetic monopoles as fundamental particles if they existed in even relatively small numbers in the early history of the Universe rather than in a quasi-state as described in the article, our Universe would be radically different today and devoid of life as we would know it.
At the time of the GUT epoch when the strong and electroweak forces were still combined when magnetic monopoles formed, the Universe had a particle horizon volume of ≈(2 x 10⁻²⁹m)³.

Even if there was only 1 magnetic monopole per 10 particle horizon volumes at the GUT epoch, the density of magnetic monopoles today assuming the Universe expanded at a linear rate is 0.1 x ((2 x 10⁻²⁹m) x (4.4 x 10²⁷))⁻³ ≈ 150m⁻³
This is considerably greater than the current proton density in the Universe 0.17m⁻³.

Fundamental monopoles have some curious properties.
According to GUT the mass is concentrated within a core of about 10⁻³⁰m surrounded by a layer populated by X leptoquark bosons which is further surrounded by a layer of W and Z bosons at a diameter of about 10⁻¹⁷m.
Hence magnetic monopoles cannot be considered point like particles like leptons and quarks.
Furthermore magnetic monopoles are superheavy that they should accumulate in the cores of stars where they may collide with protons.
Some protons may penetrate the W and Z boson layer and collide with a virtual leptoquark boson which converts the d quark in the proton according to the reaction
d + X → e+ where e+ is a positron.
Hence magnetic monopoles would destroy hadronic (protons and neutrons) matter at a faster rate than its natural decay and would greatly reduce the amount of baryonic matter in the Universe.

In the early history of the Universe rather than undergoing a linear expansion, inflation occurred which was an exponential expansion resulting in a single magnetic monopole occupying a considerably larger particle horizon volume.
This results in far lower current magnetic monopole densities which are not only below detection limits but do not affect stellar formation.

 

[usexpat97]

Help from our astro friends!

It sounds like I am more interested in this quasi-monopole observation in the lab, in a cryo environment. Why did they get tunneling?

This is good about the possible [non-]existence of monopoles as spontaneous stellar phenomena, but would it be possible, maybe, to realize a monopole in a nanotech lab environment? What kind of energy would it take for that proton to penetrate the boson layer? Are we talking smashing H+ ions at near-c velocity?

 

[sjastro]

Help from our astro friends!

It sounds like I am more interested in this quasi-monopole observation in the lab, in a cryo environment. Why did they get tunneling?

This is good about the possible [non-]existence of monopoles as spontaneous stellar phenomena, but would it be possible, maybe, to realize a monopole in a nanotech lab environment? What kind of energy would it take for that proton to penetrate the boson layer? Are we talking smashing H+ ions at near-c velocity?

Our current particle accelerators are nowhere near the energy levels to test at GUT scales.
The centre of mass energy for the Large Hadron Collider is 14 TeV = 1.4 x 10⁴ GeV.
The GUT energy scale is around 10¹⁵ GeV.

"Proposed" particle accelerators to probe at the GUT scale are at best works of science fiction.

We discuss the main factors affecting the design of accelerators aiming to investigate physics at the GUT scale. The most important constraints turn out to be the energy used and the time taken to accumulate sufficient luminosity. We propose a photon collider design, where the photons are generated by undulator radiation from high energy muon beams. This reduces the energy requirements by a factor of 107 compared to a pp collider. Much of the reduction in energy use is achieved by using a periodic magnetic field to prevent a cascade of secondary reactions at the collision points. The proposed collider would be powered by (part of) a Dyson swarm constructed around the Sun, and efficient use of energy will be important to reduce the time needed to reach the desired number of collisions.

The Undulator Radiation Collider: An Energy Efficient Design For A...

 

[essentialsaltes]

I found this about Maxwell in Wikipedia: "Gauss's law for magnetism states that there are no "magnetic charges" (also called magnetic monopoles), analogous to electric charges."

...which of course adds to my confusion. Theoretical physicists are adamant that there in fact are monopoles now.

I think it's more that condensed matter experimental physicists have called things 'magnetic monopoles' (with scare quotes).

Since around 2003, various condensed-matter physics groups have used the term "magnetic monopole" to describe a different and largely unrelated phenomenon.[18][19]

A true magnetic monopole would be a new elementary particle, and would violate Gauss's law for magnetism ∇⋅B = 0. A monopole of this kind, which would help to explain the law of charge quantization as formulated by Paul Dirac in 1931,[40] has never been observed in experiments.[41][42]

The monopoles studied by condensed-matter groups have none of these properties.

 

[essentialsaltes]

I assume, though, spin superposition states could spontaneously exist in a high-energy environment. Could they not?

It's not so much about temperature as about environment. If there are other molecules to interact with (and at high temperature there is more thermal agitation and presumably more interaction between neighboring particles) then the additional interactions would, I think, decohere any superpositions into definite states. To get superpositions, we typically need isolated systems.

"If a quantum system were perfectly isolated, it would maintain coherence indefinitely, but it would be impossible to manipulate or investigate it. If it is not perfectly isolated, for example during a measurement, coherence is shared with the environment and appears to be lost with time; a process called quantum decoherence. As a result of this process, quantum behavior is apparently lost, just as energy appears to be lost by friction in classical mechanics."

 

[usexpat97]

It's not so much about temperature as about environment. If there are other molecules to interact with (and at high temperature there is more thermal agitation and presumably more interaction between neighboring particles) then the additional interactions would, I think, decohere any superpositions into definite states. To get superpositions, we typically need isolated systems.

"If a quantum system were perfectly isolated, it would maintain coherence indefinitely, but it would be impossible to manipulate or investigate it. If it is not perfectly isolated, for example during a measurement, coherence is shared with the environment and appears to be lost with time; a process called quantum decoherence. As a result of this process, quantum behavior is apparently lost, just as energy appears to be lost by friction in classical mechanics."

Okay,

I think I'm gradually getting a better grasp of geo frustration now. Although, I think my brain is telling me it is done for the day.

However, from that same wiki page I am seeing this:

They are not sources for the B-field (i.e., they do not violate ∇⋅B = 0); instead, they are sources for other fields, for example the H-field,[5] the "B*-field" (related to superfluid vorticity),[6][43] or various other quantum fields.[44]

We have a problem in Type II Superconductors in that they tend to trap unwanted flux during the cooling process from just over Tc down to below Tc (where Tc = critical superconducting temperature). It seems possible that maybe these B* fields from these condensed-matter magnetic monopoles are what's causing unwanted flux in Type II's, and then it gets trapped inside once the material goes superconducting? If so, then maybe it's possible to manipulate the lattice structure so as to control exactly where the magnetic vortices occur? But it would require their existence in a pre-superfluid state.