A Mössbauer spectral study of some magnetic rare-earth iron intermetallic compounds

In this thesis the Mössbauer spectra of Ce2Fe17Hx, where x is 0, 1,2, 3, 4, and 5, Pr2Fe17Hx where x is 0, 1, 2, 3, 4, and 5, Dy2Fe17Hx where x is 0, 1,2, 3, and 3.8, Tb2Fe17-xSix, where x is 0, 1, 2, and 3, and Nd6Fe13X, where X is Au, Ag, Cu, and Si, have been measured between 450 to 4.2 K. Ce2Fe17 crystallizes in the R-3m space group and exhibits a helical magnetization from 225 to 90 K and a fan magnetization below 90 K. The helical magnetization was modeled in the Mössbauer spectra by a distribution of θ angles and hyperfine fields. The hyperfine parameters were found to have the expected temperature dependence and to be in good agreement with the hyperfine parameters found for the hydrides. Pr2Fe17H5 is the first R2Fe17Hx compound to show an unusual change in the Mössbauer spectra below 155 K, a change which was attributed to the freezing of the motion of the 18g hydrogen from site to site on the Mössbauer time scale. Dy2Fe17Hx crystallizes in the hexagonal P63/mcm space group and exhibits a disorder of the 4e, 4f iron and 2c, 2d dysprosium site, a disorder which was modeled in the observed Mössbauer spectra. Tb2Fe17-xSix shows an unexpected improvement in the magnetic properties with increasing silicon content in spite of a contration of the unit cell upon the substitution of iron by silicon. Nd6Fe13Ar crystallizes in the I4|mmc space group and exhibits a basal magnetization for X = Ag, Au, and Cu. Nd6Fe13Si shows a spin reorientation from axial at 155 K towards basal at 80 K --Abstract, page v

Structural, Magnetic, and Mössbauer Spectral Study of Er₂Fe₁₇ and Its Hydrides

The structural and magnetic properties of the Er2Fe17Hx compounds, where x is 0, 1, 2, 3, and 3.8, have been investigated by means of powder x-ray diffraction, thermal and ac magnetic susceptibility measurements, and iron-57 Mössbauer spectroscopy. The Er2Fe17Hx compounds crystallize in the hexagonal P63/mmc space group with the Th2Ni17-like structure, a structure which has both an iron-rich stoichiometry and disorder of the erbium and iron-iron 4f-4f dumbbell sites. The increase in the lattice parameters, the magnetic ordering temperature, the saturation magnetization, and the dependence of the Mössbauer spectral hyperfine parameters upon the hydrogen content reveal a two-step filling of the interstitial sites, with hydrogen first filling the octahedral 6h sites for x\u3c3 and then partially filling the tetrahedral 12i sites for x=3 and 3.8. Neither the Mössbauer spectra nor the ac magnetic susceptibility measurements reveal any spin reorientation in any of these compounds. In all of the compounds both the excess amount of iron and the expected disorder is confirmed by the Mössbauer spectra and the hyperfine parameters of the iron 4e site are reported herein. Finally, the Mössbauer spectra indicate that the interstitial hydrogen atoms partially occupying the tetrahedral 12i sites are jumping between these sites on the Mössbauer time scale

Mössbauer Spectral Evidence for Rhombohedral Symmetry in R₃Fe₅O₁₂ Garnets with R = Y, Eu and Dy

The iron-57 Mossbauer spectra of R3Fe5O12, where R is Y, Eu and Dy, have been measured between 4.2 and 550 K. The substantial quadrupole splittings observed in the paramagnetic spectra confirm that the local symmetry at both the tetrahedral and octahedral iron(III) sites is not cubic. The low temperature Mossbauer spectra of Dy3Fe5O12 clearly confirm the spin reorientation between 10 and 15 K and the 4.2 and 10 K spectra are consistent with the known orientation of the magnetization at 14 K in the cubic Ia3-d unit cell. The Mossbauer spectra of R3Fe5O12, where R is Y, Eu and Dy, obtained between 45 and 295 K, reveal four different tetrahedral iron(III) Mossbauer spectral components, four components which are inconsistent with a magnetization oriented along the [111] axis of a cubic Ia3-d unit cell. In contrast, these four components are consistent with a crystal symmetry which is reduced from cubic to rhombohedral R3-. The temperature dependence of the hyperfine fields in Dy3Fe5O12 indicates a small biquadratic exchange contribution to the magnetic exchange. The temperature dependence of the isomer shifts in Dy3Fe512 gives Mossbauer lattice temperatures of 405 and 505 K for the 16a and 24d sites, respectively, values which are in excellent agreement with the Debye temperature measured for Y3Fe5O12

