Gary J. Long

Magnetic and Mossbauer spectral properties of the compound Nd6Fe13Au

Abstract The magnetic properties of Nd 6 Fe 13 Au were studied by means of magnetic measurements and iron-57 Mossbauer spectroscopy. It is shown that magnetic ordering of the Fe moments occurs at temperatures much above room temperature ( T N = 408 K). At low temperatures, the net moment of Nd 6 Fe 13 Au is very small owing to mutual compensation of the contributions of the various magnetic sublattices involved. High-field measurements made at 4.2 K show that ferromagnetic alignment of the sublattice moments is reached in two steps via first-order magnetic phase transitions. The Mossbauer spectra reveal that the Zeeman splitting gradually decreases with increasing temperature and that the average hyperfine field is zero at 435 K and above. Between 325 K and 411 K the area in the Mossbauer spectra due to the ordered magnetic component decreases gradually as it is replaced by a quadrupole doublet.

Structural, Electronic, and Magnetic Properties of UFeS 3 and UFeSe 3

Black prisms of UFeS(3) and UFeSe(3) have been synthesized by solid-state reactions of U, Fe, and S or Se with CsCl as a flux at 1173 K. The structure of these isostructural compounds consists of layers of edge- and corner-sharing FeS(6) or FeSe(6) octahedra that are separated by layers of face- and edge-sharing US(8) or USe(8) bicapped trigonal prisms. The isomer shifts in the iron-57 Mössbauer spectra of both UFeS(3) and UFeSe(3) are consistent with the presence of high-spin iron(II) ions octahedrally coordinated to S or Se. The XANES spectra of UFeS(3) and UFeSe(3) are consistent with uranium(IV). Single-crystal magnetic susceptibility measurements along the three crystallographic axes of UFeSe(3) reveal a substantial magnetic anisotropy with a change of easy axis from the a-axis above 40 K to the b-axis below 40 K, a change that results from competition between the iron(II) and uranium(IV) anisotropies. The temperature dependence of the magnetic susceptibility along the three axes is characteristic of two-dimensional magnetism. A small shoulder-like anomaly is observed in the magnetic susceptibilities along the a- and b-axes at 96 and 107 K, respectively. Below 107 K, the iron-57 Mössbauer spectra of UFeS(3) and UFeSe(3) show that the iron nuclei experience a magnetic hyperfine field that results from long-range magnetic ordering of at least the iron(II) magnetic moments because the field exhibits Brillouin-like behavior. Below 40 K there is no significant change in the Mössbauer spectra as a result of change in magnetic anisotropy. The complexity of the iron-57 Mössbauer spectra and the temperature and field dependencies of the magnetic properties point toward a complex long-range magnetic structure of two independent iron(II) and uranium(IV) two-dimensional sublattices. The temperature dependence of the single-crystal resistivity of UFeSe(3) measured along the a-axis reveals semiconducting behavior between 30 and 300 K with an energy gap of about 0.03 eV below the 53 K maximum in susceptibility, of about 0.05 eV between 50 and 107 K, and of 0.03 eV above 107 K; a negative magnetoresistance was observed below 60 K.

A Structural, Magnetic, and Mössbauer Spectral Study of the Dy Co₄₋ₓFeₓB Compounds, with X=0-3

The DyCo4−xFexB compounds, with x=0, 1, 1.5, 2, 2.5, and 3, have been investigated by x-ray diffraction, magnetic measurements, and iron-57 Mossbauer spectroscopy. The substitution of cobalt by iron induces both an anisotropic increase in the unit-cell volume and a large increase in the Curie temperature. The 4.2K magnetization decreases with increasing iron content. The Mossbauer spectra of the DyCo4−xFexB compounds with x=1, 1.5, and 2 reveal rather small iron hyperfine fields of approximately 16T and large quadrupole interactions of +1.0 and −1.0mm∕s, for the 6i and 2c sites, respectively. The relative areas of the 6i and 2c spectral components indicate a strong preferential substitution of iron on the 2c site. In DyCo3FeB approximately 70% of the iron occupies the 2c site; a strong increase in the a lattice parameter and in the Curie temperature is observed between DyCo4B and DyCo3FeB and smaller increases are observed for x>1. The compensation temperature of the DyCo4−xFexB compounds decreases from 3...

Li₁₁Nd₁₈Fe₄O₃₉₋δ Revisited

The structure proposed for Li(11)Nd(18)Fe(4)O(39-δ) (Chen et al. Inorg. Chem. 2012, 51, 8073) on the basis of diffraction and Mössbauer spectral data is compared to that determined previously for Nd(18)Li(8)Fe(5)O(39) (Dutton et al. Inorg. Chem.200847, 11212) using the same techniques. The Mössbauer spectrum reported by Chen et al. has been reinterpreted. The newly refined spectral parameters differ significantly from the published values but are similar to those reported for Nd(18)Li(8)Fe(5)O(39). The relative areas of the three components indicate that iron cations occupy the 2a, 8e, and 16i sites in space group Pm3n, in disagreement with the model determined from neutron diffraction by Chen et al. in which only the 2a and 8e sites are so occupied. The relationship between Li(11)Nd(18)Fe(4)O(39-δ) and Nd(18)Li(8)Fe(5)O(39) is discussed, and it is proposed that the sample prepared by Dutton et al. is a kinetic product whereas the sample prepared by Chen et al. is the thermodynamically preferred product.

