The magnetic transition in epsilon-Fe2O3 nanoparticles: Magnetic properties and hyperfine interactions from Mossbauer spectroscopy
Abstract
The nanoparticles of epsilon-Fe2O3 enriched with 57 Fe 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 epsilon-Fe2O3 was identified as the major phase and alpha-Fe2O3 and beta-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 d(0) similar to 25 nm was derived. The rather high coercivity of similar to 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 57 Fe Mossbauer spectra measured in the temperature range of 4.2-300 K, the hyperfine parameters for one tetrahedral and three octahedral sites of epsilon-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 sub-lattices 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. (C) 2015 AIP Publishing LLC.