Zbigniew M. Stadnik

Voigt-based methods for arbitrary shape quadrupole splitting distributions (QSD's) applied to quasi-crystals

It is now well established that, as with amorphous alloys, the Mössbauer spectra of icosahedral alloys exhibit QSDs. This is evidence for intrinsic local disorder present in icosahedral structures. The shapes of the QSDs have not been unambiguously established and, as a result, it has been difficult to link these shapes with other physical properties of these novel and complex alloys. We apply a recently developed arbitrary shape static hyperfine parameter distribution method to the case of a new icosahedral alloy series: Al65Cu20Cr15-xFex. The method proves to be a powerful and useful tool in this application. It enables us to evaluate several robust features and to identify other features that are subject to large uncertainties due to extreme sensitivity to details.

Transport, magnetic, and 57Fe and 155Gd Mossbauer spectroscopic properties of GdFeAsO and the slightly overdoped superconductor Gd0.84Th0.16FeAsO

The results of x-ray diffraction, electrical resistivity, magnetic susceptibility, and (57)Fe and (155)Gd Mössbauer spectroscopy studies of polycrystalline samples of nonsuperconducting GdFeAsO and superconducting Gd(0.84)Th(0.16)FeAsO are reported. The superconductor Gd(0.84)Th(0.16)FeAsO has the onset transition temperature of 54.4(1) K. We find that Fe and Gd magnetic moments in GdFeAsO order antiferromagnetically at 132.7(1) K and 4.1(1) K, respectively. In Gd(0.84)Th(0.16)FeAsO, Fe atoms carry no magnetic moment down to 1.9 K whereas Gd magnetic moments order below 2.9(1) K. This is taken as evidence for the coexistence of Gd magnetic order and superconductivity. The Debye temperatures of GdFeAsO and Gd(0.84)Th(0.16)FeAsO are, respectively, 409(4) and 389(3) K.

Structural, Mössbauer, and transport studies of the icosahedral quasicrystals Al55Si7Cu25.5Fe12.5, Al62.5Cu24.5Fe13 and the crystalline 1/1 approximant Al55Si7Cu25.5Fe12.5

As-quenched icosahedral Al55Si7Cu25.5Fe12.5, its 1/1 approximant of the same composition, and icosahedral Al62.5Cu24.5Fe13 alloys have been studied using x-ray diffraction, scanning transmission electron microscopy and the high-angle annular dark field technique, zero-field and in-field 57Fe Mossbauer spectroscopy, and electrical resistivity. The crystal structure of the 1/1 approximant Al55Si7Cu25.5Fe12.5 has been refined with the Rietveld method and shown to be compatible with the measured high-angle annular dark field images. The distribution of the principal component of the electric field gradient tensor has a bimodal character with a dominant negative sign in the icosahedral Al–Cu–Fe system. The local order of the Fe structural environment is compared in icosahedral Al55Si7Cu25.5Fe12.5, its 1/1 approximant, and icosahedral Al62.5Cu24.5Fe13. The average quadrupole splitting decreases with temperature as T3/2 for all alloys studied, and its value is significantly larger for the icosahedral alloys. The vibrations of the Fe atoms in the alloys studied are well described by a Debye model, with characteristic Mossbauer temperatures of 468(25), 487(19), and 455(6) K for icosahedral Al55Si7Cu25.5Fe12.5, its 1/1 approximant, and icosahedral Al62.5Cu24.5Fe13, respectively. The electrical resistivity is discussed in terms of quantum interference effects and structural disorder.

Photoemission study of Al70Pd20Mn10 quasicrystal

Abstract Measurements with photoemission spectroscopy in the photon-energy range 35–150 eV have been used to measure the valence band of the stable icosahedral Al 70 Pd 20 Fe 10 . The maxima of the Pd 4d- and Mn 3d-derived states are respectively at 3.6 and 0.7 eV below the Fermi level. The strong decrease of intensity towards the Fermi level suggests the presence of a pseudogap at the Fermi level. No unusual features in the valence band of icosahedral Al 70 Pd 20 Mn 10 , which could be ascribed to its quasiperiodic nature, are observed within the resolution of the experiment.

ARE DECAGONAL QUASICRYSTALS THE HUME-ROTHERY PHASES?

