  Light carrying orbital angular momentum (OAM) has attractive applications in
the fields of precise optical measurements and high capacity optical
communications. We study the rotation of a light beam propagating in warm 87Rb
atomic vapor using a method based on magnetic-field-induced circular
birefringence. The dependence of the rotation angle on the magnetic field makes
it appropriate for weak magnetic field measurements. We quote a detailed
theoretical description that agrees well with the experimental observations.
The experiment shown here provides a method to measure the magnetic field
intensity precisely and expands the application of OAM-carrying light. This
technique has advantage in measurement of magnetic field weaker than 0.5 Gauss,
and the precision we achieved is 0.8 mGauss.
