Magnetism and superconductivity in Sb-doped binary and ternary ironchalcogenide single crystals

Dona Cherian, G.M. Nagendra, Suja Elizabeth n

Department of Physics, Indian Institute of Science, Bangalore 560012, India

a r t i c l e i n f o

Keywords:A2. Bridgman methodB1. ChalcogenidesB2. Magnetic materialsB2. Superconducting materials

a b s t r a c t

We report the single crystal growth of antimony doped Fe1þyTe and Fe1þyTe0:5Se0:5 (Fe1þySbxTe1�x

(x¼0, 2%, 5%) and Fe1þyTe0:49Se0:49Sb0:02) by a modified horizontal Bridgman method. Growthparameters are optimized to obtain high quality single crystals. The antiferromagnetic (AFM) transitionat TN¼62.2 K which is a first order transition, shifts to lower temperature on doping in Fe1þyTe.Alternately when the chalcogen site of the ternary compound Fe1þyTe0:5Se0:5 is doped with Sb,superconductivity is preserved albeit the superconducting transition temperature (TC) falls slightly anda concomitant reduction occurs in superconducting volume fraction.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Discovery of superconductivity in iron pnictides and chalcogenidesresulted in tremendous excitement for material scientists to realizenovel superconducting materials with higher TC and a variety ofphysical properties. While different iron pnictides and chalcogenidesuperconductors show striking similarity in crystal structure, they canbe broadly classified into fourmajor categories based on their chemicalcomposition ratio as 1111, 122, 111 and 11 [1]. Of these, the binary 11compounds Fe1þyCh (Ch¼Se, Te) are marked by their simple layeredstructure and interesting structural, magnetic and superconductingproperties [2,3]. At room temperature, Fe1þyCh exists as a primitivetetragonal structure with space group P4/nmm, with the basal planessharing layers of Fe2Ch2 [1]. An earlier propounded density functionaltheory predicted that among the binary Fe chalcogenides, Fe1þyTewould show superconductivity with maximum TC [4]. But later,experimental evidences confirm that Fe1þyTe does not demonstratesuperconductivity whereas FeSe1�x does, with TC¼8 K [5]. On theother hand, isovalent doping with selenium or sulfur at the telluriumsite induces superconductivity. With sulfur doping, the highest TCreported is 10 K where as with selenium, the maximum TC reaches to15 K [6,7]. FeSe reveals a large enhancement in TC, from 8 K to 37 K,with external pressure [8,9].

The parent compound Fe1þyTe is a collinear antiferromagnetbelow Néel temperature [10]. Although Fe1þyTe is not a super-conductor, the AFM parent compound attracts much attention due

to its remarkable structural and magnetic properties arising fromthe presence of excess Fe(¼y) in the lattice. At Néel temperature, itundergoes a magneto-structural transition from tetragonal toorthorhombic or tetragonal to monoclinic phase depending onthe concentration of excess Fe [11]. It would then be of interest tostudy the change in magneto-structural transition in Fe1þyTe withdoping. Various dopants have been used to replace the anion (Ch)and cation (Fe) sites to alter superconducting and magneticproperties with the assumption that chemical pressure may alsoalter the physical properties [12]. The selenium site in FeSe wasdoped with Sb [13] which resulted in superconducting transitionclose to 9 K, at low concentration of Sb (0–0.15). Ge et al. haveobserved changes in structural transition as well as superconduct-ing transition in FeSe when Sb was used as a dopant [14]. In thispaper we proceeded to dope Sb at Te site in Fe1þyTe and sought toobserve the modifications in the magneto-structural transition inFe1þyTe. We also performed co-doping at the chalcogen site ofsuperconducting Fe1þyTe0:5Se0:5. While the superconducting tran-sition was pushed slightly to lower temperature, it was interestingto note how the superconductivity was preserved at low Sb dopingconcentration levels in this material.

2. Crystal growth

Iron chalcogenide single crystals are grown by the modifiedhorizontal Bridgman method. The crystal growth unit is a home-made setup with a cylindrical furnace placed at an angle withrespect to the horizontal. The ampoule containing precursorswhen placed inside the furnace also experiences the same inclina-tion which helps in better homogenization and better controlduring nucleation. This angle is optimized and is crucial for single

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Journal of Crystal Growth

0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jcrysgro.2013.11.008

n Corresponding author. Tel.: þ91 80 2293 3461; fax: þ91 80 2360 2602.E-mail addresses: donnacherian@gmail.com,

donacherian@physics.iisc.ernet.in (D. Cherian),liz@physics.iisc.ernet.in (S. Elizabeth).

