  Exotic nuclei are characterized by a number of neutrons (or protons) in
excess relative to stable nuclei. Their shell structure, which represents
single-particle motion in a nucleus, may vary due to nuclear force and excess
neutrons, in a phenomenon called shell evolution. This effect could be
counterbalanced by collective modes causing deformations of the nuclear
surface. Here, we study the interplay between shell evolution and shape
deformation by focusing on the magnetic moment of an isomeric state of the
neutron-rich nucleus 75Cu. We measure the magnetic moment using highly
spin-controlled rare-isotope beams and achieving large spin alignment via a
two-step reaction scheme that incorporates an angular-momentum-selecting
nucleon removal. By combining our experiments with numerical simulations of
many-fermion correlations, we find that the low-lying states in 75Cu are, to a
large extent, of single-particle nature on top of a correlated 74Ni core. We
elucidate the crucial role of shell evolution even in the presence of the
collective mode, and within the same framework, we consider whether and how the
double magicity of the 78Ni nucleus is restored, which is also of keen interest
from the perspective of nucleosynthesis in explosive stellar processes.
