Mössbauer study of hyperfine interactions in EuFe2(As1−xPx)2 and BaFe2(As1−xPx)2

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Highlights

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    Presence of two paramagnetic components reflecting the As/P coordination numbers was seen using Mössbauer spectroscopy

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    Using DFT calculations for the Fe centres the experimentally obtained Mössbauer parameters were verified.

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    Coexistence of spin-density-wave type magnetism and superconductivity was observed in low-temperature Mössbauer spectra.

Abstract

The magnetic properties of the pnictide superconductors with the nominal composition of BaFe2(As0.68P0.32)2 and EuFe2(As0.8P0.2)2 were studied by 57Fe Mössbauer spectroscopy. A superconducting transition at 30 K was detected and coexistence of magnetism and superconductivity at low temperatures was observed. The Mössbauer spectra show two iron-atom surroundings, which are attributed to undoped AFe2As2 and substituted AFe2(As1−xPx)2, with at least one phosphorus atom in the tetragonal iron environment, (A = Ba or Eu). These two iron-atom surroundings were attributed to one macroscopic AFe2(As1−xPx)2 phase.

Introduction

Among iron-based superconducting materials pnictides are most intensively studied due to the variety of physical and chemical properties which these materials have. New pnictide superconductors with various properties are produced using carrier-, hole- or isovalent-doping processes of arsenic in the AFe2As2 (A = Ca, Ba, Eu, Sr) parent compound. Two of them are BaFe2(As1−xPx)2 and EuFe2(As1−xPx)2, the parent materials of which have a layered structure with Ba/Eu atoms between the Fe2As2 layers. The properties of these parent compounds are well-known from the literature. Undoped AFe2As2 exhibits an anomaly at 140–200 K in the electric resistance, due to a structural phase transition from tetragonal to orthorhombic [1], [2], [3], [4], [5]. The structural transition is accompanied by antiferromagnetic, spin-density wave (SDW) transition at the same temperature that was observed by neutron diffraction [4], [5], [6]. In substituted pnictides this SDW anomaly shifts to lower temperatures with increasing substitutional-element concentration [7], [8]. In Mössbauer spectra the broadening of the spectral lines begins below ∼140–200 K and at 77 K a completed magnetic splitting of the main paramagnetic doublet is observed. The value of the hyperfine field is around 5 T [9].

Superconductivity in undoped material can be induced by applying an external pressure. For Ba- or EuFe2As2 the required external pressure value lies in the range of 20–60 kbar, with a maximum Tc value of ∼29 K [10], [11], [12]. The most common way to induce superconductivity is hole, electron, and isovalent doping that suppresses the structural and antiferromagnetic transitions and promotes superconductivity [13], [14], [15], [16], [17], [18], [19]. In this case the obtained superconducting properties depend on the dopant concentration [7], [10], [17], [19].

By isovalent doping phosphorus atoms are replacing a part of the arsenic atoms. Superconductivity in AFe2(As1−xPx)2 appears in the narrow concentration range of x = 0.2–0.6 for A=Ba and x=0.14–0.23 for A= Eu [7], [19], [20], [21], [22], [23]. The narrowness of the concentration range may be related to the observed overlap of superconductivity and magnetism.

Previously it was thought that the most likely reason for superconductivity arising in these compounds is the chemical pressure due to the difference in radii between dopant and regular atoms, but it has been shown that the presence of a chemical pressure not always leads to superconductivity and the true reason is more complicated [8].

In this paper the hyperfine interactions of AFe2(As1−xPx)2 with x = 0.32 for A = Ba and x = 0.20 for A = Eu were examined by 57Fe Mössbauer spectroscopy. This method is sensitive to the local properties of the investigated nucleus and allows probing of phase composition, magnetic ordering, valence, and spin-state of atoms.

Section snippets

Experimental

For the preparation of samples a solid-state reaction method was used. Iron powder (99.99%), barium/europium pieces (99.9%), arsenic pieces (99.999%), and phosphorus powder were directly mixed in stoichiometric ratios and sealed into an evacuated quartz tube. For preventing reactions between barium and the tube wall during the synthesis the mixture of the elements was placed in an alumina crucible, and covered by an another crucible.

The quartz ampule with the Ba-sample mixture was slowly heated

Results and discussion

The phase contents in the polycrystalline BaFe2(As0.68P0.32)2 and EuFe2(As0.8P0.2)2 samples were analyzed by XRD and the obtained patterns are presented in Fig. 1.

For both samples the XRD data analysis revealed a tetragonal ThCr2Si2-type crystal structure (I4/mmm) of the main Ba/EuFe2(As1−xPx)2 phase and the presence of a preferred orientation along the 00l direction. This preferred orientation was taken into account during the fit procedure using the March–Dollase method integrated into the

Conclusions

Polycrystalline AFe2(As1−xPx)2 samples with x = 0.32 and 0.20 for A =Ba and Eu, respectively, were examined by XRD, SQUID and 57Fe Mössbauer spectroscopy. Two Fe environments giving rise to two 57Fe Mössbauer components were observed and assigned to the pnictide coordinations 4As and 1P3As, respectively. A broadening of the second component down to the 6 K indicates magnetic ordering of iron coordinated by 4As of the main phase. This magnetic ordering coexists with superconductivity below the

References (40)

  • Dirk Johrendt et al.

    Physica C

    (2009)
  • H. Raffius et al.

    J. Phys. Chem. Solids

    (1991)
  • A. Sklyarova et al.

    J. Magn. Magn. Mater.

    (2014)
  • F. Neese

    Coord. Chem. Rev.

    (2009)
  • Zhi Ren et al.

    Phys. Rev. B

    (2008)
  • Shugo Ikeda et al.

    J. Phys. Soc. Jpn.

    (2012)
  • A. Błachowski et al.

    Phys. Rev. B

    (2011)
  • Q. Huang et al.

    Phys. Rev. Lett.

    (2008)
  • M. Kofu et al.

    New J. Phys.

    (2009)
  • Stephen D. Wilson et al.

    Phys. Rev. B

    (2009)
  • G. Wu et al.

    Europhys. Lett.

    (2008)
  • Marianne Rotter et al.

    Phys. Rev. B

    (2010)
  • Marianne Rotter et al.

    Phys. Rev. B

    (2008)
  • Johnpierre Paglione et al.

    Nat. Phys.

    (2010)
  • Patricia L. Alireza et al.

    J. Phys.: Condens. Matter

    (2009)
  • C.F. Miclea et al.

    Phys. Rev. B

    (2009)
  • P.C. Canfield et al.

    Phys. Rev. B

    (2009)
  • C. Lester et al.

    Phys. Rev. B

    (2009)
  • S.R. Saha et al.

    J. Phys.: Condens. Matter

    (2010)
  • Shuai Jiang et al.

    J. Phys.: Condens. Matter

    (2009)
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