Please cite this article as: D. Cherian, et al., Journal of Crystal Growth (2013), http://dx.doi.org/10.1016/j.jcrysgro.2013.11.008i

Journal of Crystal Growth ∎ (∎∎∎∎) ∎∎∎–∎∎∎

crystal growth. The required high purity precursors are accuratelyweighed according to the stoichiometric composition to obtain 7 gof the product and are filled in a quartz ampoule that has a taperedend. A quartz tube of 10 mm inner diameter (ID) and 12 mm outerdiameter (OD) is used for this. The ampoule is evacuated to10�4 mbar and sealed under vacuum. Considering that the pre-cursors are very sensitive to oxygen, the sealed ampoule is placedin a second larger quartz ampoule and again sealed under vacuumto avoid any possible contamination during crystal growth. Thelarger quartz tube's typical dimensions are 15 mm ID and 18 mm

OD. The furnace is designed to attain a temperature gradientacross its length. The maximum temperature is obtained at themiddle portion along the axis of the furnace and a minimum atboth ends. During crystal growth, the furnace is translated acrossthe loaded ampoules to achieve the required temperature gradi-ent. The rate at which the gradient sweeps across the ampoule canbe controlled by varying the furnace translation rate. The gradientsweep rate across the ampoule has been optimized after tryingdifferent translation rates. Better quality, bigger crystals areobtained for a typical growth rate of 9.2 mm/h in all four sets of

Table 1Crystal growth parameters for Sb doped Fe1þyTe and Sb doped Fe1.05Se0.5Te0.5. The composition is estimated from EPMA.

Growth parameters FeTe FST02 FST05 FTSS02

TDwell (1C) 950 950 950 950tDwell (h) 12 44 44 72Furnace 9.2 9.2 9.2 9.2translation (mm/h)Growth ambiance Vacuum Vacuum Vacuum ArgonPost growth annealing – – – 350 1C (72 h)Starting composition Fe1.05Te Fe1.05Te0.98Sb0.02 Fe1.05Te0.95Sb0.05 Fe1.05Te0.49Se0.49Sb0.02Composition from Fe1.11Te Fe1.1Te0.983Sb0.017 Fe1.11Te0.098Sb0.015EPMA

Fig. 1. (a) Photographs of as-grown crystals and Laue patterns on cleaved crystals showing four-fold symmetry. (b) Bragg peaks obtained from plate XRD which shows the setof (00l) planes.

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crystals. The maximum temperature attainable for the Bridgmanfurnace is 1000 1C and translation rate varies from 0.09 mm/h to27 mm/h.

Before initiating crystal growth, the precursors are heated tomelting point and mixed thoroughly for homogenizing the melt.The ampoule is rotated within the Bridgman furnace duringhomogenization. Crystal growth is initiated by translating thefurnace at a specified translation rate. After crystal growth theampoule is cooled to room temperature at a very slow andcontrolled cooling rate. The advantage of this procedure is thatthe prior polycrystalline synthesis is completely avoided, so alsothe possibility of contamination from the quartz ampoule. Thehomogenization ensures a proper mixing and slow cooling helpsto reduce thermal strain on the grown crystal.

2.1. Fe1:05SbxTe1� x, (0,0.02,0.05)

The precursors, Fe (99.7%), Te (99.99%) and Sb (99.9999%) areaccurately weighed and are vacuum sealed in quartz ampoules. Theampoule is heated to 950 1C for homogeneous melting and mixing,during which time the ampoule is continuously rotated in theclockwise and anti-clockwise direction, for 12 h in the case ofFe1þyTe and 44 h in the case of Fe1:05SbxTe1� x. Crystal growth isthen initiated by translating the furnace at a rate of 9.2 mm/h.Through out the process, the temperature of the furnace is main-tained at 950 1C so that the gradient across the ampoule is same asthat across the furnace. Finally the furnace is cooled gradually toroom temperature. The starting composition of crystals are Fe1.05Te(FeTe), Fe1.05Sb0.02Te0.98 (FST02) and Fe1.05Sb0.05Te0.95 (FST05).

