Paper NS-MoM8
Exchange Bias in Pure and Core/Shell Structured γ-Fe₂O₃-based Nanoparticles

Monday, October 29, 2012, 10:40 am, Room 12

Iron oxide nanoparticles, Fe₃O₄ and γ-Fe₂O₃, are of great interest for applications in high-density magnetic recording media, sensor technology, and biomedicine. These systems are also excellent candidates for probing fundamental properties due to well-established synthesis methods that yield uniform, high quality particles with good control over size and shape. It has recently been shown by polarized small angle neutron scattering that Fe₃O₄ nanoparticles possess a chemically uniform, but magnetically distinct, core and canted-spin shell structure [1]. While many similarities exist in the magnetic and microstructural properties of γ-Fe₂O₃ and Fe₃O₄, the exchange bias that can be observed in nanoparticles of γ-Fe₂O₃ [2] is not present in similarly sized Fe₃O₄ [3]. Therefore, an investigation of the evolution of spin canting angle and shell thickness with temperature in γ-Fe₂O₃ is expected to yield valuable information about subtly altered spin geometries that can be correlated with the presence of exchange bias in this nanoparticle system.

In order to compare results for both chemically distinct and chemically uniform exchange-biased systems, we report the synthesis and characterization of γ-Fe₂O₃ and core/shell Fe/γ-Fe₂O₃ nanoparticles. The p articles used in the present study were synthesized by high temperature decomposition of iron organometallic compounds. X-ray diffraction and transmission electron microscopy techniques were used to study the structural and microstructural properties of the nanoparticles, which confirmed the presence of bcc iron and fcc γ-Fe₂O₃ in these particles. High-resolution TEM images evidenced monodisperse products with particle diameters of 9±0.8 nm in the pure γ-Fe₂O₃, while the core/shell particles showed 9.8±0.7 nm Fe cores surrounded by a shell of γ-Fe₂O₃ with 2±0.4 thickness. The DC magnetic properties of the samples were characterized using a vibrating sample magnetometer over a temperature range of 5-300 K, revealing a superparamagnetic behavior. Pronounced exchange bias of up to ~4100 Oe was confirmed in these particles using cooling fields of up to 5T. While the spin-glass-like phase associated with disordered surface spins in γ-Fe₂O₃ plays an important role as a fixed phase in both systems, providing the pinning force to the reversible spins, the frozen spins at the interface between Fe and γ-Fe₂O₃ are also shown to contribute to EB in the core/shell Fe/γ-Fe₂O₃ nanoparticles .