Lattice Dynamics in the FeSb₃ Skutterudite

Thin films of FeSb3 were characterized by electronic transport, magnetometry, x-ray diffraction, 57Fe and 121Sb nuclear inelastic scattering, and 57Fe Mössbauer spectroscopy. Resistivity and magnetometry measurements reveal semiconducting behavior with a 16.3(4) meV band gap and an effective paramagnetic moment of 0.57(6) B, respectively. A systematic comparison of the lattice dynamics with CoSb3 and EuFe 4Sb12 reveals that the Fe4Sb12 framework is softer than the Co4Sb12 framework, and that the observed softening and the associated lowering of the lattice thermal conductivity in the RFe4Sb12 filled skutterudites are not only related to the filler but also to the Fe4Sb12 framework

Influence of the Rare-Earth Element on the Effects of the Structural and Magnetic Phase Transitions in CeFeAsO, PrFeAsO and NdFeAsO

We present results of transport and magnetic properties and heat capacity measurements on polycrystalline CeFeAsO, PrFeAsO and NdFeAsO. These materials undergo structural phase transitions, spin density wave-like magnetic ordering of small moments on iron and antiferromagnetic ordering of rare-earth moments. The temperature dependence of the electrical resistivity, Seebeck coefficient, thermal conductivity, Hall coefficient and magnetoresistance are reported. The magnetic behavior of the materials have been investigated using Mössbauer spectroscopy and magnetization measurements. Transport and magnetic properties are affected strongly by the structural and magnetic transitions, suggesting significant changes in the band structure and/or carrier mobilities occur, and phonon-phonon scattering is reduced upon transformation to the low-temperature structure. Results are compared with recent reports for LaFeAsO, and systematic variations in properties as the identity of Ln is changed are observed and discussed. As Ln progresses across the rare-earth series from La to Nd, an increase in the hole contributions to the Seebeck coefficient and increases in magnetoresistance and the Hall coefficient are observed in the low-temperature phase. Analysis of hyperfine fields at the iron nuclei determined from Mössbauer spectra indicates that the moment on Fe in the orthorhombic phase is nearly independent of the identity of Ln, in apparent contrast to reports of powder neutron diffraction refinements

Magnetic and Electronic Properties of Eu₄Sr₄Ga₁₆Ge₃₀

Magnetization, static and ac magnetic susceptibility, nuclear forward scattering, and electrical resistivity measurements have been performed on polycrystalline Eu4Sr4Ga16Ge30, a type I clathrate that has divalent strontium and europium ions encapsulated within a Ga-Ge framework. These data are compared with those of type I clathrates Eu8Ga16Ge30 and Eu6Sr2Ga16Ge30. The ferromagnetic ordering of these Eu-containing clathrates is substantially altered by the incorporation of strontium, as compared to Eu8Ga16Ge30. Ferromagnetism, accompanied by a relatively large negative magnetoresistance, is observed below 15 and 20 K in Eu4Sr4Ga16Ge30 and Eu6Sr2Ga16Ge30, respectively. An effective magnetic moment of 7.83 µB per Eu ion is observed above 30 K for Eu4Sr4Ga16Ge30, a moment which is close to the free-ion moment of 7.94 µB per europium(II) ion