2.2. Sb doped Fe1.05Te0.5Se0.5

High purity Se (99.999%) is added to the above mentioned setof precursors in the required ratio. The precursors are sealed under3 mbar argon pressure. Selenium has high vapor pressure com-pared to other precursors. To minimize the loss of Se vapors athigh temperature and to have better control over Se compositionargon ambiance is preferred. Procedure for crystal growth is same asmentioned above except that homogenization is carried out for 72 h.After cooling to room temperature, the as-grown crystals are furtherannealed in argon at 350 1C for 72 h and cooled to room temperature.The starting composition is Fe1.05Te0.49Se0.49Sb0.02 (FTSS02).

The composition of the crystals is determined by an electronprobe micro-analyzer (EPMA), JEOL JXA-8530F using wavelengthdispersive spectroscopy (WDS). The details of crystal growth,growth parameters and composition are given in Table 1.

3. Results and discussion

3.1. Characterization of single crystals

The as-grown crystals are cleaved and Laue diffraction isrecorded in transmission geometry. Fig. 1(a) shows the photo-graphs and Laue pattern of as-grown crystals. Formation of singlegrain and crystal symmetry is confirmed by Laue pattern. A four-fold symmetry is clearly seen here. The X-ray diffraction obtainedfrom a cleaved plate confirms the orientation as (00l). Fig. 1(b) showsthe set of (00l) planes.

3.2. Crystal structure and refinement

The crystallographic phase formation of the compounds is deter-mined by powder X-ray diffraction (XRD) patterns using a Bruker D8advanced powder diffractometer. Cleaved pieces of the as-growncrystals are crushed into fine powder. Rietveld refinement is

employed to identify and confirm the crystallographic phase and toobtain the refined lattice parameters. All compounds are refined toP4/nmm with space group number 129. The refined powder patternsfor the above discussed crystals are shown in Fig. 2. Sb dopedFe1þyTe samples give an impurity peak which is identified as FeSb2.But this does not show any signature in the experimental investiga-tions of magnetic and transport behavior. Lattice parameters increasewith Sb concentration in Fe1:05SbxTe1�x. The details of refinementare summarized in Table 2.

3.3. Magnetic properties

Magnetization measurements on Fe1:05SbxTe1� x single crystals areperformed using a vibrating sample magnetometer using the fieldcooled warming protocol (FCW) with applied field perpendicular toc-axis. Magnetization measurements on Sb doped Fe1.05Te0.5Se0.5 are

Fig. 2. Powder XRD patterns obtained from crushed single crystals. Observedpattern, calculated pattern and possible Bragg peaks are shown. All the patterns arerefined in tetragonal structure with P4/nmm symmetry.

Table 2Crystal structure refinement and refined lattice parameters for as-grown crystals.Magnetic and superconducting transition temperature for different crystals areestimated from bulk measurements. Linear fits to inverse susceptibility yield ΘCW

values.

Name FeTe FST02 FST05 FTSS02

Space group P4/nmm P4/nmm P4/nmm P4/nmmLattice parameters a¼3.824 Å a¼3.826 Å a¼3.835 Å a¼3.796 Å

c¼6.283 Å c¼6.274 Å c¼6.305 Å c¼6.001 ÅTransition temperature 62.2 K (TN) 56.5 K (TN) 56 K (TN) 8 K (TC)ΘCW (K) �150 �152 �228 –

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performed using a SQUID magnetometer in zero filed cooling(ZFC) protocol with applied field parallel to c-axis. Fig. 3(a) showsmagnetization as a function of temperature performed at 1000 Oe.The transition temperature is calculated from the peak corre-sponding to the transition in the derivative plot (not shown).In Fe1þyTe a sharp transition is observed at TN¼62.2 K whichcorresponds to the antiferromagnetic transition [11]. This isidentified as a first order magneto-structural transition, belowwhich the sample transforms to lower symmetry [15,16].In Fe1þyTe crystal the transition is very sharp with ΔT ¼ 0:8 K.On doping with 2% antimony, the transition falls to TN¼56.5 K. At5% doping level, the transition occurs at TN¼56 K. The broadeningof the transition width may be attributed to the combined effect ofincrement in the value of excess Fe in addition to the presenceof Sb in the crystal lattice. The contribution of Fe(y) on the magneto-structural transition is previously documented [16]. To clarify therole of Sb in the lattice we compared the data of Sb doped sampleswith Fe1þyTe at different y concentrations (figure not shown). It isconfirmed that Sb doped samples show a marked difference fromthe parent Fe1þyTe. A detailed study of TN and its dependence on ydoes not form the scope of this work.

