Mössbauer spectroscopy studies of oxypnictides superconductors

Physica C 469 (2009) 485–490

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Mössbauer spectroscopy studies of oxypnictides superconductors Israel Nowik, Israel Felner * Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel

a r t i c l e

i n f o

Article history: Available online 20 March 2009 PACS: 78.80.+y 74.25.Dw 75.20.EN 75.30.Fv

a b s t r a c t The contribution of Mössbauer spectroscopy to the study of conventional superconducting materials, Chevrel phases, rare earth rhodium borides, high Tc cuprates, and Ru-based superconducting-magnetic systems are very shortly reviewed. The newly discovered oxypnictides, magnetic and superconducting (mostly the Fe–As) systems, are discussed in great detail. Ó 2009 Elsevier B.V. All rights reserved.

Keywords: Fe–As-based superconductors Mössbauer spectroscopy Spin density waves Impurity phases

1. Introduction

2. Mössbauer spectroscopy

Since the discovery of Mössbauer effect spectroscopy (MS) 50 years ago, superconductivity (SC) has been one of the subjects, among many others, which was investigated by this method. Mössbauer probes in conventional superconductors yield relatively little information; however, in compounds that display both superconductivity and magnetic phenomena, MS has contributed extensively. The recently discovered new Fe-based systems (the oxypnictides) in which SC emerges when doping a magnetic mother compound with electrons or holes and thereby, suppressing the magnetic order, have triggered intense MS studies in both SC and magnetic states. Since the crystal structures, as well as the magnetic properties of the oxypnictides are widely described by others in this volume, we shall concentrate here on MS studies only. This paper is organized as follows. In Section 2 we discuss the general properties of solids for which MS is able to yield useful information, stressing the fields of magnetism and superconductivity. In Section 3 we discuss briefly, early trials in the study of conventional superconducting materials, the Chevrel phases, Rare Earth Rhodium Borides, Cubased high Tc SC systems and the superconducting-magnetic ruthenium-based systems. In Section 4, the spin density wave (SDW)–superconducting oxypnictides systems are discussed in detail and, in Section 5, the general conclusions are presented.

MS of solids, glasses or even highly viscous materials is capable of yielding information concerning the following subjects: (1) The electron density at the position of the Mössbauer isotope is derived from the isomer shift (IS). (2) The electric field gradient acting on that nucleus is derived from the quadrupole splitting (QS). In some cases, the local symmetry at the position of the Mössbauer probe can be extracted from the measured QS. (3) The magnetic field acting on the studied nucleus is derived from the magnetic Zeeman splitting. (4) The orientation of the magnetic field relative to the electric field gradient axis affects the electron–nucleus Spin-Hamiltonian, changing the Mössbauer spectrum shape. This yields the easy axis of magnetization in a powder sample. (5) The presence of crystallographic and/or magnetic inequivalent sites and their relative occupation, determined by the positions of the absorption lines and their spectral intensity. (6) Spin relaxation rates in systems in which the dynamic phenomena are in the Mössbauer Larmor frequency window. These spin fluctuations change the observed spectrum completely. (7) Charge fluctuation phenomena, in mixed or intermediate valence systems, measured through the various hyperfine interaction parameters, in particular the isomer shifts. (8) Phase transitions: crystallographic, magnetic order–disorder, spin reorientation, metallic-insulator and valence changes. (9) Local vibration modes including anisotropy, derived from the spectral intensity, which yields the recoil free Mössbauer fraction and thus the Debye–Waller factor. (10) Fluctuations in position of the isotope probe, due to jumps in a cage, diffusion or bound diffusion, are well observed by drastic changes in the spectral shape.

* Corresponding author. Tel.: +972 2 6585752; fax: +972 2 6586347. E-mail address: [email protected] (I. Felner). 0921-4534/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2009.03.034

