Background: The region around $N=40$ in the Ni isotopes has attracted considerable interest both from an experimental and theoretical point of view over the last few decades after ${}^{68}\mathrm{Ni}$ was proposed as a doubly magic nucleus. However, the experimental evidence has proven contradictory and inconclusive. Measurements of magnetic dipole moments in the adjacent ${}^{67,69,70}\mathrm{Ni}$ would be sensitive to the purity of the nuclear wave function in the vicinity of ${}^{68}\mathrm{Ni}$.

Purpose: Investigate the single-particle structure of several $\nu g_{9/2}$ isomeric states in neutron-rich Ni isotopes, namely the $9/2^{+}$ isomer in ${}^{67}\mathrm{Ni}$ ($E_x=1007$ keV, $T_{1/2}=13.3\left(2\right)\mu \mathrm{s}$, expected $\nu g_{9/2}$ configuration), the ($17/2^{-}$) isomer in ${}^{69}\mathrm{Ni}$ ($E_x=2700$ keV, $T_{1/2}=439\left(3\right)\mathrm{ns}$, expected $\nu p_{1/2}^{-1}{g}_{9/2}^2$ configuration), and the $8^{+}$ isomer in ${}^{70}\mathrm{Ni}$ ($E_x=2861$ keV, $T_{1/2}=232\left(1\right)\mathrm{ns}$, expected $\nu g_{9/2}^2$ configuration), via $g$-factor measurements.

Methods: The isomers were populated and spin-aligned in an intermediate-energy projectile-fragmentation reaction of a ${}^{76}\mathrm{Ge}$ beam in an experiment at NSCL, MSU. The $g$ factors of the Ni isomeric states were determined using the time-dependent perturbed angular distribution technique.

Results: The half-lives of the isomers in ${}^{67,70}\mathrm{Ni}$, as well as that of the $\left(6^{-}\right)$ isomer in ${}^{72}\mathrm{Cu}$, were confirmed, the precision on the half-life of the ($19/2^{-}$) isomer in ${}^{71}\mathrm{Cu}$ was improved, and a new value was obtained for the half-life of the isomer in ${}^{69}\mathrm{Ni}$. The $g$ factors of the isomers in ${}^{69,70}\mathrm{Ni}$ were determined for the first time, while the remeasured $g$ factor of the $9/2^{+}$ isomer in ${}^{67}\mathrm{Ni}$ resolves an existing discrepancy between experiment and theory.

Conclusions: The obtained results and the systematics of the $\nu g_{9/2}$ $g$ factors in the region were compared to state-of-the-art shell-model calculations using the JUN45 and the LNPS interactions. Excluding proton excitations from the ${}^{56}\mathrm{Ni}$ core, as in the JUN45 interaction, requires a significant reduction of the single-nucleon spin $g$ factors to reproduce the data ($g_s^{\mathrm{eff}}\le 0.75g_s^{\mathrm{free}}$). Including proton excitations across the $Z=28$ shell closure, as in the LNPS interaction, reproduces the $g$ factors of the Ni and Zn isotopes in the region by using a small reduction of 0.9 $g_s^{\mathrm{free}}$.

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• Isomer decays
• Lifetimes & widths
• Nuclear fragmentation
• Nuclear structure & decays
• Spectroscopic factors & electromagnetic moments
• 59 ≤ A ≤ 89
• Radioactive beams
• Shell model