  The shapes of neutron-rich exotic Ni isotopes are studied. Large-scale shell
model calculations are performed by advanced Monte Carlo Shell Model (MCSM) for
the $pf$-$g_{9/2}$-$d_{5/2}$ model space. Experimental energy levels are
reproduced well by a single fixed Hamiltonian. Intrinsic shapes are analyzed
for MCSM eigenstates. Intriguing interplays among spherical, oblate, prolate
and gamma-unstable shapes are seen including shape fluctuations, $E$(5)-like
situation, the magicity of doubly-magic $^{56,68,78}$Ni, and the coexistence of
spherical and strongly deformed shapes. Regarding the last point, strong
deformation and change of shell structure can take place simultaneously, being
driven by the combination of the tensor force and changes of major
configurations within the same nucleus.
