  We study the development of collectivity in the neutron-rich nuclei around
$N=40$, where experimental and theoretical evidences suggest a rapid shape
change from the spherical to the rotational regime, in analogy to what happens
at the {\it island of inversion} surrounding $^{31}$Na. Theoretical
calculations are performed within the interacting shell model framework in a
large valence space, based on a $^{48}$Ca core which encompasses the full $pf$
shell for the protons and the $0f_{5/2}$, $1p_{3/2}$, $1p_{1/2}$, $0g_{9/2}$
and $1d_{5/2}$ orbits for the neutrons. The effective interaction is based on a
G-matrix obtained from a realistic nucleon-nucleon potential whose monopole
part is corrected empirically to produce effective single particle energies
compatible with the experimental data. We find a good agreement between the
theoretical results and the available experimental data. We predict the onset
of deformation at different neutron numbers for the various isotopic chains.
The maximum collectivity occurs in the chromium isotopes, where the large
deformation regime starts already at $N=38$. The shell evolution responsible
for the observed shape changes is discussed in detail, in parallel to the
situation in the $N=20$ region.