Structural Chemistry and Magnetic Properties of Ln₁₈Li ₈Rh₅₋ₓFeₓO₃₉ (Ln = La, Nd)

Polycrystalline samples of Ln(18)Li(8)Rh(5-x)Fe(x)O(39) (Ln = La, Nd; 0.5 < or = x < or = 5) have been synthesized by a solid-state method and studied by a combination of dc and ac magnetometry, neutron diffraction, and Mossbauer spectroscopy. All compositions adopt a cubic structure (space group Pm3n, a(0) approximately 12 A) based on intersecting 111 chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Ln-O framework. At low values of x, iron preferentially occupies the smaller (2a) of the two distinct octahedral sites as low-spin Fe(IV). The Rh(III) on the larger (8e) octahedral site is replaced by high-spin Fe(III). Nd-containing compositions having x > 1 show spin-glass-like behavior below approximately 5 K. La-containing compositions having x > 1 show evidence of a magnetic transition at approximately 8 K, but the nature of the transition is unclear. This contrasting behavior demonstrates that, although the structural chemistry of the two systems is essentially the same, the magnetic character of the Ln cations plays an important role in determining the properties of these compounds.

Mössbauer Spectral Study of Two Layered Honeycomb Molecular Magnets: PPh4FeIIFeIII(ox)3 and NBu4FeIIFeIII(ox)3

The Mossbauer spectra of the layer ferrimagnets PPh4FeIIFeIII(ox)3 and NBu4FeIIFeIII(ox)3 have been measured between 1.9 and 315 K. The paramagnetic spectra exhibit both high-spin iron(II) and iron(III) doublets. The iron(II) quadrupole splittings are negative and decrease substantially upon cooling as a result of a low-symmetry crystal field splitting. In contrast, the iron(III) splittings are small, positive, and virtually independent of temperature. The respective magnetic ordering temperatures of 30 and 42.5 K agree well with the phase observed by bulk magnetometry and the magnetic sextets yield hyperfine fields of ca. 60 kOe for iron(II) and 540 kOe for iron(III). The unusually low iron(II) field results from the exceptionally large orbital and dipolar contributions, while the iron nuclei experience a static hyperfine field at 1.9 K and a relaxing hyperfine field between ca. 10 and 30 K.

X-ray-absorption spectral study of the R{sub 2}Fe{sub 17{minus}x}M{sub x} solid solutions (R=Ce,Nd and M=Al,Si)

The x-ray-absorption near-edge structure (XANES) spectra obtained at the cerium L{sub III} edge of the Ce{sub 2}Fe{sub 17{minus}x}Al{sub x} solid solutions and Ce{sub 2}Fe{sub 14}Si{sub 3} show two absorption peaks characteristic of the 4f{sup 1} and 4f{sup 0} configurations of cerium, peaks which indicate that cerium is in a mixed valent state in these compounds. All the XANES spectra have been consistently and excellently fit with one sigmoidal function and two Gaussian-broadened Lorentzian functions. The cerium spectroscopic valence obtained from the relative areas of the two peaks decreases from 3.64 to 3.43 between x=0 and 9 in Ce{sub 2}Fe{sub 17{minus}x}Al{sub x}, and correlates linearly with the cerium site volume. This correlation confirms that the cerium valence is strongly dependent upon steric effects. In contrast, the cerium valence obtained from the XANES spectrum of Ce{sub 2}Fe{sub 14}Si{sub 3} is not determined by steric effects and indicates, in agreement with other measurements and calculations, that silicon is more covalently bonded with its near-neighbor cerium atoms than is aluminum. The neodymium L{sub III}-edge XANES spectra of the Nd{sub 2}Fe{sub 17{minus}x}Al{sub x} solid solutions, where x is 0, 3, and 8, reveal the presence of only trivalent neodymium and an increase of the emptymorexa0» 5d state density when aluminum is substituted in place of iron. XANES measurements at the iron K edge of the Ce{sub 2}Fe{sub 17{minus}x}Al{sub x} and Nd{sub 2}Fe{sub 17{minus}x}Al{sub x} solid solutions show changes in the relative intensity of the multiple scattering peaks, changes which are related to the changing composition of the first three neighbor shells with increasing aluminum content. {copyright} {ital 1997} {ital The American Physical Society}«xa0less