In the Quantum Mechanics, the origination and essence of the spin angular momentum remain a mystery. Recently some viewpoints are in the violent competition.
The first viewpoint deems that the spin angular momentum is intrinsic, and is a quantum property without detailed classical analogy [Copenhagen interpretation]. Of course, not everyone accepts the idea.
The second viewpoint argues that the spin angular momentum is not intrinsic, and is merely the property of wave-field [F. J. Belinfante, “On the spin angular momentum of mesons”, Physica, 6 (7-12) (1939) 887--898. H. C. Ohanian, “What is spin?” Am. J. Phys., 54 (6) (1986) 500--505.], which is independent of the internal structure of particle.
The third viewpoint considers some suppressed precessional motions as the spin motions. In the complex octonion space, the precessional equilibrium equation infers the angular velocity of precession. The external electromagnetic strength may induce a new precessional motion, generating a new term of angular momentum, even if the orbital angular momentum is zero. This new term of angular momentum can be regarded as the spin angular momentum, and its angular velocity of precession is different from that of revolution. [“Spin Angular Momentum of Proton Spin Puzzle in Complex Octonion Spaces”, International Journal of Geometric Methods in Modern Physics, 14 (6) (2017) 1750102. “Two different types of precessional angular velocities in the complex-octonion space”, April, 2017]
Furthermore, there may be certain other viewpoints. Undeniably these different viewpoints violate the physical model of spin angular momentum. And these conundrums will intrigue and faze some scholars continually.