# Spin magnetic moment direction of a particle

Is the spin magnetic moment of a fundamental particle like an electron always aligned along the direction of the spin angular momentum (meaning that the magnetic moment and the spin operators have the same eigenstates)? Is it true for composite particles like protons, neutrons, pions, etc.? Do the loop corrections from QFT change the direction of spin magnetic moment from that of the net spin angular momentum in the fundamental and/or composite cases?

• 0-th order they are (anti)parallel, for the electron. So in which direction would loop-corrections move $\vec{\mu}$ away from $\vec{S}$? What would break the azimuthal symmetry? – JEB Jan 18 '18 at 1:21