Karel Zaveta

The δ and θ Al ₂ O ₃ phases well stabilized Fe( iii ) in T _d or O _h coordination, which were identified as the active species in high temperature decomposition of N ₂ O in a complex gas mixture produced by oxidation of ammonia.

We studied the effect of interfaces character in 57Fe/Au thin layers on iron hyperfine parameters using Mössbauer spectroscopy. Four samples of bilayers (57Fe covered by Au) were prepared on MgO substrate by pulsed laser deposition (PLD) with varying 57Fe layer thickness (∼10–95 nm), covered by Au layer with constant thickness of ∼8.7 nm. Mössbauer spectra were acquired in transmission and conversion electron geometry. The hyperfine parameters of α–Fe layer, Fe/Au and MgO/Fe interfaces were determined.

Low temperature (4.2 K) magnetic behavior of Fe₇₆Mo₈Cu₁B₁₅ metallic glass was studied by ⁵⁷Fe Mössbauer spectrometry (MS) and ⁵⁷Fe NMR. Distributions of hyperfine magnetic fields P(B) were determined for as-quenched and annealed (nanocrystalline) samples with relative fraction of the grains about 43%. P(B) distributions were derived for both the amorphous matrix and nanocrystalline grains. NMR of alloys with natural and ⁵⁷Fe enriched Fe enabled to assess the contribution of ¹¹B to the total NMR signal. P(B) distribution of the as-quenched alloy derived from MS matches reasonably well the one from NMR of the enriched sample. NMR signal from the sample with natural Fe exhibits contributions from ¹¹B nuclei. The principal NMR lines of the annealed alloys at 47 MHz correspond to bcc Fe nanocrystals. Small asymmetry of the lines towards higher frequencies might be an indication of possible impurity atoms in the bcc structure. The observed differences between natural and enriched samples are attributed to higher sensitivity in the latter. Positions of the lines attributed to bcc Fe nanocrystals obtained from MS and NMR are in perfect agreement.

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.

The research and utilization of the magnetic nanoparticles in, e.g., biology and medicine have been preferentially oriented on magnetite Fe ₃ O ₄ and maghemite γ-Fe ₂ O ₃ . The complex oxides allow a better control of the magnetic properties in a desirable way. We demonstrate this approach on an example of the La _1-x Sr _x MnO ₃ perovskites from the point of view of their use in self-controlled magnetic fluid hyperthermia and increase of contrast in magnetic resonance imaging. With materials possessing suitable values of transition temperature T _C , coercivity and magnetization, the heating power generated by the nanoparticles was estimated by two independent methods and they were finally employed in vitro and in vivo hyperthermia experiments.