In-gas-cell laser spectroscopy for magnetic dipole moment of $^{199}$Pt toward $N=$ 126

Magnetic dipole moment and mean-square charge radius of $^{199}$Pt ($I^{pi}=$ 5/2$^-$) have been evaluated for the first time from the investigation of the hyperfine splitting of the $lambda_1=$ 248.792 nm transition by in-gas-cell laser ionization spectroscopy. Neutron-rich nucleus $^{199}$Pt was produced by multi-nucleon transfer reaction at the KISS where the nuclear spectroscopy in the vicinity of $N=$ 126 is planed from the aspect of an astrophysical interest as well as the nuclear structure. Measured magnetic dipole moment $+$0.63(13)$mu_{rm N}$ is consistent with the systematics of those of nuclei with $I^{pi}=$ 5/2$^-$. The deformation parameter $|<beta_2^2>^{1/2}|$ evaluated from the isotope shift indicates the gradual shape change to spherical shape of platinum isotopes with increasing neutron number toward $N=$ 126.