Mössbauer Spectral Study of the Magnetic Properties of Ce₂Fe₁₇Hₓ (x=0, 1, 2, 3, 4, and 5)

The Mössbauer spectra of Ce2Fe17Hx, where x=0, 1, 2, 3, 4, and 5, have been measured and analyzed between 4.2 and 295 K. Because Ce2Fe17 exhibits a helical magnetic order between 225 and 90 K and a fan magnetic order below 90 K, its Mössbauer spectra were fit with a distribution of hyperfine fields and θ angles for four of the eight magnetically inequivalent sites. Because the hydrides exhibit a magnetization within the basal plane of the hexagonal cell, their Mössbauer spectra were fit with seven sextets. The four isomer shifts correlate with the Wigner-Seitz cell volume, the hyperfine fields correlate with the number of iron near neighbors and give estimates of the individual iron magnetic moments ranging from 0.91B to 2.13B, and the quadrupole splittings are in agreement with a point charge calculation. The temperature dependence of the hyperfine fields in Ce2Fe17 is insensitive to the transition from the fan to the helical magnetic structure, a transition which does not modify to any extent the iron electronic structure. As a result of the sudden ferromagnetic ordering which results from the presence of as little as one hydrogen per formula unit, the increase of 70 kOe in the weighted average hyperfine field upon hydrogenation is the largest between x = 0 and 1. The temperature dependence of the weighted average hyperfine field in the hydrides shows Brillouin behavior, a behavior which is slightly different for Ce2Fe17. The temperature dependence of the isomer shift yields a Debye temperature of 345 K. The variation of the hyperfine parameters upon hydrogenation confirms that first the 9e octahedral site is filled by hydrogen, and second the 18g tetrahedral hydrogen site is filled

A Mössbauer Spectral Study of the Jilin Meteorite

The iron-57 Mössbauer spectra of three different samples of the Jilin meteorite have been measured at 78 and 295 K. Five iron containing major components are identified, two magnetic components, kamacite and troilite, and three non-magnetic components, olivine, pyroxene, and an iron(III) component. The relative absorption areas of these five components show that sample A contains a larger fraction of magnetic components, ca. 50 percent, than samples B and C, which contain ca. 30 percent. This difference indicates a significant compositional inhomogeneity in the Jilin meteorite. The fit of the troilite component sextet is extensively discussed in the paper and requires the adjustment of not only the isomer shift and hyperfine field, but also of the quadrupole interaction, the asymmetry parameter of the electric field gradient tensor, and the orientation of the hyperfine field in the principal axes of the electric field gradient tensor. The smaller isomer shift and hyperfine field of the kamacite mineral in sample B indicate that this sample contains less nickel than the kamacite in samples A and C, in which the amount of nickel is estimated to be ca. 9 percent. On the basis of its hyperfine parameters, the iron(III) component is assigned to iron(III) substituted on the M1 site of pyroxene

