K. Ruebenbauer

Shape of spin density wave versus temperature in AFe 2As 2 (A=Ca, Ba, Eu): A Mössbauer study

Parent compounds AFe2As2 (A=Ca, Ba, Eu) of the 122 family of the iron-based superconductors have been studied by 57Fe Moessbauer spectroscopy in the temperature range 4.2 K - ~300 K. Spin density waves (SDW) have been found with some confidence. They are either incommensurate with the lattice period or the ratio of the respective periods is far away from ratio of small integers. SDW shape is very unconventional i.e. differs from the sinusoidal shape. Magnetic order starts with lowered temperature as narrow sheets of the significant electron spin density separated by areas with very small spin density. Magnetic sheets are likely to be ordered in the alternate anti-ferromagnetic fashion as the material as whole behaves similarly to collinear anti-ferromagnet. Further lowering of temperature simply expands sheet thickness leading to the near triangular SDW. Finally, sheets fill the whole available space and almost rectangular shape of SDW is reached. Substantial maximum amplitude of SDW appears at the temperature just below the magnetic onset temperature, and this maximum amplitude increases slightly with lowering temperature. The square root from the mean squared hyperfine field behaves versus temperature according to the universality class (1, 2), i.e., with the electronic spin space having dimensionality equal unity and the real space having dimensionality equal two. The more or less pronounced tail above transition temperature due to the development of incoherent SDW is seen.

A neutron diffraction and119Sn Mössbauer study of PbSnF4 and BaSnF4

Abstract119Sn Mössbauer data have been recorded for α- and β-PbSnF4 and BaSnF4. A temperature dependent neutron diffraction study on PbSnF4 clearly indicates the phase transition temperatures. BaSnF4 is confirmed to have the tetragonal P4/nmm structure.

Effect of Pd Impurity on Charge and Spin Density in Metallic Iron Studied by Mössbauer Spectroscopy

Random iron-palladium alloys of BCC structure and containing up to 10.59 at.% of Pd have been investigated by means of 14.4 keV Mossbauer spectroscopy in 57 Fe at ambient temperature versus Pd concentration. It has been found that the perturbation of the iron hyperfine field caused by Pd impurity extends to the third co-ordination shell around an iron atom. Perturbations caused by single Pd atom located in the first, second and third co-ordination shell, respectively, were determined. It was found that they depend very weakly on the average Pd concentration in the range of concentrations investigated. On the other hand, similar perturbations of the isomer shift extend to greater distances from the impurity. Individual contributions due to Pd impurity in the first, second and third shell, respectively, are seen up to about 5 at.% of Pd. However they are small compared to the change of the average isomer shift due to more distant impurities. One sees the change of the average shift solely for alloys having higher concentration of Pd than 5 at.%. On the other hand, a contribution to the hyperfine field remains constant beyond the third shell at all Pd concentrations investigated. It is the same as the field in pure iron.

Interplay between magnetism and superconductivity in EuFe2−xCoxAs2studied by57Fe and151Eu Mössbauer spectroscopy

The compound EuFe(2-x)Co(x)As2 was investigated by means of the 57Fe and 151Eu Moessbauer spectroscopy versus temperature (4.2 - 300 K) for x=0 (parent), x=0.34 - 0.39 (superconductor) and x=0.58 (overdoped). It was found that spin density wave (SDW) is suppressed by Co-substitution, however it survives in the region of superconductivity, but iron spectra exhibit some non-magnetic component in the superconducting region. Europium orders anti-ferromagnetically regardless of the Co concentration with the spin re-orientation from the a-axis in the parent compound toward c-axis with the increasing replacement of iron by cobalt. The re-orientation takes place close to the a-c plane. Some trivalent europium appears in EuFe(2-x)Co(x)As2 versus substitution due to the chemical pressure induced by Co-atoms and it experiences some transferred hyperfine field from Eu2+. Iron experiences some transferred field due to the europium ordering for substituted samples in the SDW and non-magnetic state both, while the transferred field is undetectable in the parent compound. Superconductivity coexists with the 4f-europium magnetic order within the same volume. It seems that superconductivity has some filamentary character in EuFe(2-x)Co(x)As2 and it is confined to the non-magnetic component seen by the iron Moessbauer spectroscopy.

Crystal structure and Mössbauer study of FeAl2O4

Abstract In this work the synthesis of hercynite from Fe2O3 and Al2O3 powders was carried out by arc-melting method under the protective argon atmosphere. The obtained material was characterized with the use of powder X-ray diffractometry (XRD) and Mössbauer spectroscopy (MS). A Mössbauer effect in hercynite obtained by the arc-melting method indicated the cations distribution in the spinel structure among the tetrahedral and octahedral interstices. The presence of Fe2+ ions was detected in both tetrahedral and octahedral sites while Fe3+ ions occupied only the octahedral interstices. The approximate formula of the obtained iron-aluminate spinel was as follows (Fe2+0.77Al3+0.23) (Fe3+0.07Fe2+0.05Al0.88)2O4.

