  The spin degree of freedom of charge carriers in halide-perovskite
semiconductors can be highly useful for information photonics applications. The
Faraday effect is known to be the best indicator of paramagnetism of the
material and of the spin-light interaction. In this work, the Faraday effect is
demonstrated, for the first time, in a hybrid organic-inorganic halide
perovskite MAPbI3 (MA+=CH3NH+3). The Faraday rotation and birefringence were
measured across the tetragonal-cubic phase transition at 327 K. The Faraday
rotation is strongly suppressed below the phase transition temperature due to
anisotropy (linear birefringence) of the tetragonal crystal phase. The
situation changes drastically above the phase transition temperature, when the
crystal becomes optically isotropic. The emerging Faraday rotation obeys the
Curie law, demonstrating its population-related paramagnetic nature. This
observation opens new prospects for application of these systems and for their
investigations using methods of the polarization noise spectroscopy applicable
to optically anisotropic materials.
