Cobalt substitution induced magnetodielectric enhancement in multiferroic Bi2Fe4O9
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.