Nanoscale size effect on surface spin canting in iron oxide nanoparticles synthesized by the microemulsion method

Abstract

Uniformly sized and crystalline iron oxide nanoparticles (IONPs) with spinel structure and mean diameters of about 3, 6 and 9 nm were synthesized in high yield using the microemulsion route at room temperature. The nanoparticles (NPs) were stabilized in situ by organic surfactant molecules which acted both as a stabilizer of the microemulsion system and as a capping layer of the NP surface. NP size control was attained by careful adjustment of the preparation conditions. The structure, morphology and NP size distribution were investigated by x-ray diffraction, transmission electron microscopy and scanning electron microscopy. A particular effort was devoted in this work to study the effect of size and capping of these NPs on their magnetic structure by in-field Mössbauer spectroscopy at 4.2 K. The mean canting angle (relative to the applied field direction) of the Fe spins was observed to increase with decreasing NP size due to the enhanced surface-to-volume ratio. Comparing bare and capped NPs of the same diameter, we verified that the spin canting was not affected by the organic capping. This implied almost identical magnetic orientations of bare and capped NPs. Simultaneously, the capping material was capable of preventing agglomeration effects which can occur in case of direct particle contact. Using a core/shell model, we showed that spin canting originated from the surface shell of the NPs. Furthermore, the Mössbauer spectral parameters provided evidence for the existence of a high fraction of Fe3O4 (magnetite) in the IONP.

The dipole moment of the spin density as a local indicator for phase transitions

“…In our NPs, spin canting has also been experimentally observed as a finite slope at high magnetic fields in magnetization hysteresis measurements with XMCD44 and with Mößbauer spectroscopy as described in Ref. [45]. Thus, a reduced VT temperature seems to be reasonable and can explain the reduced transition temperature measured with XMCD.…”

Investigation of structural and magnetic properties of Ni0.5Zn0.5Fe2O4 nano powders prepared by self combustion method

“…Some of the commonly observed size effects are lowering of saturation magnetization, increase in coercivity, surface spin canting, exchange anisotropy and inter particle interactions [12][13][14][15][16][17]. Particle size and size distribution are known to be sensitive to the processing techniques/conditions and is one of the main challenges in the synthesis of nano-sized particles.…”

Evolution of the magnetic structure with chemical composition in spinel iron oxide nanoparticles

“…The symmetry breaking induces changes in the topology of the superficial magnetic moments and consequently in exchange integrals (through superexchange angles and/or distances between moments), thus leading to a different local surface anisotropy. [9][10][11][12] This feature has also been confirmed by polarized neutron scattering 13 and Mössbauer experiments 14 in cobalt and copper ferrites, respectively. with spinel structure have attracted much interest, not only because of their technological applications, but also because the rich crystal chemistry of spinels offers excellent opportunities for fine tuning the magnetic properties.…”