Magnetism is often perceived as the spin of subatomic particles that "makes" the magnetic dipole of these particles. Sometimes this complicates the understanding of magnetism on a macroscopic level as well as in the intraatomic interactions.
Classically (means, not in the QED‘s view) one may treat electrons and protons as having the two intrinsic properties charge and magnetic moment (electric field and magnetic field of the electron and the proton), that exist in the same way. Indeed, both properties are separable on the macroscopic level. The electric field by separating electrons from the nucleus and magnetic field by aligning atoms by their magnetic dipole moments. To put it directly: electrons and protons are just as much charges as magnets.
I know that spin is needed for defining the magnetic moment of any particle, and I have also read that the spin actually is the reason why some materials are magnetic.
Considering the above, the spin is - without considering the QED - a synonym for the magnetic moment. The aligned magnetic dipols of the involved subatomic particles are the reason for the macroscopic magnetic field.
What I want to know is whether spin is necessary for the some interactions in the electromagnetic field... in classical electromagnetic field theory, the electric and the magnetic fields could be considered as some combinations of partial derivatives of the vector potential (A𝜇). Any particle couples with the field and interacts with other particles through it.
There is a difference between the mathematics of an electro-magnetic-field dealt with in physics and the interactions resulting from these two phenomena. Particles as well as macroscopic bodies interact with their magnetic fields and with their electric fields. For example, an electron and a proton are attracted by their electric fields and aligned by their magnetic fields, but never the electric field interacts with the magnetic field.
Physics knows nothing about the internal structure of electric and magnetic fields. A current consists of electrons, water consists of H2O molecules, but we are not interested in the internal structure of E and B fields. The only thing we do is to represent them by field lines. And without caring about the components, we have introduced the model of virtual photons for the interactions. This makes things more difficult than they are. It is better to replace virtual photons by the interaction of charges and magnetic dipoles.
The QED was developed for processes on the atomistic level. The interaction between the nucleus and the electrons and between the electrons in the shells are both: electrical and magnetic interactions. But also here the term "spin" is interchangeable with the term „magnetic dipoles“. Somehow it is easier to imagine the paired electrons from the Paulis principle through paired magnets than through antiparallel spin.
Moving on, if we consider the quantum field theory version, we have two particles coupled with the electromagnetic field, which then interact with the exchange of bosons (photons). My question is: how big of a role does spin play in the interactions which happen through the electromagnetic field? Are there some interactions which spinless particles cannot have, but those with spin can?
Not having a spin for a particle is the same as not having a magnetic dipole. But perhaps the inner structure of spinless particles get influenced by an external magnetic field and induces a magnetic dipole. Only in this case the external field will deflect these moving particles (the Lorentz force) from the trajectory like electrons do.