It seems that the spin was discovered in 1922 from the results of experiments by Zeemann and Stern et al. The effect of magnetic fields on light was discovered in 1896 by Zeemann (Netherlands). He was born in a small town, he may have been interested in biology, physics and astrophysics from an early age. When he was 17, he saw the aurora, which is rare in the Netherlands, and he probably felt that the atoms were emitting light. The Zeeman effect have found the experimental fact that the line splits when the sodium atom is made to emit light in a magnetic field.
The existence of spins is determined experiments by Stern (Germany) and Gernach (Germany) in 1922. They evaporate silver in a hot pot and slit silver atoms to create atomic rays. When the atomic beam passed through a non-uniform magnetic field, the atomic beam image split into two. From the Larmor angular frequency model, it was predicted that the atomic beam image would split into odd numbers due to the magnetic field. However, since there are two experimental facts and cannot be explained by the conventional idea of quantum mechanics, we introduced the rotation motion of electrons as a new concept and named it spin.
1. P. Zeeman. Nature. 1897, vol. 55, p. 347.
2. W. Gerlach, O. Stern. Z. Phys. 1922, vol. 1, pp. 349-352.