Origin of the magnetic transition at 100 K inε-Fe₂O₃nanoparticles studied by x-ray absorption fine structure spectroscopy

Abstract

We present a study of the correlation between the magnetic phase transition and the structural distortion observed at 100 K in ε-Fe2O3. For this purpose, we have designed a novel one-pot sol-gel method assisted by glycerol, which reproducibly provides samples with a nominal 100% concentration of ε-Fe2O3 nanoparticles embedded in a SiO2 matrix. The high crystallinity of the samples and the absence of other iron oxide polymorphs has allowed us to perform, for the first time, temperature-dependent X-ray absorption fine structure spectroscopy experiments, with the aim of investigating the origin of the magnetic quenching anomaly observed at 100 K. The deformation of the structure at a local scale, where the tetrahedral and octahedral Fe sites undergo distortions of different intensities, has been simulated to fulfill the long-range order. Our results point to a local structure distortion accompanied by the magnetism quenching through a magneto-elastic coupling.

Stability and nature of the volume collapse of ε-Fe2O3 under extreme conditions

“…From the CS (0.35(2) mm∙s −1 ), we can assign it to a paramagnetic high-spin state of Fe 3+ in octahedral coordination, whose abundance increases with P 49 . The CS and QS values of this doublet are similar to those found in the literature 50,51 at room conditions, where they were tentatively assigned to the presence of hematite or superparamagnetic (SPM) ε-Fe 2 O 3 . In our case, however, the presence of the hematite is excluded by XRD technique and synchrotron Mössbauer spectroscopy, where the hematite, sextet characterized by a large hyperfine magnetic splitting, is not observed.…”

Control of the Length of Fe73.5Si13.5Nb3Cu1B9 Microwires to Be Used for Magnetic and Microwave Absorbing Purposes

“…No changes in the energy position at the absorption edge of the XANES spectra are observed (first peak located at the same position in the first-derivative spectra in inset of Figure 7a). These results evidence the same valence state for as-cast MWs, after annealing performed at 550 ºC and Fe foil (oxidation state 0) [46][47][48][49]. The short range structure of Fe ions for as-cast amorphous MWs and after the annealing process is also evaluated by EXAFS measurements.…”

Effect of gadolinium on magnetic circular dichroism and electron magnetic resonance of ε-Fe2O3 nanoparticles formed in borate glasses

“…This maximum can be indicative of the transition of the magnetic state of the nanoparticles from the superparamagnetic to the blocked one. The occurrence of the wide maximum in the ZFC curve characteristic of superparamagnetic nanoparticles is similar to that reported by López-Sánchez et al [42] and can be explained by a large spread in the nanoparticle sizes.…”

“…Substantial changes have been reported in magnetic characteristics of fine (3.4 nm) ε-Fe 2 O 3 nanoparticles prepared by impregnating silica gel and aluminium oxide with FeSO 4 solutions [23][24][25]41]. Coercivity of ε-Fe 2 O 3 particles of sizes ranging from 7 to 15 nm, embedded in SiO 2 films, was 0.009 T [42].…”