Neutron magnetic moment

The neutron carries no net charge, and a genuinely neutral point particle cannot be magnetic. Yet the neutron has a substantial magnetic moment, −1.913 nuclear magnetons, first measured by Luis Alvarez and Felix Bloch in 1940. The only way to reconcile the two facts is internal structure: charged constituents whose charges cancel but whose currents and spins do not. The quark model gets it right: a down-heavy composition predicts a moment ratio μₙ/μₚ of −2/3, against the measured −0.685.

The sign is negative, meaning the moment points against the spin. It is what makes neutron beams steerable and polarisable with magnetic fields, and it underlies neutron scattering as a probe of magnetic order in solids: neutrons see magnetic structure that X-rays, which scatter from charge, are blind to.