  We report on the growth of nanostructured rods fabricated by electrostatic
co-assembly between iron oxide nanoparticles and polymers. The nanoparticles
put under scrutiny, {\gamma}-Fe2O3 or maghemite, have diameter of 6.7 nm and
8.3 nm and narrow polydispersity. The co-assembly is driven by i) the
electrostatic interactions between the polymers and the particles, and by ii)
the presence of an externally applied magnetic field. The rods are
characterized by large anisotropy factors, with diameter 200 nm and length
comprised between 1 and 100 {\mu}m. In the present work, we provide for the
first time the morphology diagram for the rods as a function of ionic strength
and concentration. We show the existence of a critical nanoparticle
concentration and of a critical ionic strength beyond which the rods do not
form. In the intermediate regimes, only tortuous and branched aggregates are
detected. At higher concentrations and lower ionic strengths, linear and stiff
rods with superparamagnetic properties are produced. Based on these data, a
mechanism for the rod formation is proposed. The mechanism proceeds in two
steps : the formation and growth of spherical clusters of particles, and the
alignment of the clusters induced by the magnetic dipolar interactions. As far
as the kinetics of these processes is concerned, the clusters growth and their
alignment occur concomitantly, leading to a continuous accretion of particles
or small clusters, and a welding of the rodlike structure.