Abstract Photoemission spectroscopy measurements in the photon energy range 40–100 eV have been used to make the first determinations of the valence bands of the high quality decagonal alloys Al 65 Co 15 Cu 20 and Al 70 Co 15 Ni 15 . Within the energy resolution our results reveal that, contrary to the prediction of recent band-structure calculations and to the widespread qualitative interpretation of various electronic transport data, there is no pseudogap in the density of states at the Fermi level. This suggests that decagonal alloys differ significantly in their electronic structure from icosahedral alloys and that a Hume-Rothery mechanism does not play an important role in accounting for their stability and electronic transport properties.

A Mossbauer effect study of single crystals of the non-superconducting parent compound Fe1.09Te and the superconductor FeSe0.4Te0.6

The results of a (57)Fe Mössbauer spectroscopy study between 2.0 and 297 K of the parent compound Fe1.09Te and the superconductor FeSe0.4Te0.6 are reported. It is shown that in both compounds the magnitude of the quadrupole splitting increases with decreasing temperature and is well described by a T(3/2) power-law relation. The presence of incommensurate spin-density-wave antiferromagnetism in Fe1.09Te is demonstrated, with the Néel temperature T(N) = 71.1(6) K. A theoretical prediction (Zhang et al 2009 Phys. Rev. B 79 012506) of the Fe magnetic moment at the 2c sites being significantly larger than that at the 2a sites in the parent compound is confirmed experimentally by showing that these moments at 4.4 K are, respectively, 3.20(4) and 1.78(3) μ(B). The absence of magnetic order in FeSe0.4Te0.6 down to 2.0 K is confirmed. The Debye temperatures of Fe1.09Te and FeSe0.4Te0.6 are found to be 290(1) and 233(1) K, respectively.

Magnetic properties and 155Gd Mössbauer spectroscopy of the rare-earth Heusler compound Cu2GdIn

The results of x-ray diffraction, magnetic susceptibility, and 155Gd Mossbauer spectroscopy studies of the rare-earth Heusler compound Cu2GdIn are reported. The studied compound has the L 21 crystal structure with the lattice constant of 0.666 43(3) nm. Cu2GdIn is an antiferromagnet with the Neel temperature TN = 9.6(1) K. The temperature dependence of the magnetic susceptibility above TN follows the Curie–Weiss law with the effective magnetic moment of 7.98(4) µB per Gd atom and the paramagnetic Curie temperature of −41.2(9) K. The Debye temperature of Cu2GdIn is 229(5) K. The temperature dependence of the hyperfine magnetic field follows a J = 7/2 Brillouin function. It is shown that the total contribution to the hyperfine magnetic field at 155Gd nuclei in Gd-containing Heusler compounds resulting from the conduction electron polarization is +9.8(2.5) T.

Mössbauer and transport studies of amorphous and icosahedral Zr-Ni-Cu-Ag-Al alloys

The alloy Zr65Al7.5Ni10Cu7.3Fe0.2Ag10 in the amorphous and icosahedral states, and the bulk amorphous alloy Zr65Al7.5Ni10Cu7.5Ag10, have been studied with Fe-57 Mossbauer spectroscopy, electrical r ...

Mössbauer spectroscopy of quasicrystals

A critical review is presented of the application of Mössbauer spectroscopy in the study of quasicrystals. The importance of a correct analysis of the Mössbauer spectra is emphasized. Current progress associated with Mössbauer studies of quasicrystals is discussed. The urgency for theoretical calculations of the distributions of quadrupole splittings for various possible structural models of quasicrystals is stressed. Suggestions for future work are given.

Magnetic behaviour and hyperfine interactions in AlFeCe metallic glasses

Abstract 57 Fe Mossbauer effect and magnetic susceptibility measurements have been performed on the new glassy alloys Al 90 Fe 5 Ce 5 , Al 87 Fe 6.7 C 6.3 , Al 87 Fe 8.7 Ce 4.3 , and Al 87 Fe 9.3 Ce 3.7 . It is found that Fe atoms are not magnetically ordered in the Fe-rich glasses down to 4.2 K. The disorder inherent to glasses is manifested in the distribution of the electric quadrupole splittings, which is shown to be well described by a Gaussian shape. The magnetic susceptibility of the alloys follows the Curie-Weiss law between 4.2 K and room temperature. The small effective magnetic moment values suggest mixed-valence behaviour.