  We explore possibility of tuning photonic crystal properties via
order-disorder transition. We fabricated a photonic bandgap material consisting
of a three-dimensional array of conducting magnetizable spheres. The spheres
self-assemble into ordered state under external magnetic field, in such a way
that the crystalline order can be continuously controlled. We study mm-wave
transmission through the array as a function of magnetic field, i.e. for
different degrees of order. This was done for the regular crystal, as well for
the crystal with the planar defect which demonstrates resonance transmission at
a certain frequency. We observe that in the ordered, "crystalline" state there
is a well-defined stopband, while in the completely disordered, glassy or
"amorphous" state, the stopband nearly disappears. We relate the disappearance
of the stopband in the disordered state to the fluctuations in the particle
area density. We develop a model which predicts how these fluctuations depend
on magnetic field and how they affect electrodynamic properties of the whole
sample. The model describes our results fairly well.
