Effect of Holmium substitution on the magnetic and magnetodielectric properties of multiferroic Bi_2Fe_4O_9

摘要

In the quest for deriving new multiferroics from the existing ones, we have prepared and studied polycrystalline Bi_(2(1-x))Ho_(2x)Fe_4O_9 (0 ≤ x ≤ 0.02) ceramics. A substantial increase in the Néel temperature (T_N) from 250 K (x = 0, BFO) to 266 K (x = 0.02, BHFO2) is observed for Ho-substituted samples. The magnetization measurements suggest that Ho~(3+) goes as isolated magnetic defects and interaction among them is reflected only when the temperature goes below 70 K. Interestingly, the Néel temperature is not clearly visible in the magnetization-temperature plot due to its masking by the high moment of isolated Ho~(3+) ions, but T_N is very clearly reflected in the dielectric plot, thus indicating a plausible coupling between the magnetic and electric order parameters. Also, a dielectric crossover at T ∼ 200 K is observed for Ho~(3+) substituted samples and explained using a mean-field approximation model, thereby validating the presence of isolated defects arising due to Ho~(3+) substitution. At the same time, an enhanced magnetodielectric (MD) effect at 200 K i.e., ∼-1.6% (∼530 times) for BHFO2 is discerned as compared to BFO. Furthermore, confirmation to this coupling is drawn from MD% versus T plot and MD% versus H plot, where the latter is found to obey (H)m behaviour.