Spin Magnetic Moment of a Free Relativistic Electron

Using eigenvalues of the Dirac equation solved for a uniform magnetic field, it is demonstrated that the spin magnetic moment of a free relativistic electron is ${\mu }_B\left(\mathcal{E}\right)=\frac{e\hslash }{2m\left(\mathcal{E}\right)c}$, where $m\left(\mathcal{E}\right)$ is the Lorentz mass.