The coherent spin dynamics of electrons are investigated for ${\mathrm{CsPbI}}_3$ perovskite nanocrystals in a glass matrix using time-resolved Faraday ellipticity. In nanocrystals with a diameter of about 11 nm, the Larmor precession frequency has a linear dependence on the magnetic field corresponding to the electron Landé $g$ factor of 2.07. We find a finite Larmor precession frequency at zero magnetic field, corresponding to the electron spin splitting of 0.8 $\mu \mathrm{eV}$. This splitting is explained by the hyperfine interaction with nuclear spin fluctuations. Our model analysis shows that the hyperfine interaction for the conduction-band electrons is contributed both by the $p$ orbitals of the lead atoms and by the $s$ orbitals of the iodine atoms, with the leading contribution to the hyperfine field fluctuations coming from iodine. This fact agrees well with the 9% iodine contribution to the Bloch amplitude of the conduction band, obtained by density functional theory calculations. From these findings, the atomic hyperfine constant for the $5s$ orbital of iodine is evaluated as 190 $\mu \mathrm{eV}$.

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• Magneto-optics
• Optical pumping
• Spin dynamics
• Spintronics
• Quantum dots
• Ultrafast pump-probe spectroscopy