No abstract

The nanoparticles of ε-Fe2O3 enriched with 57Fe isotope in amorphous silica matrix were prepared by sol-gel technique starting from a single molecular precursor for both Fe2O3 and silica. From the X-ray powder diffraction pattern ε-Fe2O3 was identified as the major phase and α-Fe2O3 and β-Fe2O3 were observed as minor iron oxide phases. Using the log-normal distribution for fitting the experimental data from the TEM micrographs, the characteristic size of particles d0 ∼ 25 nm was derived. The rather high coercivity of ∼2.1 T at room temperature was confirmed for our nanoparticle system. From the dependences of magnetization on temperature a two-step magnetic transition spread between 100 K and 153 K was indicated. From the 57Fe Mössbauer spectra measured in the temperature range of 4.2–300 K, the hyperfine parameters for one tetrahedral and three octahedral sites of ε-Fe2O3 structure were identified. The in-field spectra in the external magnetic fields up to 6 T were taken both above and below the indicated two-step magnetic transition. Their dependence on temperature and external magnetic field suggests that the first step in the temperature range of 153 K–130 K is related to the spin reorientation of the local magnetic moments in the magnetic sublattices and the second step in temperatures 130 K–100 K may be associated with the intermediate spin–high spin state transition of Fe3+ cation in the tetrahedral sublattice expressed in the change of the hyperfine magnetic field.

No abstract

No abstract

The influence of added sucrose on the phase composition of iron oxide nanoparticles was studied in the Fe 2 O 3 / SiO 2 nanocomposites. The nanocomposites were prepared by impregnation of mesoporous silica SBA-15 with iron nitrate containing various amounts of sucrose and their subsequent annealing at 1100 and 1200 °C. The phase composition of the iron oxide nanoparticles in the prepared nanocomposites was analyzed by X-ray diffraction and 57 Fe Mossbauer spectroscopy on polycrystalline samples. Profile analysis of the X-ray diffraction maxima was used to determine the apparent crystallite size of the different polymorphs. The content of β-Fe 2 O 3 in the samples increased with an increasing amount of sucrose, and no β-Fe 2 O 3 was observed in the samples without sucrose. These samples contained only ε-Fe 2 O 3 and hematite. The apparent crystallite size was found to be similar for both hematite and β-Fe 2 O 3 . Therefore, sucrose was identified as the primary variable, which directs the ε-Fe 2 O 3 transformation to either hematite or β-Fe 2 O 3 . Our findings shift the way we understand the β-Fe 2 O 3 formation from ε-Fe 2 O 3 in the Fe 2 O 3 /SiO 2 system from the size-directed to the starting materialdirected transformation.