In the last years, the hybrid metal-halide perovskites have attracted a great interest in the scientific community due to their excellent photo-physical properties and their outstanding optoelectronic and photovoltaic properties. In particular, formamidinium lead triiodide (FAPbI₃) is the best candidate for single-junction HMP photovoltaics. It shows the highest theoretical photoconversion efficiency due to his energy bandgap, that is the lowest in the family of Pb-based halide perovskites (HPs). The very promising a-phase of FAPbI₃ is however difficult to stabilize at room temperature. Recently, different groups work to implement a strategy to make it stable and some of them have obtained promising results [1]. The difficulties to stabilize the material have so far prevented exploring experimentally the whole peculiarities and characteristics of FAPbI₃ beyond photovoltaic applications. However, FAPbI₃, as other member of HP family, are highly promising material in the spintronic domain due to their large spin-orbit coupling and its spin-dependent optical selection rules.

In this work, we have studied, at 2K, the coherent evolution of the electronic spins in a polycrystalline film of FAPbI₃ by using a picosecond pump-probe experimental set-up and measuring the photo-induced Faraday rotation technique (PFR) in transverse magnetic field. We identified two contributions to the PFR signal that we associated to localized electrons and holes respectively. We measured long spin coherence times of localized electrons (holes) of 3.1ns (1.6 ns) at 1.49 eV. These values are shorter than the values obtained in FA_0.9Cs_0.1PbI_2.8Br_0.2 [2], MAPI [3, 4,5] and CsPbBr₃ [6] but are of the same order of magnitude. We have also obtained the value of g Landé factors for electrons 3.479 and for holes 1.137 that compares very well with previous results on FA_0.9Cs_0.1PbI_2.8Br_0.2 [2]. From the measured hole g factors and ellipsometry results on FAPbI₃ we are able to obtain the Kane energy parameter of FAPbI₃, Ep=13.5 eV, that is a central parameter to describe the band-structure and optical properties in the framework of the effective mass approximation [7]. This experimentally estimated value is smaller than the theoretical value [8]. All this results are of prime importance to evaluate the potentialities of the FAPbI₃ as future actor in engineered HP spin-optronic devices.