  Antiferromagnetic Bi_{2}Fe_{4}O_{9} (BFO), lightly substituted by cobalt is
studied for magnetodielectricity. The substitution causes a substantial
decrease in the Neel temperature (T_N) from 250 K (in parent sample, BFO) to
152 K (in 2% Co substituted sample). At the same time, the substituted samples
display a pronounced irreversibility in the ZFC-FC magnetization data for T $<$
370 K and opening of hysteresis in the M-H plot, thus signifying the onset of
weak ferromagnetism (FM) and magnetic glassiness. The induced magnetic
glassiness is found to slow down the dynamics such that the magnetization decay
follows. The dielectric measurement in the same temperature window shows
unusual oppression in, for T$\sim$T_N and contrasting nature of tan loss for
temperatures above and below T_N, thus hinting a plausible coupling between the
magnetic and electric order parameters. A confirmation to this coupling is seen
in the magnetodielectric (MD) results, in which it is found that the
substitution induces an additional component in the MD, apart from the usual
components in BFO. This additional component of MD is found to obey behaviour,
with the n values being comparable to 1-p of magnetization. The temperature
variation of MD also shows a contrasting behaviour for the parent and 2% Co
substituted sample with an enhancement of two times in MD value. In summary,
our study shows ME coupling introduced by the magnetic glassiness and its
behaviour is very much different from the intrinsic one.