Based on the assumption that magnetism is predominantlyparamagnetic above Néel temperature in the region 175–300 K,the experimental data of inverse susceptibility is fitted to astraight line. Fig. 3(b) illustrates the fitted curve for three sampleswhere in the Curie–Weiss temperature, ΘCW is calculated. Thesevalues along with the transition temperatures are listed in Table 2.The results reveal that antimony doping brings about noticeablechanges in the magneto-structural transition and AFM behavior inTeFe1þy Te at 2% concentration levels. Further increment in Sbcontent (up to 5%) does not yield any appreciable change intransition temperature. Even though sample FST05 has the nom-inal composition of 5% Sb, the as-grown crystal behaves in closeresemblance to FST02 which is supported by magnetization data.

We confirmed this by doping with 7% and 10% Sb concentration atthe Te site in these crystals. In both cases, single crystals did notform but mixed phases of polycrystalline material with high valuesof excess Fe were left behind.

In Fig. 3(c), the superconducting transition observed in ZFCmagnetization measurements at 100 Oe on 2% Sb dopedFe1.05Te0.5Se0.5 is shown. The superconducting transition is notentirely suppressed by Sb doping, but a considerable reductionin transition temperature is noticeable with Tonset

C ¼ 8 K. A dropin magnetization to negative side mark the onset of superconduc-tivity. The inset shows a magnified view of transition regionin units of 4πχ and computed superconducting volume fraction,9%. Fig. 3(d) illustrates the behavior of magnetization with respectto applied magnetic field. As field increases, the moment yieldsnegative values. At a threshold level, lower critical field(HC1¼200 Oe), it reverses in magnitude and direction. In presenceof Sb, the transition temperature and volume fraction decreasewhen compared to Fe1.05Se0.5Te0.5 [17].

3.4. Electrical transport measurements

Fig. 4(a) shows the electrical transport measurements on FeTe,FST02 and Fig. 4(b) shows measurement on FTSS02 crystals.Resistivity measurements in zero magnetic field on Fe1þyTeregister a sharp drop in resistivity at TN, whereas a broad transitionis discernible in FST02 (see Fig. 4(a)). This is in support to thebehavior observed in magnetization measurements. In FTSS02 adrop in resistivity near 10 K indicates the onset of superconductingtransition. At 7 K zero resistance is achieved which corresponds tobulk superconductivity. At this temperature the magnetizationalso becomes negative according to magnetization measurements.Investigating further, resistivity measurements were preformed inapplied magnetic field (H¼2 T, 3 T, 4 T and 5 T) parallel to c-axisfrom 4.5 K to 40 K. At 5 T, resistivity does not reach zero till 4.5 K

Fig. 3. (a) Magnetization vs. temperature measurements on Fe1þyTe1�xSbx (x¼0, 2%, 5%) at 1000 Oe. (b) The Curie–Weiss fit of FeTe, FST02 and FST05 crystals. Red linesrepresent the linear fit. (c) The magnetization measurements on Fe1.05Te0.49Se0.49Sb0.02 at 100 Oe. The inset shows the magnified transition region. (d) The moment responseto applied magnetic field. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)

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even though it commences to drop. The upper critical fields (HC2)are likely to be lower in comparison with other iron chalcogenideswhich are known to possess high values [18].

4. Conclusions

In summary, single crystals of Fe1þyTe were grown together withSb doped variants of Fe1þyTe and Fe1þyTe0:5Se0:5 by the modified

horizontal Bridgman method and the influence of dopant onmagnetic and superconducting properties has been investigated.A lower concentration level (2%) alone was conducive to crystalgrowth and at higher doping concentration ðSb45%Þ crystalformation did not occur in pure phase. At 2% Sb doping, theantiferromagnetic Fe1þyTe demonstrated a decline of TN from62.2 K to 56.5 K for the doped compound. Sb doped Fe1.05Te0.5Se0.5demonstrated a superconducting transition at TC¼8 K. Lowercritical filed is calculated from magnetization measurements.

Acknowledgments

The authors wish to express their gratitude to DST, India, forfinancial support through project grant.

