Is spin a relativistic effect

The Schrodinger equation in its original form did not contain spin. To describe spin, Pauli [45] added an ad hoc term to the SE in the presence of a magnetic field. Shortly afterwards it was shown that spin results naturally without any ad-hoc assumptions from the Dirac equation. Therefore spin was for a long time regarded as a relativistic effect. 

It took some time until it was realized that the Dirac theory describes the spin correctly because it is a spinor-field theory, and not because it is relativistic [16]. In fact, if one takes the nonrelativistic limit of the Dirac equation, spin survives, and this is consistent with the observation that the Galilei group has spinor representations as well. So, without any doubt, spin is not a relativistic effect. 

The question, whether the interaction of spin with an external magnetic field is a relativistic effect, is more subtle. The answer that it is 

The situation is more subtle for the spin-orbit interaction. One may try to explain it by noting that a moving electron in a one-electron atom induces a magnetic field (see e.g. Ref. [7], p. 180, or [42], p. 375) 

There are a few serious objections against this nonrelativistic derivation of the spin-orbit interaction  

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