  For many important applications, the performance of polymer-anisotropic
particle nanocomposite materials strongly depends on the orientation of the
nanoparticles. Using the very peculiar magnetic properties of goethite
({\alpha}-FeOOH) nanorods, we produced goethite-poly(hydroxyethyl methacrylate)
nanocomposites in which the alignment direction and the level of orientation of
the nanorods could easily be tuned by simply adjusting the intensity of a
magnetic field applied during polymerization. Because the particle volume
fraction was kept low (1-5.5 vol \%), we used the orientational order induced
by the field in the isotropic phase rather than the spontaneous orientational
order of the nematic phase. At the strongest field values (up to 1.5 T), the
particles exhibit almost perfect antinematic alignment, as measured by optical
birefringence and small-angle X-ray scattering. The results of these two
techniques are in remarkably good agreement, validating the use of
birefringence measurements for quantifying the degree of orientational order.
We also demonstrate that the ordering induced by the field in the isotropic
suspension is preserved in the final material after field removal. This work
illustrates the interest, for such problems, of considering the field-induced
alignment of anisotropic nanoparticles in the isotropic phase, an approach that
is effective at low filler content, that avoids the need of controlling the
nematic texture, and that allows tuning of the orientation level of the
particles at will simply by adjusting the field intensity.
