Particle size dependent magnetic properties and phase transitions in multiferroic BiFeO3 nano-particles

The particle size effect in the range 10-150 nm on the magnetic properties and phase transitions in BiFeO3 samples prepared by spray pyrolysis method has been studied. The phase purity and structure have been investigated by XRD and FTIR spectroscopy analysis. The FTIR peaks of the nanoparticles shift to lower wave number due to increase surface area and grain boundaries. The Fe-SEM and TEM images show that the particles are uniform, dense and of nearly spherical shape nanoparticles. The significant enhancement in magnetization with finite coercive field has been observed in 12 nm particle size samples. The increase in magnetization is about four times larger than that of the bulk samples. It has been attributed to the suppression of the cycloidal spin structure due to large uncompensated spins of Fe+3 ions at the surface of the particle and the adsorbate induced spin polarization screening in BFO nanoparticles. The phase transitions above room temperature have been investigated by DTA measurements and show that Neel temperature (T-N) and Curie temperature (T-c) increase with particle size. The shift in T-N and T-c values with particle size are fitted well to the finite scaling models. The microscopic parameters like correction length, characteristic microscopic dimension of the system and critical particle size have been evaluated which provide more physical insight in the finite scaling effect in the nanoparticle samples. (C) 2012 Elsevier B.V. All rights reserved.