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Spin dynamics of excitons and carriers in mixed-cation MA$_{x}$FA$_{1-x}$PbI$_{3}$ perovskite crystals: alloy fluctuations probed by optical orientation

Optical spin orientation measured by time-resolved photoluminescence provides a powerful tool to probe the spin dynamics of excitons and charge carriers in perovskite semiconductors. The impact of alloy fluctuations on the spin dynamics of mixed-cation \MAFAPI{} perovskite single crystals is studied here experimentally. The optical orientation is measured under nonresonant excitation for crystals with $x = 0.1$, $0.4$, and $0.8$ at cryogenic temperatures and compared with data on \MAPI{} crystals. The high degree of exciton optical orientation of $75-80$\% for $x = 0.1$ and $0.8$ reduces to about 60\% for $x = 0.4$. A similar trend is observed for the carrier spin optical orientation. This behavior is attributed to enhanced scattering of free excitons and carriers in the alloys with increased compositional and structural disorder. From the Larmor spin precession measured from spin dynamics in an external magnetic field applied in the Voigt geometry, the electron and hole $g$-factors are evaluated. Their dependence on the band gap energy in \MAFAPI{} crystals follows the universal trend previously established for lead halide perovskites.