What causes the neutron to have a magnetic moment , and is it paramagnetic or diamagnetic.
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....
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.
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
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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