Particle size dependence of magnetization and phase transition near T_N in multiferroic BiFeO3

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 BiFeO3 system. Weemployed electrical, thermal, and temperature dependent X-ray diffraction (XRD)studies in order to characterize the transition in a host of samples. We alsocarried out detailed magnetic measurements over a temperature regime 2-300 Kunder a magnetic field 100-10000 Oe both on bulk and nano-crystalline systems.While in the bulk system a sharp endothermic peak at T_N together with a broadfeature, ranging over nearly ~150 K (Delta_T), could be observed incalorimetry, the nanoscale systems exhibit only the broad feature. Thecharacteristic dielectric anomaly, expected at T_N, is found to occur both atT_O and T_N across Delta_T in the bulk sample. The Maxwell-Wagner component dueto interfaces between heterogenous regions with different conductivities isalso present. The magnetic properties, measured at lower temperature,corroborate our observations in calorimetry. The metastability increases in thenanoscale BiFeO3 with divergence between zero-field cooled (ZFC) and fieldcooled (FC) magnetization below ~100 K and faster magnetic relaxation.Interestingly, in nanoscale BiFeO3, one also observes finite coercivity atlower temperature which points out that suitable design of particle size andshape may induce ferromagnetism. The inhomogeneous distribution of Bi/Fe-ionsand/or oxygen non-stoichiometry seems to be giving rise to broad features inthermal, magnetic as well as in electrical responses.