  The nuclear magnetic moment of the ground state of $^{55}$Ni
($I^{\pi}=3/2^{-}, T_{1/2}=204$ ms) has been deduced to be
$|\mu$^{55}Ni)$|=(0.976 \pm 0.026)$ $\mu_N$ using the $\beta$-NMR technique.
Results of a shell model calculation in the full \textit{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 \textit{sd} shell.
The present theoretical interpretations of both $\mu(^{55}$Ni) and $<\sum
\sigma_z>$ for the $T=1/2$, A=55 mirror partners support the softness of the
$^{56}$Ni core.
