Perovskite nanocrystals are a convenient model system for optical spin orientation and manipulation. However, their real potential might be underestimated due to the incomplete knowledge on spin relaxation times, which are obscured by the limited sensitivity of measurement techniques as well as by the insufficient understanding of the spin relaxation mechanisms in perovskites. In this work, we study the spin relaxation of charge carriers in perovskite nanocrystals both experimentally and theoretically. We address the electron and hole spins in ${\mathrm{CsPbI}}_3$ nanocrystals embedded in a glass matrix by the resonant spin inertia technique based on optically detected magnetic resonance. It allows us to determine the longitudinal spin relaxation time $T_1$ separately for electrons and holes, the $g$ factors, and the effective Overhauser field of the nuclear spin bath. At a temperature of 1.6 K, the $T_1$ time for electrons can be as long as 0.9 ms. We reveal the effect of the time-varying nuclear field fluctuations, which enhance the electron spin relaxation at low magnetic fields, and measure a rather long nuclear spin correlation time of about 56 $\mu \mathrm{s}$. We develop a model of the spin relaxation in nanocrystals based on a two LO phonon Raman process, which explains the observed temperature dependence of the time $T_1$.

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• Spin relaxation
• Spintronics
• Nanocrystals
• Perovskites
• Optically detected magnetic resonance
• Pump-probe spectroscopy