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 ⁵⁵Ni (I^π=3/2^-, T_1/2=204 ms) has been deduced to be |μ(⁵⁵Ni)|=(0.976±0.026) μ_N using the β-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 ⁵⁵Co, the isoscalar spin expectation value was extracted as 〈∑σ_z〉=0.91±0.07. The 〈∑σ_z〉 shows a trend similar to that established in the sd shell. The present theoretical interpretations of both μ(⁵⁵Ni) and 〈∑σ_z〉 for the T=1/2, A=55 mirror partners support the softness of the ⁵⁶Ni core.