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These abilities of Mössbauer spectroscopy were utilized in the research of SC systems by using 57Fe, 99Ru, 119Sn, and 151Eu as the isotope probes and also several other magnetic rare earth isotopes. All these isotopes are very sensitive to their magnetic local environment and respond in different ways. Iron is generally a magnetic probe. In a magnetic environment, its spin will order through the exchange interactions with the magnetic environment. Ruthenium can be magnetic or nonmagnetic. On the other hand, Sn is always nonmagnetic and exhibits transferred magnetic hyperfine interactions, when the magnetic environment polarizes its closed shells or the conduction electrons. Europium ions are magnetic (Eu2+, S = 7/2) or nonmagnetic (Eu3+, J = 0), and they interact with the environment accordingly. Eu3+ will exhibit induced magnetic hyperfine interactions when acted upon by exchange fields. 3. Previous Mössbauer spectroscopy studies of superconductors To learn about SC the earliest trial was done on the tin element, in 1962 by using Mössbauer spectroscopy [1]. These studies did not reveal any noticeable changes in the hyperfine interaction parameters or vibrational modes in crossing the superconducting to normal state transition. Much more useful information was obtained from studies of the Chevrel phases concerning the local vibrational modes of the 119Sn nucleus in SnMo6S8 [2]. Studies of 151Eu in doped Chevrel phases such as EuxSn1xMo6S8, enabled the observation that the external magnetic field applied, is compensated by a negative spin polarization of the conduction electrons which interact weakly with the localized magnetic moments. This leads to a small decrease in Tc and to a net enhancement in Hc2. The electron–conduction electron interaction was obtained from the measured spin relaxation rates in the 151Eu2+ Mössbauer spectra [2]. The RRh4B4 are systems (R = rare earth) in which magnetic order and SC compete with each other. Mössbauer spectra of R = 151Eu, 155Gd, 160Dy, 161Dy, 166Er, and 169Tm reveal the magnetic properties of the R ions and their saturation magnetic hyperfine fields which scale to their magnetic moments. The local moment interactions with the conduction electrons were determined through the measurement of their spin relaxation [3]. MS has made a considerable contribution to determine the magnetic state of the high Tc superconductors of the RBa2Cu3O7d family. The Cu(2) magnetic state in oxygen deficient systems was easily observed by doping 57Fe as a Mössbauer probe. MS revealed the high temperature antiferromagnetic (AFM) order of the Cu(2) and the competition between the SC and the magnetic states [4–6]. In the rutheniumbased (Ru-1222) magneto-superconducting materials, several isotopes 57Fe, 99Ru, and 119Sn revealed the Ru magnetic ordered state [7]. As mentioned above, in all these studies MS was sensitive to the magnetic state only, and little information was gained for the SC state. 4. Oxypnictides superconductors The recent discovery that some Fe–As oxypnictides are superconducting [8], with relatively high Tc (up to 54 K), led to great interest in the Mössbauer community. The reason for this is that in the oxypnictides, SC is confined to the Fe–O planes, thus Fe may serve as a direct Mössbauer probe to sense the SC properties. So far all three Fe-based oxypnictides systems: (i) LaFeAsO1x, LaFeAsO1xFx (assigned as 1111) [9–12], (ii) Sr1xKxFe2As2, Ba1xKxFe2As2, and EuFe2As2 (assigned as 122) [13–18] and (iii) LiFeAs (assigned as 111) [19], have been studied by 57Fe Mössbauer spectroscopy. EuFe2As2 [13] and EuFe2xNixAs2 [17] were also studied by 151Eu Mössbauer spectroscopy. In the 1111 and 122 systems, the parent materials are magnetically ordered in terms of a

spin density wave (SDW) state. Iron has no localized magnetic moment of its own, like in most of RFe2Si2 and RFe2Ge2 compounds [20]. Mössbauer studies reveal the SDW magnetic order, and the suppression of this state when SC is induced. We shall consider each of these three systems separately, but first we pay attention to two alternative ways by which SDW spectra may be analyzed. We also consider the possible Fe–As impurities in the samples. A Mössbauer spectrum of a commensurate SDW should exhibit a well-defined sextet, similar to that of a well-localized magnetic moment. On the other hand, in the random incommensurate case one should observe a hyperfine field (Heff) distribution according to the density formula PðHÞ ¼ ð1=pÞðH20  H2 Þ1=2 , where Ho is the saturation value of Heff and Ho < H < Ho, though the negative fields display the same Mössbauer spectrum. For a system that contains vacancies or doped foreign ions, it would be possible that the Mössbauer spectrum is composed of a superposition of two subspectra, corresponding to commensurate and incommensurate field distributions. In Fig. 1 we display such theoretical spectra for various ratios of the two types of subspectra (where Ho = 8.5 T, C = 0.25 mm/s and e = 0.06 mm/s, as in EuFe2As2) and the incommensurate SDW (Inc) fraction is indicated in the middle plots. It is clear that the commensurate subspectrum may have a hyperfine field different from Ho, allowing one more free parameter to fit experimental spectra. Alternatively, it is possible that all iron ions exhibit the same constant commensurate field (or localized field HL) and the hyperfine field is composed of HL and an additional ripple of an incommensurate field distribution for which Ho = HT  HL, where HT is the maximal hyperfine field. Here the negative part of the incommensurate field plays a crucial role. A relatively small incommensurate part changes the spectrum drastically, Fig. 2. Many of the experimentally observed spectra have these intermediate mixed subspectra shapes. However, they were interpreted either as some kind of arbitrary distribution of hyperfine fields or alternatively by using transmission integral fits, assuming thick absorbers, allowing an extra parameter which fictitiously takes care of real broadening phenomena or the presence of impurities, as discussed below. Generally speaking, the Mössbauer spectrum of all 1111 and 122 Fe–As-based materials studied, regardless of whether they are SC or SDW above their magnetic phase transition, is composed

Incomm.

