Doubly-magic nature of ${}^{56}\mathrm{Ni}$: Measurement of the ground state nuclear magnetic dipole moment of ${}^{55}\mathrm{Ni}$

The nuclear magnetic moment of the ground state of ${}^{55}\mathrm{Ni}$ ($I^{\pi }=3/2^{-}$, $T_{1/2}=204$ ms) has been deduced to be $|\mu ({}^{55}\mathrm{Ni})|=(0.976\pm 0.026)\hspace{0.5em}{\mu }_N$ using the $\beta$-ray detecting nuclear magnetic resonance technique. Results of a shell model calculation in the full $\mathit{fp}$ shell model space with the GXPF1 interaction reproduce the experimental value. Together with the known magnetic moment of the mirror partner ${}^{55}\mathrm{Co}$, the isoscalar spin expectation value was extracted as $\langle \sum {\sigma }_z\rangle =0.91\pm 0.07$. The $\langle \sum {\sigma }_z\rangle$ shows a trend similar to that established in the $\mathit{sd}$ shell. The present theoretical interpretations of both $\mu ({}^{55}\mathrm{Ni})$ and $\langle \sum {\sigma }_z\rangle$ for the $T=1/2$, $A=55$ mirror partners support the softness of the ${}^{56}\mathrm{Ni}$ core.

Figure 1

(a) $A_{\beta }P$ measured by the $H_0$ on/off technique. (b) NMR spectrum of ${}^{55}\mathrm{Ni}$ in NaCl, where $A_{\beta }P$ was determined as a function of applied rf. Data were taken at $H_0=(0.4491\pm 0.0005)$ T and with $\mathrm{FM}=\pm 25$ kHz shown as a horizontal bar at each point. The squares are the experimental values and the band is the baseline obtained from a weighted average of all the data except the resonance point at 955 kHz.