  Metal halide perovskites make up a promising class of materials for
semiconductor spintronics. Here we report a systematic investigation of
coherent spin precession, spin dephasing and spin relaxation of electrons and
holes in two hybrid organic-inorganic perovskites MA0.3FA0.7PbI3 and
MA0.3FA0.7Pb0.5Sn0.5I3 using time-resolved Faraday rotation spectroscopy. With
applied in-plane magnetic fields, we observe robust Larmor spin precession of
electrons and holes that persists for hundreds of picoseconds. The spin
dephasing and relaxation processes are likely to be sensitive to the defect
levels. Temperature-dependent measurements give further insights into the spin
relaxation channels. The extracted electron Land\'e g-factors (3.75 and 4.36)
are the biggest among the reported values in inorganic or hybrid perovskites.
Both the electron and hole g-factors shift dramatically with temperature, which
we propose to originate from thermal lattice vibration effects on the band
structure. These results lay the foundation for further design and use of lead-
and tin-based perovskites for spintronic applications.
