The magnetic transition in ε-Fe2O3 nanoparticles: Magnetic properties and hyperfine interactions from Mössbauer spectroscopy

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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.

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