⁵⁷Fe Mössbauer Spectral Study of Gd₂Fe₁₇Hₓ (for x = 0, 3, and 5) and Gd₂Fe₁₇Hₓ

The Mossbauer spectra of Gd2Fe17Hx (for x = 0, 3, and 5) and Sm2Fe17D5 have been measured between 85 and 295 K and have been analysed with a model which takes into account the basal orientation of the iron magnetic moments, the near-neighbor environment of the four crystallographically inequivalent iron sites, and the structural changes occurring upon hydrogen or deuterium insertion. The temperature dependences of the individual iron site isomer shifts and hyperfine fields follow the expected second-order Doppler shift and Brillouin law behavior, respectively, and provide support for the adequacy of the fitting model. The increases in the isomer shifts upon going from R2Fe17, to R2Fe17H3, to R2Fe17H5, and finally to R2Fe17N3, where R is a rare-earth atom, correlate well with the observed increases in the unit-cell volume and the iron Wigner-Seitz cell volumes upon hydrogen and nitrogen insertion. The 85 K weighted average hyperfine field in R2Fe17 and R2Fe17H3 is at a maximum for the gadolinium compounds in agreement with their higher Curie temperatures. Pr2Fe17H3 and Sm2Fe17N3, which both exhibit axial magnetization, show large 85 K weighted average hyperfine fields than the remaining R2Fe17H3 and R2Fe17N3 compounds, respectively. Finally, the differences in the 18h iron site environment, due to the insertion of the fourth and fifth hydrogen atoms into R2Fe17H5, where R is Nd, Sm, and Gd, are not observed in the Mossbauer spectra, and hence the hydrogen atoms on the 18g tetrahedral interstitial sites must be rapidly moving on the Mossbauer timescale. The magnetization curves of Sm2Fe17 and Sm2Fe17D5 have been measured at 5 and 300 K. The increase in the saturation magnetization upon deuterium insertion is well explained by the increase in the Curie temperature and correlates very well with the increase in the 85 K weighted average hyperfine field

Hydrogen dynamics in the hydrides of Pr₂Fe₁₇ as revealed by Mössbauer spectroscopy

The rhombohedral Pr2Fe17Hx compounds with the Th2Zn17 structure have been prepared for x =0-5. Their lattice parameters and Curie temperatures have been determined from powder x-ray diffraction and thermomagnetic measurements, respectively, and their Mössbauer spectra have been measured between 4.2 and 295 K. The Mössbauer spectra for x=0, 1, and 2, obtained between 4.2 and 295 K, and those of Pr2Fe17H3, obtained above 90 K, have been analyzed with a seven sextet model, indicative of a basal magnetization in these compounds. The Mössbauer spectra of Pr2Fe17H3 below 90 K, of Pr2Fe17H4 between 4.2 and 295 K, and of Pr2Fe17H5 above 155 K, have been analyzed with a four sextet model, indicative of an axial magnetization in these compounds over the indicated temperature ranges. The axial magnetic anisotropy results from a combination of lattice expansion upon hydrogenation and contraction upon cooling, and the relative importance of the praseodymium Stevens coefficients. A magnetic phase diagram for the Pr2Fe17Hx compounds is proposed on the basis of their magnetic Mössbauer spectra. The Mössbauer spectra of Pr2Fe17H5 indicate that, above 155 K, the two hydrogen atoms occupying one third of the tetrahedral 18g sites are rapidly jumping, on the Mössbauer time scale of 100 ns, between the six available 18g positions, a jumping which slows down or ceases below 155 K. The compositional dependence of the hyperfine parameters of the Pr2Fe17Hx compounds indicates an initial filling of the interstitial 9e sites by the first three hydrogen atoms and then a subsequent filling of the interstitial 18g sites by the last two hydrogen atoms

Mössbauer Spectral Study of CeFe₁₁Ti and CeFe₁₁TiH

The Mossbauer effect spectra of CeFe11Ti and CeFe11TiH have been measured between 85 and 295 K and analyzed in terms of a model which takes into account the local environment of iron on the 8f, 8i, and 8j crystallographic sites, the influence of the random distribution of titanium on the 8i site, and the insertion of hydrogen into the 2b site of the structure. The presence of titanium in the near-neighbor environment of iron decreases both the hyperfine field and the isomer shift relative to that observed in its absence. A synergistic decrease in the hyperfine fields upon multiple titanium substitution is also observed. Upon hydrogenation, both the hyperfine fields and the isomer shifts increase in agreement with the unit cell expansion and the increased magnetization and Curie temperature of CeFe11TiH as compared to CeFe11Ti