A Mössbauer Spectral Study of the GdCo₄₋ₓFeₓB Compounds

The iron-57 Mössbauer spectra of the GdCo4-xFexB compounds, where x is 0.10, 0.15, 0.20, 0.25, 1, 2, 2.5, and 2.6, have been measured at room temperature and reveal relatively small iron hyperfine fields of approximately 12-18 T, relatively large quadrupole interactions of approximately +0.9 and -1 mm/s, and three very different types of spectra for x=0.10 and 0.15, x=0.25, 1, and 2, and x=2.5 and 2.6. The differences result from both the different easy magnetization directions in these compounds and the different cobalt and/or iron occupancies of the crystallographic 2c and 6i sites. The spectra have been fitted by calculating the spectral absorption with the complete iron-57 nuclear excited state Hamiltonian for the iron 2c and 6i sites. The fits have used an asymmetry parameter eta and Euler angles θ and φ that relate the hyperfine field to the iron electric field gradient axes of each crystallographic site in an orientation that is consistent with the structural and magnetic properties of the site. The results of the fits indicate both that the full Hamiltonian approach is required for physically reasonable spectral fits and that the small observed fields result from the presence of large orbital contributions which subtract from the Fermi contact contributions to the magnetic hyperfine fields of the two sites. The iron 2c occupancy obtained from the Mössbauer spectral area has been used to model the compositional dependence of the magnetic anisotropy constant in the GdCo4-xFexB compounds

Magnetic and Mössbauer Spectral Study of ErFe₁₁Ti and ErFe₁₁TiH

The influence of the insertion of hydrogen into the crystal lattice of ErFe11Ti upon its spin reorientation was investigated. It was found that the insertion of hydrogen into ErFe11Ti to form ErFe11TiH expanded the lattice by 1% and increased the Curie temperature by 56 K. The presence of one titanium near neighbor in the environment of an iron site decreased the hyperfine field by ~2T and changed the isomer shift by ~0.05 mm/s

Crystal Chemistry of the Hydrothermally Synthesized Na₂(Mn₁₋ₓFeₓ²⁺)₂Fe³⁺(PO₄)₃ Alluaudite-Type Solid Solution

Several compounds of the Na2(Mn1-xFex2+)2Fe3+(PO4)3 solid solution have been hydrothermally synthesized at 400 °C and 1 kbar; pure alluaudite-like compounds have been obtained for x = 0.00, 0.25, 0.50, 0.75, and 1.00. Rietveld refinements of the powder X-ray diffraction patterns indicate the presence of Na + at the A1 and A2\u27 sites, Mn2+ and Fe 2+ at the M1 site, and Mn2+ Fe2+ and Fe3+ at the M2 site. The presence of small amounts of Na + at the M1 site and Mn2+ at the A1 site indicates a partially disordered distribution of these cations. An excellent linear correlation has been established between the M1-M2 distance and the energy of the infrared band attributed to the M 2+-O vibrations. The Mössbauer spectra, measured between 85 and 295 K, were analyzed in terms of a model which includes the next-nearest neighbor interactions at the M2 and M1 crystallographic sites. Fe2+ and Fe3+ isomer shifts are typical of the alluaudite structure and exhibit the expected second-order Doppler shift. The derived iron vibrating masses and Mössbauer lattice temperatures are within the range of values expected for iron cations in an octahedral environment. The Fe2+ and Fe3+ quadrupole splittings are typical of the alluaudite structure, and the temperature dependence of the Fe2+ quadrupole splitting was fit with the Ingalls model, which yielded a ground state orbital splitting of ca. 460 to 735 cm-1 for the Fe2+ sites. The isomer shifts and quadrupole splitfings of Fe2+ at the M1 site are larger than those of Fe2+ at M2, indicating that the M1 site is both larger and more distorted than the M2 site

⁵⁷Fe Mössbauer Spectral and Muon Spin Relaxation Study of the Magnetodynamics of Monodispersed γ-Fe₂O₃ Nanoparticles

