Influence of Surface Effects on Magnetic Behavior of Hematite Nanoparticles Embedded in Porous Silica Matrix

Unique magnetic properties of superparamagnetic iron oxide (hematite) nanoparticles prepared by nanocasting in a periodic nanoporous silica (PNS) matrix are described in this work. The nanoparticles were prepared via a novel approach, when the external surface of PNS was modified to become more hydrophobic. The prepared composite sample was characterized by the synchrotron related techniques small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and X-ray absorption near edge spectra (XANES) measurements, by nitrogen adsorption/desorption, and by high resolution transmission electron microscopy (HRTEM). The investigation of the magnetic properties by superconducting quantum interference device (SQUID) magnetometry at the temperatures 2−300 K shows the superparamagnetic relaxation of the particles with a blocking temperature of TB ∼32 K. Below TB, the ferromagnetic interactions are present as suggested by coercivity HC ∼1900 Oe. The value of magnetic moment mP = 296 μB of the hematite particle was estimated by distribution of Langevin functions. This magnetic moment originates in uncompensated surface spins of Fe3+ ions due to the small size of hematite particles (5 nm) and due to the loading of nanoparticles into silica matrix. The magnetic behavior of the Fe2O3@PNS nanocomposite is mainly related to the surface effects (spin canting and different surface to volume spin ratio

Silica-Coated and Bare Akaganeite Nanorods: Structural and Magnetic Properties

We report on structural and magnetic properties of uniform silica-coated akaganeite nanorods with length of <i>L</i> ∼ 80 ± 15 nm and diameter <i>D</i> ∼ 15 ± 5 nm as well as silica shell thickness of about 5 nm. Unexpected negative difference between field-cooled (FC) and zero-field-cooled (ZFC) magnetization Δ<i>M</i> = <i>M</i>_FC – <i>M</i>_ZFC < 0, room temperature ferromagnetism, and exchange bias effect have been found. The nanorods are investigated by X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) measurements. The magnetic measurements were also performed on bare akaganeite nanorods in order to discriminate the effects of silica coating on the magnetic properties. The measured coercivity and exchange bias effect of bare β-FeOOH nanorods are much lower compared with same properties of SiO₂@β-FeOOH nanorods, emphasizing the effect of silica coating on the magnetic properties. These results are discussed considering the core–shell structure of akaganeite nanorods; i.e., the inner part of the akaganeite nanorod has antiferromagnetic ordering, whereas the nanorod surface exhibits some disorder spin state