Cobalt substitution induced magnetodielectric enhancement in multiferroic Bi_2Fe_4O_9

摘要

Antiferromagnetic Bi_2Fe_4O_9 (BFO) lightly substituted by cobalt is studied for magnetodielectricity. The substitution causes a substantial decrease in the Néel temperature (T_N) from 250 K (in the parent sample, BFO) to 152 K (in the 2% Co substituted sample). At the same time, the substituted samples display a pronounced irreversibility in the zero field cooled-field cooled magnetization data for T < 370 K and opening of hysteresis in the M-H plot, thus signifying the onset of weak ferromagne-tism and magnetic glassiness. The induced magnetic glassiness is found to slow down the dynamics such that the magnetization decay follows M(t) ∝ exp[—(t)~(1-p)]. The dielectric measurement in the same temperature window shows the unusual oppression in ε' for T～T_N and the 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 ∝ exp (ω)~n behaviour, with the values being comparable to "7-/?" of magnetization. The temperature variation of MD also shows a contrasting behaviour for the parent and 2% Co substituted samples with an enhancement of two times in the MD value. In summary, our study shows that magnetoelectric coupling introduced by the magnetic glassiness and its behaviour is very much different from the intrinsic one.