Zeeman-Doppler imaging (ZDI) is the use of a spectropolarimeter to image the magnetic field of a star, making use of the Zeeman effect.
The Zeeman effect is the division of what would be a single emission line into multiple spectral lines from atoms within a magnetic field. Or it can be the complimentary effect on absorption lines. The field interacts with orbiting electrons, slightly affecting the atom's energy levels depending on orientation of the electrons.
The polarization of the resulting lines differ, so by measuring both the polarity and the wavelength of lines from a stellar atmosphere, the star's magnetic field can be reconstructed through tomography.
Instruments currently used for this:
The Bcool project uses the technique.