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 Ni-55 (I-pi=3/2(-), T-1/2=204 ms) has been deduced to be |mu(Ni-55)|=(0.976 +/- 0.026) mu(N) using the beta-ray detecting nuclear magnetic resonance technique. Results of a shell model calculation in the full fp shell model space with the GXPF1 interaction reproduce the experimental value. Together with the known magnetic moment of the mirror partner Co-55, the isoscalar spin expectation value was extracted as <>=0.91 +/- 0.07. The <> shows a trend similar to that established in the sd shell. The present theoretical interpretations of both mu(Ni-55) and <> for the T=1/2, A=55 mirror partners support the softness of the Ni-56 core. .