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

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.