The Mössbauer spectra of monodispersed iron oxide nanoparticles with diameters of 4, 7, 9, and 11 nm have been measured between 4.2 and 315 K and fitted within the formalism for stochastic fluctuations of the hyperfine Hamiltonian. In this model, the hyperfine field is assumed to relax between the six ±x, ±y, and ±z directions in space with a distribution of relaxation rates that is temperature dependent. Muon spin relaxation measurements have been carried out on the 9 nm particles between 4.2 and 295 K. Both techniques reveal three regimes in the magnetic dynamics of these nanoparticles. In the low-temperature regime, between 4.2 and ~30 K, the nanoparticle magnetic moments are blocked and a spin-glass-like state is observed with nearly static hyperfine fields, as is indicated by the well resolved magnetic Mössbauer spectra and the slow exponential decay of the muon asymmetry functions. In the high-temperature regime, above ~125 K, the nanoparticle magnetic moments and, hence, the hyperfine fields, relax rapidly and a typical thermally activated superparamagnetic behavior is observed, as is indicated by the Mössbauer doublet line shape and the muon asymmetry functions that are unquestionably characteristic of monodispersed nanoparticles. In the intermediate regime between ~30 and 125 K, the Mössbauer spectra are the superposition of broad sextets and doublets and the muon asymmetry functions have been fitted with a sum of two terms, one relaxing term similar to that observed at and above 125 K and one term characteristic of static local fields. Hence, in this intermediate regime, the sample is magnetically inhomogeneous and composed of nanoparticles rapidly and slowly relaxing as a result of interparticle interactions. The magnetic anisotropy constants determined from both the Mössbauer spectral and magnetic susceptibility results decrease by a factor ~4 with increasing diameter from 4 to 22 nm and increase linearly with the percentage of iron(III) ions present at the surface of the nanoparticles. The interparticle interaction energy is estimated to be between 89 and 212 K from the temperature dependence of the magnetic hyperfine field measured on the 9 nm nanoparticles

Magnetic Properties of Iron Nitride-Alumina Nanocomposite Materials Prepared by High-Energy Ball Milling

The structural and magnetic properties of the granular iron nitride-alumina composite materials, (FexN)0.2(Al 2O3)0.8 and (FexN) 0.6(Al2O3)0.4, fabricated using high-energy ball milling have been determined by using X-ray diffraction, Mössbauer spectroscopy, and magnetization measurements. The Mössbauer spectra, fit with a distribution of hyperfine fields between zero and 40 T, indicate that the weighted average field decreases with increasing milling time. The isomer shift increases with milling time because of a reduced iron 4s-electron density at the grain boundaries. Coercive fields as high as 325 and 110 Oe are obtained for (FexN)0.2(Al2O 3)0.8 at 5 and 300 K, respectively; the increase in the coercive field upon cooling indicates the presence of superparamagnetic particles. The coercive field increases with milling time because of the reduced particle size. The decrease in the magnetization results from the increase in both the superparamagnetic fraction and the concentration of surface defects with increased milling time

Weak Ferromagnetism in Fe₁₋ₓCoₓSb₂

Weak ferromagnetism in Fe1-xCoxSb2 is studied by magnetization and Mössbauer measurements. A small spontaneous magnetic moment of the order of ~ 10-3µB appears along the b axis for 0.2 ≤ x ≤ 0.4. Based on a structural analysis, we argue against extrinsic sources of weak ferromagnetism. We discuss our results in the framework of the nearly magnetic electronic structure of the parent compound FeSb2

Mössbauer Spectral Study of the Magnetocaloric FeMnP₁₋ₓAsₓ Compounds

The magnetic phase transitions in the FeMnP1-xAsx compounds with x= 0.25, 0.35, 0.45, 0.50, and 0.55, have been studied by iron-57 Mössbauer spectroscopy. The ferromagnetic and antiferromagnetic spectra have been analyzed with a model that takes into account the random distribution of the P and As near-neighbor anions of a given iron site. This distribution is a binomial distribution of the contributions to the spectra of each iron with n As near neighbors. A magnetostriction model has been used to characterize the temperature induced paramagnetic to ferromagnetic first-order phase transition and order parameters, ηF=2.6, 2.3, 2.0, 1.57, and 1.43 have been obtained for x=0.25, 0.35, 0.45, 0.50, and 0.55, respectively. A detailed phase diagram has been derived from the Mössbauer spectral analysis and reveals a magnetic triple point at x= ~0.35 and ~210 K. A model that takes into account the random binomial P and As distribution and the contribution from the iron and manganese magnetic sublattices yields excellent fits of the spectral components assigned to the ferromagnetic and incommensurate antiferromagnetic components for the x=0.25 and 0.35 compounds at all temperatures