  The interparticle interactions in a magnetic nanocomposite play a dominant
role in controlling the phase transitions superparamagnetic to superspinglass
to superferromagnetic. These interactions can be tuned by controlling the size
and number density of nanoparticles. Aerogel composites, 0.3 Ni-BC and 0.7
Ni-BC, consisting of Ni nanoparticles distributed in bacterial cellulose have
been used as model systems to study the effect of interparticle interactions
with increasing fraction of Ni in the composite. Contrary to conventional
approach, the size of Ni nanoparticles is not controlled and was allowed to
form naturally in the bacterial cellulose template. The structural
characterization using x-ray diffraction and electron microscopies indicates
the presence of only Ni and cellulose with no other phases. The uncontrolled
growth of Ni in cellulose matrix results in the formation of nanoparticles with
3 different size distributions - 10 nm particles mainly distributed along the
length of fibrils, 50 nm particles present in the intermediate spaces between
the fibrils and 100 nm particles present in voids formed by the reticulate
structure. At room temperature the composites exhibit a weakly ferromagnetic
behavior with a coercivity of 40 Oe which increases to 160 Oe at 10 K. The
transition from weakly ferromagnetic state at room temperature to a
superferromagnetic state at low temperatures is mediated via a superspinglass
state at intermediate temperatures. A temperature dependent microstructural
model has been developed combining the structural and magnetic results obtained
from the two composite aerogels.
