Doubly-magic nature of 56Ni: measurement of the ground state nuclear magnetic dipole moment of 55Ni

The nuclear magnetic moment of the ground state of ${}^{55}$Ni ($I^{\pi }=3\mathrm{/}{2}^{-},T_{1\mathrm{/}2}=204$ ms) has been deduced to be $\mathrm{\mid }\mu$^{55}Ni)$\mathrm{\mid }=(0.976\pm 0.026)$ ${\mu }_N$ using the $\beta$-NMR technique. Results of a shell model calculation in the full $\mathit{\text{fp}}$ shell model space with the GXPF1 interaction reproduce the experimental value. Together with the known magnetic moment of the mirror partner ${}^{55}$Co, the isoscalar spin expectation value was extracted as $<\sum {\sigma }_z>=0.91\pm 0.07$. The $<\sum \sigma_z>$ shows a similar trend as that established in the $\mathit{\text{sd}}$ shell. The present theoretical interpretations of both $\mu {(}^{55}$Ni) and $<\sum \sigma_z>$forthe$T=1/2$,A=55mirrorpartnerssupportthesoftnessofthe$^{56}$Ni core.