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

Abstract

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 T B ∼32 K. Below T B, the ferromagnetic interactions are present as suggested by coercivity H C ∼1900 Oe. The value of magnetic moment m P = 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).

Magnetic properties of Fe2O3 nanoparticles embedded in hollows of periodic nanoporous silica

“…The calculated value is comparable with the value determined from Scherrer formula ͑d =5 nm͒. 12 For potential applications of magnetic nanocomposite in magnetic recording media, it is necessary to know how different parameters affect the relaxation time in order to avoid spontaneous data erasure in the lifetime of the device ͑caused by thermal fluctuation͒. Because of the ongoing effort to increase the information/volume ratio, it is of special importance to know how the dipolar interactions of the nanoparticles affect the relaxation time.…”

“…For detailed assessment of the pore size the nitrogen adsorption/desorption experiments were used. 12 The pore size, as calculated from adsorption branch of the isotherms according to BarrettJoyner-Halenda ͑BJH͒ model ͑D BJH = 7.1 nm͒, was in good agreement with the HRTEM observations.…”

“…The detailed structural study was described elsewhere. 12 The results of the HRTEM ͑see Fig. 1͒ confirm the regular honey-comb structure of the composite sample.…”

“…The calculated value for hematite nanoparticles was K ef f = 2.4ϫ 10 6 erg/ cm 3 . This value is larger than the value for bulk hematite ͑K ef f =8ϫ 10 4 erg/ cm 3 ͒, 16 and can be related to the spin canting in the surface layer of nanosized antiferromagnetic hematite particles 12 and to the encapsulation of particles into the porous matrix. Similarly to above results, the Vogel-Fulcher analysis indicates that only weakly interacting superparamagnetic particles are present in Fe 2 O 3 @ SiO 2 nanocomposite.…”

“…12 The x-ray diffraction data were used for estimation of the particle size by Scherrer formula. The calculated value of the crystallite sizes was d = 5 nm.…”