Magnetic anisotropy and lattice dynamics in FeAs studied by Mössbauer spectroscopy

Iron mono-arsenide in the powder form has been investigated by transmission 57 Fe Mossbauer spectros- copy in the temperature range 4.2-1000 K. Additional spectra have been obtained at 20 K and 100 K applying external magnetic field of 7 T. It was found that the spin spiral propagating along the c-axis leads to the complex variation of the hyperfine magnetic field amplitude with the spin orientation vary- ing in the a-b plane. The magnitude of the hyperfine field pointing in the direction of the local magnetic moment depends on the orientation of this moment in the a-b plane. Patterns are vastly different for iron located in the (0 k 0) positions and for iron in the (0 k + 1 2 0) positions within the orthorhombic cell set to the Pnma symmetry. Lattice softens upon transition to the paramagnetic state at 69.2 K primarily in the a-c plane as seen by iron atoms. This effect is quite large considering lack of the structural transition. Two previously mentioned iron sites are discernible in the paramagnetic region till 300 K by different electron densities on the iron nuclei. The anisotropy of the iron vibrations developed at the transition to the para- magnetic state increases with the temperature in accordance with the harmonic approximation, albeit tends to saturation at high temperatures indicating gradual onset of the quasi-harmonic conditions. It seems that neither hyperfine fields nor magnetic moments are correct order parameters in light of the determined static critical exponents. Sample starts to loose arsenic at about 1000 K and under vacuum. 2013 Elsevier B.V. All rights reserved.

Goldanskii-Karyagin effect in the absence of inversion centres

Abstract A density matrix formalism describing perturbation of the Mossbauer line intensities caused by the anisotropic recoilless fraction has been published recently. It dealt with the point group symmetries having inversion centres. The present paper extends this formalism to the point groups lacking inversion centres. It is shown, that the experimental sensitivity to the cubic anisotropy can be dramatically improved by applying circularly polarized radiation, i.e. ferromagnetic sources magnetized to saturation either parallel or antiparallel to the beam direction.

Properties and microstructure of the (Fe, Ni)-Cu-(P, Si, B) melt-spun alloys.

The work presents the microstructure characterization of the new (Fe, Ni)–Cu–(P, Si, B) melt‐spun glass forming alloys investigated by means of transmission electron microscope. The results are compared with the data obtained by other complementary methods such as XRD and Mössbauer spectroscopy. The phases occurring during the crystallization of the glassy matrix are identified and characterized in terms of a long‐range order and a short‐range order. The thermal stability of the alloy is characterized by differential scanning calorimetry. The study describes the mechanical and magnetic properties of the new alloys at room temperature as well as characteristics resulting from heating the as‐cast melt‐spun alloy at elevated temperatures. The changes of the properties of the alloy at elevated temperatures are correlated with the microstructural changes.

Spin reorientation in the Er2−xFe14+2xSi3 single crystal studied by the Fe57 Mössbauer spectroscopy and magnetic measurements

Spin reorientation in the single crystal of Er2−xFe14+2xSi3 with x=0.25(5) has been studied in the temperature range of 4–300K by means of the magnetic measurements and Mossbauer spectroscopy on Fe57 by using the 14.41keV resonant transition. The bulk magnetic moment has been measured versus applied field up to ±8.9T along the c axis of the P63∕mmc cell at 4K. The hysteresis loop has been measured at 300K for the external field applied along the c axis. The bulk moment has been measured versus temperature in the moderate external field of 0.1T applied along the c axis. The ac susceptibility has been measured for several frequencies and amplitudes of the ac field applied along the c axis versus temperature either in the null external field or in the external field of 0.1T along the c axis. Mossbauer measurements were performed versus temperature on the powder sample and single crystal with the radiation beam oriented along the c axis. The spin reorientation from the [a−b] plane onto the c axis occurs for a...

Expansion of the Lamb-Mössbauer factor into semi-invariants

It is proposed to use a formalism of semi-invariants to expand a general time-averaged vibrational self-correlation function into either Cartesian or spherical irreducible tensors describing a recoilless fraction (Lamb-Mossbauer factor) for an arbitrary wave-vector transfer. The formalism allows to estimate the number of independent components for a particular symmetry and an expansion order. A cut-off of the expansion series on the lowest (second) order is provided automatically by the above formalism for Gaussian self-correlation functions.