H. Stepankova

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 .

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...

Analysis of Cationic Impurity Impact on Hyperfine Interactions in Magnetite

The Al, Ga, Ti and Zn impurities were studied in high quality single crystal samples of magnetite (Fe3O4). The cases when the valence of the substitution ion differs from that of the original iron ion are of a particular interest. The measurement of nuclear magnetic resonance (NMR) of 57Fe was chosen as the experimental method for its sensitivity to Fe magnetic and electronic structure and to local ordering in the neighborhood of resonating nuclei. Local electronic structure and hence also the hyperfine interactions are affected by the presence of the substitution and thus resonance frequencies of nuclei in the vicinity of the substitution are shifted due to the modified hyperfine field, therefore satellite signals are observed in the NMR spectra. Temperature dependences of spectra above the Verwey transition were measured in a zero external magnetic field. Subsequently, the dependences of frequencies of main lines and satellite signals in the spectra on temperature were fitted with spin-wave model dependences and compared for different types of the substitution. Furthermore, a mean-field model was applied on the substituted magnetite system and calculated temperature dependences were confronted with the experimental data. The agreement of mean field model with the experimental temperature dependences of satellite resonances is better for those induced by the cationic substitutions at octahedral sites.

NMR study of Ga and Al substituted hexagonal ferrites with magnetoplumbite structure

Fe NMR spectra and relaxation rates in lead hexaferrite single crystals PbFe12019, PbFelz-,Ga,Olg, (x=0.42, 1.2) and PbFel~-,Al,Olg, (x=0.21) have been measured at 4.2 K using spin echo technique. Besides the main lines corresponding to the nonequivalent positions of Fe3+ ions in the magne- toplumbite structure, the satellite lines caused by sub- stitution in the vicinity of resonating nuclei have been observed. The dependence of the NMR spectrum on the external magnetic Aeld has been used to separate the overlapped satellites from different iron sites. The most distant satellites have been assigned to the res- onance of iron nuclei at 4f1 site with substitution at 12k and 2a sites in their vicinity. A strong preference of 2a site for A1 substitution has been found.