Nuclear Magnetic Moment of the ⁵⁷Cu Ground State

The nuclear magnetic moment of the ground state of ⁵⁷Cu(I^π=3/2^-,T_1/2=196.3  ms) has been measured to be |μ(⁵⁷Cu)|=(2.00±0.05)μ_N using the β-NMR technique. Together with the known magnetic moment of the mirror partner ⁵⁷Ni, the spin expectation value was extracted as ⟨Σσ_z⟩=-0.78±0.13. This is the heaviest isospin T=1/2 mirror pair above the ⁴⁰Ca region for which both ground state magnetic moments have been determined. The discrepancy between the present results and shell-model calculations in the full fp shell giving μ(⁵⁷Cu)∼2.4μ_N and ⟨Σσ_z⟩∼0.5 implies significant shell breaking at ⁵⁶Ni with the neutron number N=28.