Particle size dependence of magnetization and phase transition near T_N in multiferroic BiFeO_3

摘要

We report results of a comprehensive study of the phase transition at T_N (～643 K) as a function of particle size in multiferroic BiFeO_3 system. We employed electrical, thermal, and temperature dependent x-ray diffraction studies in order to characterize the transition in a host of samples. We also carried out detailed magnetic measurements over a temperature regime of 2-300 K under a magnetic field of 100-10 000 Oe both on bulk and nanocrystalline systems. While in the bulk system a sharp endothermic peak at T_N together with a broad feature, ranging over nearly ～100 K (ΔT), could be observed in calorimetry, the nanoscale systems exhibit only the broad feature. The characteristic dielectric anomaly, expected at T_N, is found to occur both at T_O and T_N across ΔT in the bulk sample. The Maxwell-Wagner component due to interfaces between heterogenous regions with different conductivities is also present. The magnetic properties, measured at lower temperature, corroborate our observations in calorimetry. The metastability increases in the nanoscale BiFeO_3 with divergence between zero-field cooled and field cooled magnetizations below ～100 K and faster magnetic relaxation. Interestingly, in nanoscale BiFeO_3 one also observes finite coercivity at lower temperature, which points out that suitable design of particle size and shape may induce ferromagnetism. The inhomogeneous distribution of Bi/Fe ions and/or oxygen nonstoichiometry seems to be giving rise to broad features in thermal, magnetic as well as electrical responses.