J. Kohout

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

The nanoparticles of e-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 e-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 Mossbauer spectra measured in the temperature range of 4.2–300 K, the hyperfine parameters for one tetrahedral and three octahedral sites of e-Fe2O3 structure were identified. The in-field spectra in the external magnetic fields up to 6 T were taken both above and below the indicate...

NMR study of Ti-Co, Ti-Mg and Ti substituted hexagonal ferrites with magnetoplumbite structure

Abstract Domain 57 Fe NMR spectra of Ti-Co, Ti-Mg and Ti substituted hexagonal ferrites have been measured at 4.2 K using the spin echo technique. Characteristic satellite lines corresponding to the substitution in the vicinity of the resonating nucleus appeared in the NMR spectra. The dependence of the relative intensities of the NMR lines on the amount of the substitution is determined and the preference of the substitutions for the different crystallographic sites is discussed.

Hyperfine fields in nanocrystalline Fe–Zr–B probed by F57e nuclear magnetic resonance spectroscopy

In nanocrystalline Fe90Zr7B3, hyperfine fields belonging to the amorphous residual matrix are distinguished from those of bcc-Fe nanograins by nuclear magnetic resonance spectroscopy on F57e nuclei. With this technique, the nanograins located in magnetic domains can be distinguished from those positioned in domain walls. Structural features of core and surface regions of both types of nanograins are described. Presence of small (∼0.2%) inclusions of Zr was identified in the core of nanograins.

NMR of 57Fe above the Verwey transition in Al-substituted magnetite

Abstract Temperature dependence of 57 Fe NMR was measured above the Verwey transition in three Fe 3− x Al x O 4 ( x =0, 0.005, 0.03) single crystals. The satellite lines corresponding to Fe (A) which have a single Al (B) nearest neighbor were detected in the NMR spectra of x =0.005 and 0.03 compounds. Temperature dependence of NMR linewidth for x =0.03 indicates that the electron density becomes inhomogeneous as T → T V .

Applicability of NMR to impurities detection in YIG thin films

Abstract An impurity in the vicinity of the 57 Fe nucleus induces a specific structure of satellite lines in the NMR spectrum, the intensities of which are proportional to the impurity concentration. Recently achieved sensitivity makes it possible to detect as little as 0.02% impurity. Spectra of nominally pure YIG epitaxial films, containing Pb, Ga, Al, Si, Pt, Ca impurities and ‘antisite’ defects are presented to illustrate the approach.

Anisotropy of the Hyperfine Field on 57Fe in Pure and Substituted Yttrium Iron Garnet

Using NMR, anisotropy of the hyperfine field on 57Fe nuclei was measured in Y3Fe5O12 with nonmagnetic, trivalent substitutions on d (Ga3+) and a (Y3+ - yttrium antisite defect) sites which are occupied by ferric ions in a pure, stoichiometric system. The measurements were performed by the spin-echo method at liquid helium temperature. Thorough analysis of the hyperfine field anisotropy on the 57Fe nuclei in an ideal environment and in an environment where one of Fe3+ nearest cation neighbours is replaced by the nonmagnetic impurity is given. When the Fe3+ on which the NMR is measured is on the a site and the impurity is Ga3+(d), the results may be interpreted in terms of the superposition model. On the other hand, the results for Fe3+ on the d site and Y3+(a) are in clear disagreement with the predictions of this model.

57Fe NMR in BaFe2-W hexagonal ferrite

Abstract 57 Fe NMR spectra from magnetic domains of BaFe 2 2+ Fe 16 3+ O 27 hexaferrite single crystal were measured at 4.2K using the conventional spin echo technique. External magnetic fields up to 3 T parallel and perpendicular to the hexagonal axis were applied to separate the overlapped NMR lines and to find their correspondence to the seven nonequivalent lattice sites occupied by Fe 3+ ions. A pronounced satellite structure of lines from tetrahedral sites was found. These results were analyzed with respect to the distribution of Fe 2+ ions among the iron sites.

Low temperature behavior of hyperfine fields in amorphous and nanocrystalline FeMoCuB

Low temperature (4.2 K) magnetic behavior of Fe76Mo8Cu1B15 metallic glass was studied by 57Fe Mossbauer 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 highe...

57Fe Mössbauer spectroscopy of synthesized ɛ-Fe2O3 nanoparticles

Magnetic nanoparticles have promising applications in many areas, for example optics, electronics, biology, medicine, etc. The primary goal of this study was to synthesize nanoparticles of magnetic ɛ-Fe2O3 embedded in amorphous SiO2. The structure and properties of the final product have been found to depend strongly on the initial conditions of preparation. The Mössbauer spectroscopy analysis of the samples was complemented by the study of X-ray powder diffraction, magnetic measurements, and high-resolution transmission electron microscopy.

Temperature dependence of NMR spectra in YIG with nonmagnetic impurities

Abstract NMR spectra on 57 Fe nuclei in YIG containing Ga 3+ (d) and Y 3+ (a) impurities were measured in the wide temperature region 4.2–370 K in zero external magnetic field. The temperature dependence of satellite lines caused by Y 3+ (a) and Ga 3+ (d) impurities was determined. Experimental results are compared with the theoretical calculation, based on the molecular field approximation.