Fabrication and characterization of SmO 0.7 F 0.2 FeAs bulk with a transition temperature of 56.5 K

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1 RARE METALS Vol. 30, No. 5, Ot 2011, p DOI: /s y Fabriation and haraterization of SmO 0.7 F 0.2 FeAs bulk with a transition temperature of 56.5 K LIU Zhiyong a, MA Lin a, ZHAO Junjing b,, YAN Binjie a, and SUO Hongli a a College of Materials Siene and Engineering, Beijing University of Tehnology, Beijing , China b Department of Physis, University of Illinois at Chiago, Chiago, IL 60607, Ameria Materials Siene Division, Argonne National Laboratory, Argonne, IL 60439, Ameria Reeived 21 Otober 2010; reeived in revised form 24 Deember 2010; aepted 10 January 2011 The Nonferrous Metals Soiety of China and Springer-Verlag Berlin Heidelberg 2011 Abstrat The superondutivity of iron-based superondutor SmO 0.7 F 0.2 FeAs was investigated. The SmO 0.7 F 0.2 FeAs sample was prepared by the two-step solid-state reation method. The onset resistivity transition temperature is as high as 56.5 K. X-ray diffration (XRD) results show that the lattie parameters a and are and nm, respetively. Furthermore, the global J was more than A/m 2 at T = 10 K and H = 9 T, whih was alulated by the formula of J = 20ΔM/[a(1 a/(3b))]. The upper ritial fields, H T (T = 0 K), was determined aording to the Werthamer-Helfand-Hohenberg formula, indiating that the SmO 0.7 F 0.2 FeAs was a superondutor with a very promising appliation. Keywords: iron-based superondutor; SmO 0.7 F 0.2 FeAs; solid-state reation 1. Introdution Sine the disovery of layered opper oxide superondutors [1-2], some researhes have been foused on exploring higher T materials. MgB 2, whih was disovered in the early 2001 [3-4], aroused people to searh for new superondutors due to its unique performane ompared with the traditional low-temperature and high-temperature oxide ondutors. Reently, superondutivity in iron and nikelbased layered quaternary ompounds has been reported: They were LaOFeP (T -4 K) [5], LaONiP (T -3 K) [6], and LaOFeAs (T -26 K) [7]. Moreover, the T of iron-based superondutors has been greatly improved. It has been inreased as high as 50 K in ReO 1 x F x FeAs, with the replaement of La by other rare-earth elements suh as Pr, Nd, Sm, Ce, and Gd [8-16]. These quaternary superondutors have the tetragonal layered ZrCuSiAs struture, P4/nmm symmetry, and alternating FeAs and ReO layers [7-8, 17], similar to uprates. In this paper, we reported the preparation and superondutivity of SmO 0.7 F 0.2 FeAs, whose onset resistivity transition temperature is as high as 56.5 K. 2. Sample preparation The Sm hip, As, Fe, and Fe 2 O 3 powders were all with purity better than 99.99%; the purity of FeF 3 was 97%. The preursor SmAs was home-synthesized by reating mixtures of Sm hips and As powder in an evauated quartz tube (the vauum was better than 10-5 Pa) at 600 ºC for 5 h, then followed by heating to 900 ºC holding for 20 h. The exess of 1 wt.% was added to ompensate for the loss of As by volatilization [14]. The SmO 0.7 F 0.2 FeAs sample was prepared by the two-step solid-state reation method with the starting materials Fe, Fe 2 O 3, FeF 3, and the obtained SmAs powders wiht the ratio being 14:7:2:30. The stoihiometri mixture of the starting materials was ground thoroughly and pressed into pellets. The obtained pellets were sintered in an evauated quartz tube at 900ºC for 5h, then 1200ºC for 40 h, and then the samples were furnae-ooled to room temperature. The vauum in the evauated quartz tube was at the order of 10-5 Pa. The phase purity and strutural identifiation were haraterized by powder X-ray diffration (XRD) analysis on an MXP18A-HF type diffratometer with Cu K α radiation from 20 to 80 with a step of The resistivity of the sample was measured by the standard four-probe method from 30 to 300 K. The DC magnetization was measured using a Quantum Design MPMS XL-1 system. Corresponding author: SUO Hongli honglisuo@bjut.edu.n

