A. Błachowski

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.

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.

The influence of acid treatments over vermiculite based material as adsorbent for cationic textile dyestuffs

The influence of different acid treatments over vermiculite was evaluated. Equilibrium, kinetic and column studies have been conducted. The results showed that vermiculite first treated with nitric acid and then with citric acid has higher adsorption capacity, presenting maximum adsorption capacities in column experiments: for Astrazon Red (AR), 100.8 ± 0.8 mg g(-1) and 54 ± 1 mg g(-1) for modified and raw material, respectively; for Methylene Blue (MB) 150 ± 4 mg g(-1) and 55 ± 2 mg g(-1) for modified and raw material, respectively. Materials characterization by X-ray diffraction, UV-vis-diffuse reflectance spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy, X-ray fluorescence, N2 adsorption and CEC determination, has been performed. The results suggest the existence of exchange of interlayer cations, leaching of metals from vermiculites sheets and formation of an amorphous phase in the material. Adsorption follows pseudo 2(nd) order model kinetics for both dyestuffs and equilibrium occurs accordingly to Langmuirs model for AR and Freundlichs model for MB. In column systems Yans model is the best fit. The enhanced properties of acid treated vermiculite offer new perspectives for the use of this adsorbent in wastewater treatment.

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.

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...

On the activation energy of the σ-phase formation in a pure and Ti-doped Fe–Cr alloy

The σ-phase formation in Fe53.8Cr46.2 and Fe53.8Cr46.2-0.3at%Ti by isothermal annealing in the temperature interval of 923–1003 K was investigated in situ by 57Fe Mossbauer spectroscopy. The process of transformation was succesfully analyzed using the Johnson–Avrami–Mehl equation, and it was quantitatively described in terms of the activation energy, E which was determined by several methods. The best-value of E as average of all data was found to be equal to 196±2 kJ/mol for the Fe–Cr sample and to 153±2 kJ/mol for that doped with Ti.

Hybrids of Iron-Filled Multiwall Carbon Nanotubes and Anticancer Agents as Potential Magnetic Drug Delivery Systems: In Vitro Studies against Human Melanoma, Colon Carcinoma, and Colon Adenocarcinoma

Cell type, morphology, and functioning are key variables in the construction of efficient “drug-vehicle” hybrids in magnetic drug delivery. Iron-encapsulated multiwall carbon nanotubes (Fe@MWCNTs) appear as promising candidates for theranostics due to in situ chemical catalytic vapor deposition (c-CVD) synthesis, straightforward organic functionalization, and nanoneedle (1D) behavior. Here, model hybrids were synthesized by exploring C-sp2 chemistry ((1

Mössbauer studies of the peculiar magnetism in parent compounds of the iron-based superconductors

A review of the magnetism in the parent compounds of the iron-based superconductors is given based on the transmission Mössbauer spectroscopy of 57Fe and 151Eu. It was found that the 3d magnetism is of the itinerant character with varying admixture of the spin-polarized covalent bonds. For the ‘122’ compounds, a longitudinal spin density wave (SDW) develops. In the case of the EuFe2As2, a divalent europium arranges in an anti-ferromagnetical order at a much lower temperature as compared with the onset of SDW. These two magnetic systems remain almost uncoupled one to another. For the non-stoichiometric Fe1+xTe parent of the ‘11’ family, one has a transversal SDW and magnetic order of the interstitial iron with relatively high and localized magnetic moments. These two systems are strongly coupled one to another. For the ‘grand parent’ of the iron-based superconductors FeAs, one observes two mutually orthogonal phase-related transversal SDW on the iron sites. There are two sets of such spin arrangements due to two crystallographic iron sites. The FeAs exhibits the highest covalency among the compounds studied, but it has still a metallic character.