References

[1] D.C. Johnston, Adv. Phys. 59 (2010) 803.[2] S. Margadonna, Y. Takabayashi, M.T. McDonald, K. Kasperkiewicz, Y. Mizuguchi,

Y. Takano, A.N. Fitch, E. Suard, K. Prassides, Chem. Commun. 43 (2008) 5607.[3] S. Li, C. de la Cruz, Q.H.Y. Chen, J.W. Lynn, J. Hu, Y.L. Huang, F.C. Hsu, K.W. Yeh,

M.K. Wu, P. Dai, Phys. Rev. B 79 (2009) 054503.[4] W. Bao, Y. Qiu, Q. Huang, M.A. Green, P. Zajdel, M.R. Fitzsimmons,

M. Zhernenkov, S. Chang, MinghuFang, B. Qian, E.K. Vehstedt, J. Yang,H.M. Pham, L. Spinu, Z.Q. Mao, Phys. Rev. B 78 (2009) 134514.

[5] F.C. Hsu, J.Y. Luo, K.W. Yeh, T.K. Chen, T.W. Huang, P.M. Wu, Y.C. Lee, Y.L. Huang,Y.Y. Chu, D.C. Yan, M.K. Wu, PNAS 105 (2008) 14262.

[6] Y. Mizuguchi, F. Tomioka, S. Tsuda, T. Yamaguchi, Y. Takano, Appl. Phys. Lett. 94(2009) 12503.

[7] K.W. Yeh, T.-W. Huang, Y.L. Huang, T.K. Chen, F.-C. Hsu, P.M. Wu, Y.C. Lee,Y.Y. Chu, C.L. Chen, J.Y. Luo, D.C. Yan, M.K. Wu, Eurphys. Lett. 84 (2008) 37002.

[8] S. Margadonna, Y. Takabayashi, Y. Ohishi, Y. Mizuguchi, Y. Takano, T. Kagayama,T. Nakagawa, M. Takata, K. Prassides, Phys. Rev. B 80 (2009) 64506.

[9] S. Medvedev, T.M. McQueen, I.A. Troyan, T. Palasyuk, M.I. Eremets, R.J. Cava,S. Naghavi, F. Casper, V. Ksenofontov, G. Wortmann, C. Felser, Nature Mater. 8(2009) 630.

[10] F. Ma, W. Ji, J. Hu, Z.Y. Lu, T. Xiang, Phys. Rev. Lett. 102 (2009) 177003.[11] W. Bao, Y. Qiu, Q. Huang, M.A. Green, P. Zajdel, M.R. Fitzsimmons,

M. Zhernenkov, S. Chang, MinghuFang, B. Qian, E.K. Vehstedt, J. Yang,H.M. Pham, L. Spinu, Z.Q. Mao, Phys. Rev. Lett. 102 (2009) 247001.

[12] Y. Mizuguchi, F. Tomioka, S. Tsuda, T. Yamaguchi, Y. Takano, J. Phys. Soc. Jpn. 78(2009) 74712.

[13] Sudesh, S. Rani, G.D. Varma, Physica C 485 (2013) 137.[14] J. Ge, S. Cao, S. Shen, S. Yuan, B. Kang, J. Zhang, Phys. C 485 (2013) 137.[15] G.F. Chen, Z.G. Chen, J. Dong, W.Z. Hu, G. Li, X.D. Zhang, P. Zheng, J.L. Luo,

N.L. Wang, Phys. Rev. B 79 (2009) 140509.[16] S. Rößler, D. Cherian, W. Lorenz, M. Doerr, C. Koz, C. Curfs, Y. Prots, U.K. Roßler,

U. Schwarz, S. Elizabeth, S. Wirth, Phys. Rev. B 84 (2011) 174506.[17] S. Rößler, D. Cherian, S. Harikrishnan, H.L. Bhat, S. Elizabeth, J.A. Mydosh,

L.H. Tjeng, F. Steglich, S. Wirth, Phys. Rev. B 82 (2010) 144523.[18] H. Lei, R. Hu, E.S. Choi, J.B. Warren, C. Petrovic, Phys. Rev. B 81 (2010) 094518.

Fig. 4. (a) The resistivity vs. temperature for Fe1þyTe1� xSbx (0, 2%). (b) Theresistivity behavior of Fe1.05Te0.49Se0.49Sb0.02 in zero and applied magnetic field.Transition region is given in the inset for clarity.

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