Neutron Magnetic Moment: Paramagnetic or Diamagnetic

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Discussion Overview

The discussion revolves around the magnetic moment of the neutron, exploring its causes and whether it is classified as paramagnetic or diamagnetic. Participants delve into the internal structure of the neutron, the role of quarks, and the implications of its magnetic properties.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that the neutron's magnetic moment arises from its internal structure, specifically the presence of quarks, which are electrically charged particles.
  • It is noted that a neutron is a spin 1/2 particle, suggesting it could have a magnetic moment due to its angular momentum, but questions remain about the necessity of a non-zero gyromagnetic ratio.
  • One participant discusses how the spins of the quarks combine to yield the overall spin of the neutron, referencing the Pauli exclusion principle and the concept of singlet states.
  • Another participant mentions that while the naive constituent quark model predicts the ratio of magnetic moments well, relativistic corrections complicate the picture and lead to discrepancies with observed values.
  • There is a contention regarding the classification of neutrons as paramagnetic or diamagnetic, with some arguing that free neutrons exhibit paramagnetic behavior due to unpaired spins, while others assert that neutrons do not fit into these categories as they are not materials with atomic structure.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether neutrons can be classified as paramagnetic or diamagnetic, with multiple competing views presented regarding the implications of their magnetic moment and internal structure.

Contextual Notes

Participants express uncertainty about the contributions of quark spins, gluon spins, and orbital angular momentum to the neutron's overall magnetic properties, indicating that these aspects are still under investigation.

cragar
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What causes the neutron to have a magnetic moment , and is it paramagnetic or diamagnetic.
 
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the neutron consists of electrically charged particles, quarks, and it has a finite "size" (or rahter a density distrubition)

only materials are paramagnetic, diamagnetic and so on...
 
cragar said:
What causes the neutron to have a magnetic moment , and is it paramagnetic or diamagnetic.

A neutron is a spin 1/2 particle, which means that it is *possible* that it would have a magnetic moment, since a non-zero angular momentum is required. However, it is also required that the neutron have a non-zero gyromagnetic ratio, and it is less clear why this should be the case, since that is predicted to be non-zero only for charged particles. Therefore it seems that the non-zero magnetic moment of the neutron must mean that it has internal structure, and that the composite particles must have charges that sums to zero. As I understand it, this was one of the initial observations that led to the theory of quarks, which are the particles that make up neutrons and protons.

I am not sure how the angular momenta of the quarks, which are also spin-1/2 fermions, combine to give a resultant spin that is always 1/2 for the proton and neutron. Perhaps it is just that there are always two identical quarks in either of these particles (neutron = 1 "up" and two "down" quarks, while a proton = 2 "up" and 1 "down"), then those two just pair their spins in the anti-symmetric "singlet" state according to the Pauli exclusion principle, and the spin of the remaining quark gives the overall spin of the particle.
 
Last edited:
SpectraCat said:
Perhaps it is just that there are always two identical quarks in either of these particles (neutron = 1 "up" and two "down" quarks, while a proton = 2 "up" and 1 "down"), then those two just pair their spins in the anti-symmetric "singlet" state according to the Pauli exclusion principle, and the spin of the remaining quark gives the overall spin of the particle.
Already in the "naive" constituent quark model, relativistic corrections spoil this simple picture. Note that this "naive" model predicts rather well the ratio of the magnetic moments. But for absolute values, the relativistic corrections from this model do not agree with observations. This is not a simple story, and how the different contributions from quarks spin, gluons spin, and orbital angular momentum add up still under investigation.

Understanding the proton's spin structure
 
ansgar said:
the neutron consists of electrically charged particles, quarks, and it has a finite "size" (or rahter a density distrubition)

only materials are paramagnetic, diamagnetic and so on...

by materials you mean stuff with atoms , so neutrons are neither paramagnetic or diamagnetic.
 
cragar said:
by materials you mean stuff with atoms , so neutrons are neither paramagnetic or diamagnetic.

Well, since they are unpaired spins, free neutrons are definitely paramagnetic in the strict sense of the term, but it is a rather trivial case.
 

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