  The ability to control the light matter interaction and simultaneous tuning
of both structural order and disorder in materials, although are important in
photonics, but still remain as major challenges. In this paper, we show that
optical path length dictates light-matter interaction in the same crystal
structure formed by the ordering of magnetic nanoparticle self-assembled
columns inside magnetic nanofluid under applied field. When the optical path
length (L=80 {\mu}m) is shorter than the optical (for wavelength,
{\lambda}=632.8 nm) coherence length inside the magnetic nanofluid under
applied field, a Debye diffraction ring pattern is observed; while for longer
path length (L=1mm), a corona ring of scattered light is observed. Analysis of
Debye diffraction ring pattern suggests the formation of 3D hexagonal crystal
structure, where the longitudinal and lateral inter-column spacings are 5.281
and 7.344 microns, respectively. Observation of speckles within the Debye
diffraction pattern confirms the presence of certain degree of structural
disorder within the crystal structure, which can be tuned by controlling the
applied field strength, nanoparticle size and particle volume fraction. Our
results provide a new approach to develop next generation of tunable photonic
devices, based on simultaneous harnessing of the properties of disordered
photonic glass and 3D photonic crystal.
