%0 Journal Article %J Phys. Rev. Lett. %D 2006 %T Nuclear magnetic moment of the $^{57}$Cu ground state (I=3/2$^-$, T$_{1/2}$=199.4 ms) %A Minamisono, K. %A Mantica, P.F. %A Mertzimekis, T. J. %A Davies, A.D. %A Hass, M. %A Pereira, J. %A Pinter, J.S. %A Rogers, W. F. %A Stoker, J.B. %A Tomlin, B.E. %A Weerasiri, R.R. %X The nuclear magnetic moment of the ground state of Cu57(I?=3/2-,T1/2= 196.3ms) has been measured to be |?(Cu57)|=(2.00±0.05)?N using the ?-NMR technique. Together with the known magnetic moment of the mirror partner Ni57, the spin expectation value was extracted as ??z =-0.78±0.13. This is the heaviest isospin T=1/2 mirror pair above the Ca40 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 ?(Cu57)?2.4?N and ??z ?0.5 implies significant shell breaking at Ni56 with the neutron number N=28. © 2006 The American Physical Society. %B Phys. Rev. Lett. %C Department of Physics, Westmont College, Santa Barbara, CA 93108, United States %V 96 %P 1–4 %G eng %R 10.1103/PhysRevLett.96.102501