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75% Inc.

50% Inc.

25% Inc.

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VELOCITY (mm/s) Fig. 1. Theoretical spin density wave (SDW) Mössbauer spectra. The various fractional amounts of the incommensurate field distribution subspectrum are indicated.

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HL=0.7H T HL=0.8HT HL=0.85HT HL=0.9HT HL=0.95H T -2

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Sr0.9 K 0.1 Fe2 As2 RT

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VELOCITY (mm/s) Fig. 2. Theoretical spectra corresponding to hyperfine fields composed of a sum of a constant field (HL) and an incommensurate field distribution for which Ho = HT  HL.

Fig. 4. Room temperature Mössbauer spectrum of the SC Sr0.9K0.1Fe2As2 sample. The impurity phase is Fe2As (TN = 343 K), from [21].

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of two subspectra. The intense subspectrum is a singlet (or a quadrupole doublet), with IS of 0.55–0.59 mm/s, and corresponds to the iron nuclei of the main phase. Its relative intensity is sample dependent. The hyperfine parameters of this subspectrum do not change at Tc. In addition, almost all samples studied contained some amount of foreign Fe–As phases, thus a major consideration in analyzing the spectra is the amount of these extra phases present in the sample. We have shown [21] that FeAs2 (TN < 5 K), FeAs (TN = 77 K) and Fe2As (TN = 345 K) [22–24], are frequently present as extra phases. In Figs. 3–5 we show the presence of these impurities in some of the studied samples.

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4.1. The LaFeAsO (1111) samples Several groups have studied the parent LaFeAsO compound in great detail [9–12]. In [9] in addition to 57Fe Mössbauer spectra (Fig. 6), muon-spin relaxation, magnetization and resistivity were measured. It was shown that below TN = 138 K a commensurate AFM SDW static order with an iron magnetic moment of only 0.25 lB is formed, while the tetragonal–orthorhombic crystallographic phase transition occurs at 156 K. The possible presence of impurities and partly commensurate incommensurate regions were not considered. In Ref. [10] LaFeAsO has been studied by many experimental techniques. These include beside MS, temperature dependent Xray and neutron diffraction, ultra-sound spectroscopy, transport properties and heat capacity. The effort was to clarify the nature of the crystallographic transition at 160 K, and the AFM one at

Fig. 5. Mössbauer spectra of the SC SmAsFeO0.9F0.1 sample at 90 K and 4.2 K, above and below the magnetic transition of the impurity phase Fe–As, from [21].

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140 K

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Fig. 6. Mössbauer spectra of LaFeAsO as a function of temperature [9].

SmAsFeO -2

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VELOCITY (mm/s) Fig. 3. Mössbauer spectrum of commensurate SDW SmAsFeO, below and above the magnetic transition. The impurity phase is FeAs (TN = 77 K), from [21].

145 K. The Mössbauer spectra (Fig. 7) were analyzed rightfully in terms of pure LaFeAsO and an impurity of FeAS FeAs. The spectra displayed broadening phenomena, and ratios among the absorption lines differed from 3:2:1. These ratios were interpreted as due to

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the texture effect, causing some magnetic alignment in the plane of the absorber. It might be, that mixed commensurate and incommensurate fields discussed above are probably the proper explanation for the observed phenomena. The low temperature magnetic moment of iron deduced from the Mössbauer spectra is 0.35 lB. In [11] the oxygen deficient compound LaFeAsO0.90 has been studied. The spectrum at 95 K was analyzed in terms of an artificial asymmetric Gaussian hyperfine field distribution. In Fig. 8 we show an analysis in terms of the model of a sum of two subspectra, discussed earlier (Fig. 1), and a presence of the nonmagnetic impurity FeAs2 (12 %). The spectrum at 200 K confirms the presence of 12 % FeAs2 impurity [21]. In [12] relatively very clean samples of LaFeAsO and LaFeAsO0.89F0.11 were investigated in the temperature range 4.2– 298 K. The SC LaFeAsO0.89F0.11 sample (Tc = 26 K) displays a single broad Mössbauer line (probably due to a small quadrupole splitting) at all temperatures. The sample was also studied at 4.2 K under a magnetic field of 7 T (Fig. 9) and displays a magnetic hyperfine field identical to the applied field, confirming that the Fe ion has effectively no magnetic moment. The study of the LaFeAsO sample (Fig. 10) exhibits a magnetic phase transition at 140 K and a Fe moment in saturation of 0.35 lB. Here again the spectra were analyzed in terms of a distribution of hyperfine fields, and intensities as if the hyperfine field in the sample is partly aligned, perpendicular to the c-ray.

