Structural and Magnetic Properties of ultra-low density BiFeO3 nanoparticles Produced by Pulsed Laser Deposition

Ultra-low-density BiFeO3nanoparticles have been prepared by pulsed laser deposition and its structure and magnetic properties have been studied. Annealing increases crystallinity and size of the particles leading to alteration of magnetic properties, observed from magnetic studies and evaluated using HRTEM, selected area electron diffraction (SAED) and XRD patterns analysis. Transmission electron microscope results show that the BiFeO3as-deposited nanoparticles and annealed up to 400C exhibit a orthorhombic distorted perovskite structure without secondary phase with diameter varying from 9 nm (as-deposited) to 17 nm (annealed at 4000C). Magnetic data exhibit exchange bias and magnetic blocking effects at low temperatures and typical superparamagnetic behavior at high temperatures. Meanwhile, the BiFeO3nanoparticles annealed at 5000C exhibit a rhombohedrally distorted perovskite structure with typical antiferromagnetic properties and diameter of about 56nm. The analysis of magnetic relaxation time using Arrhenius equation suggests a superparamagnetic blocking process of ferromagnetic cluster on the surface of the nanoparticles at low temperature. The magnetic properties are discussed considering interaction between nanoparticles and the co-existence of different magnetic phases within the nanoparticles: an ordered antiferromagnetic core and ferromagnetic clusters on the surface.