Intuition of spin of photon I read that intrinsic spin is a quantum property aka just a number, charged particle such as electron has magnetic dipole moment which can be detected using magnetic field like the Stern–Gerlach experiment to observe the deflection and neutron is due to the sum of quarks magnetic dipole moment. What about photon which is spin 1? Because I cannot apply angular momentum in this case unless it is spin 0 for no angular momentum.
 A: Spin is a particle's angular momentum in its rest frame. Massless particles like photons don't have a rest frame so it doesn't really make sense to talk about spin. Instead, we have helicity, which is the angular momentum of the particle in its direction of motion.
To have a deeper understanding, you would need to study the theory of irreducible representation of the rotation group, and then the Lorentz group. For a massive particle in its rest frame, its states form an irreducible representation of the rotation group, characterized by a number which we call spin. It follows that spin of a massive particle can only be either integer or half integer.
For massless particles, the problem is more complicated. Instead of the simple rotation group in a rest frame, you will find yourself having to deal with a group that leaves a preferred four-momentum invariant, known as the "little group". It is less straightforward but the little group of massless particles is also characterized by a number, the helicity, which also has to be either integer or half-integer. When people talk about photons with spin 1, what they really mean is that its helicity is $\pm1$.
After you have learned all that, you can turn the argument around to give a definition of particles. A particle is "something" whose states form an irreducible representation of the Poincare group (Lorentz + spacetime translation). Each particle can be characterized by the invariant of its four-momentum, defined to be its mass, and its spin/helicity as described.