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LaFeAsO 0.90 200 K

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LaAsFeO 0.90 95 K -2

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VELOCITY (mm/s) Fig. 8. Mössbauer spectrum of LaFeAsO0.90 at 200 K and 95 K analyzed in terms of a sum of commensurate and incommensurate subspectra.

4.2. The system (122) EuFe2xNixAs2 The MS studies of tetragonal 122 system in the ThCr2Si2 type structure are discussed in another paper in this volume. However,

Fig. 9. Mössbauer spectrum of the SC LaFeAsO0.89F0.11 material at 4.2 K in 7 T, from [12].

we present here some of our own MS results concerning the same samples of EuFe2xNixAs2 studied recently [17]. Pure EuFe2As2 and EuNi2As2 have been studied in very great detail by both 57Fe and 151 Eu Mössbauer isotopes [13]. The facts to emphasize here are the saturation hyperfine magnetic fields for divalent Eu (26.0 T and 36.5 T, respectively) and for Fe in EuFe2As2 (8.5 T). Our 57 Fe and 151Eu MS measurements for EuFe2As2 show spectra (Fig. 11) identical to those seen in [13]. However for EuFe1.8Ni0.2As2 we observe (Fig. 12) a pure quadrupole doublet at 90 K, a magnetic field acting on 57Fe of 1.37 T only (pointing at an angle of 36° relative the axis of the electric field gradient), at 4.2 K (analyzed by full diagonalization of the Spin-Hamiltonian) while the field acting on the divalent 151Eu is 28.0 T. The magnetization studies show [17] that Eu in EuFe2As2 is AFM ordered at TN = 20 K, while in EuFe1.8Ni0.2As2 the iron ions are nonmagnetic and the Eu sublattice orders ferromagnetically, which explain well our observations. It appears that 10% Ni (i) suppress the magnetic order of iron and (ii) induce ferromagnetism in the Eu sublattice with a larger hyperfine field than in pure EuFe2As2 [13]. The Fe low temperature Heff (1.37 T) is just the transferred field from the Eu sublattice, like the relatively larger fields acting on dilute iron, transferred from Mn in 57 Fe:RMn2Si2xGex [20]. 4.3. The system (111) LiFeAs

Fig. 7. Mössbauer spectra of LaFeAsO as a function of temperature, from [10].

The preparation procedure for the stoichiometric LiFeAs which is SC at Tc = 18 K is described in detail in [25]. SC in 111 is unique among the Fe–As-based superconductors. (i) It

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Fig. 10. Mössbauer spectra of LaFeAsO as a function of temperature, from [12].

1.000

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EuFe2 As 2

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RT

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EuFe 2 As 2

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LiFeAs 4.2 K 0.96

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-1

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VELOCITY (mm/s) Fig. 13. Mössbauer spectra above (90 K) and below (4.2 K) Tc = 18 K of LiFeAs and the magnetic transition of the impurity Fe–As phase [19].

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pound [25]. Fig. 13 presents our MS of this sample at 90 K and 4.2 K. In contrast to the 1111 and 122 systems described above, the major subspectrum in 111 at 90 K is a doublet (IS = 0.58 mm/ s and QS = 0.59 mm/s). The IS value is similar to those of the other Fe–As systems. In addition, FeAs (TN = 77 K) is present as an extra phase. The doublet (11%) at 90 K is magnetically split at 4.2 K as in SmAsFeO0.9F0.1 (Fig. 5).

0.99 0.98

EuFe

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Ni

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As

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5. Conclusions

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EuFe

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Ni

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Fe Mössbauer spectra of EuFe1.8Ni0.2As2 at 90 K and 4.2 K.

exists in a stoichiometric compound and there is no need of doping and (ii) it does not emerge from a SDW parent com-

The 57Fe Mössbauer studies of Fe–As-based SDW–SC systems have the great advantage that the Fe is a natural constituent of the compound and is not introduced as a foreign probe, an in the studies of the Cu-based superconducting systems [6]. This enables extensive studies of the SDW state in the parent materials, and changes when the composition is altered by substitutions or depletion, as demonstrated in the examples described above. The appearance of the superconducting state is well observed by the disappearance of magnetic hyperfine interactions. Mixed phases are easily observed by spectra composed of a superposition of

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magnetic and non-magnetic subspectra. Possible new methods for the analysis of SDW Mössbauer spectra are offered.

[10]

Acknowledgement [11]

This research is supported by the Klachky Foundation for Superconductivity.

[12] [13]

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