2 2 RARE METALS, Vol. 30, No. 5, Ot Results and disussion Fig. 1 shows the omparison of XRD patterns for both nominal SmOFeAs and SmO 0.7 F 0.2 FeAs samples; the nominal SmOFeAs sample was also synthesized by the two-step solid-state reation method. The XRD results indiate that the main phase of non-doped and F-doped samples have the same struture with slight impurity phases. The major impurity phases suh as unreated SmAs are observed in our present samples, in addition to slight impurity phase of SmOF in F-doped SmO 0.7 F 0.2 FeAs sample. The impurity phases have been identified to be some by-produts, whih do not have superondutivity. The lattie parameters for the nominal SmO 0.7 F 0.2 FeAs are a = nm, = nm, whih are smaller than those of nominal SmO 0.7 F 0.3 FeAs in Ref. [18]. For the SmO 0.7 F 0.2 FeAs, due to the lak of F, its lattie onstants are even smaller. lattie onstant of SmO 0.7 F 0.2 FeAs sample desribed above. The SmO 0.7 F 0.2 FeAs sample has a metalli behavior when the temperature goes up to 300 K. The residual resistivity ratio RRR =ρ (300 K) /ρ (55 K) was about 5.3, and the transition width ΔT is about 3 K, whih shows that the SmO 0.7 F 0.2 FeAs sample has a high purity, but the fish tail indiate the existene of some non-superonduting impurities. Fig. 2. Temperature dependene of resistivity for the nominal SmO 0.7 F 0.2 FeAs sample. The inset shows a learer superonduting transition image. Fig. 1. Typial XRD patterns for the non-doping SmOFeAs and F-doped SmO 0.7 F 0.2 FeAs samples. In Fig. 2, it an be seen from the inset that a lear superonduting onset transition (T (onset)) ourred at 56.5 K, and a zero resistivity transition (T (zero)) at 53 K for the nominal SmO 0.7 F 0.2 FeAs was obtained, whih is about 1.5 K higher than that of Ref. [12]. This may be due to the smaller For an experimental yle, the SmO 0.7 F 0.2 FeAs sample was ooled down to 30 K in zero field ooling (ZFC), and the data were gathered when warmed in an applied field, then ooled again under an applied field (field ooling, FC), and measured when warming up. The DC-suseptibility data (measured under a magneti field of A/m) of the SmO 0.7 F 0.2 FeAs are shown in Fig. 3. The magneti onset T for the SmO 0.7 F 0.2 FeAs sample is at 56.5 K, whih is onsistent with the resistivity result. The existene of a superonduting phase is onfirmed by the Meissner effet by ooling it in a magneti field while the resistane goes to zero. Fig. 3. Temperature dependene of the DC-suseptibility (a) and the differential ZFC urve (b) for the nominal SmO 0.7 F 0.2 FeAs

3 Liu Z.Y. et al., Fabriation and haraterization of SmO 0.7 F 0.2 FeAs bulk with a transition temperature of 56.5 K 3 sample. Fig. 4. M-H loops at various temperatures for SmO 0.7 F 0.2 FeAs sample at 10 K (a) and 30 K (b). Figs. 4(a) and 4(b) show the typial M-H hysteresis loops of SmO 0.7 F 0.2 FeAs at 10 and 30 K, respetively. The gap in the loop drops dramatially in the low-field region with inreased applied field, but it rapidly reahes a slowly dereasing state over a wide range of applied magneti field. The rapid derease in the wide hysteresis loop is a typial abnormal behavior for granular superondutors, sine the weak superondutivity in the grain boundaries is extremely sensitive to the magneti field. The global J of the samples is alulated on the basis of the following formula: 20 M J a(1 a (3 b)) where ΔM is the height of the magnetization loop and a and b are the dimensions of the sample perpendiular to the magneti field with a < b. Fig. 5 presents the J -H urves of SmO 0.7 F 0.2 FeAs at 10 and 30 K, respetively. It shows that J drops drastially in the low-field region with the inrease of the applied field from 0 to 3 T, whih illustrates that the grain boundaries are a weak-link behavior for the granular SmO 0.7 F 0.2 FeAs sample, but J rapidly redues and then beomes stable over a wide range of applied magneti field from 3 T to 9 T. The J of SmO 0.7 F 0.2 FeAs is more than A/m 2 at 10 K and has a very weak dependene on the field, whih shows that SmO 0.7 F 0.2 FeAs has a fairly large pinning fore in the grain. But at high temperature, J drops drastially with the inrease of the applied field. This shows that the magneti flux pinning ability has been badly damaged in the sample at the high emperature. Fig. 6 shows the temperature dependene of eletrial resistivity of the superonduting SmO 0.7 F 0.2 FeAs sample at different magneti fields. An empirial riterion for the onset of superondutivity at a given applied field would be the deviation from the apparent linear behavior of the normal-state resistivity (ρ n ). Using this riterion, we define on mid transition temperatures T, T, and 10 T orresponding to 90%, 50%, and 10% of ρ n, respetively. With the inrease of the magneti field from 0 to 9 T, both the onset transition Fig. 5. J -H urves for the SmO 0.7 F 0.2 FeAs sample at 10 and 30 K. on point ( T temperature, with ) and the zero-resistane point shift toward lower on T by 2 K, while the latter by 10 k. This is understandable sine the latter is determined by the weak links between the grains and the vortex flow of magneti flux lines, while the former is ontrolled by the upper ritial field of the individual grains. The temperature dependene of H 2, H irr, and H peak determined from the 90%, 50%, and 10% of ρ n riteria is shown in Fig. 7. The values of (dh 2 /dt) T for 10%, 50%, and 90% of ρ n, are 1.3, 2.4, 4.1 TK 1, respetively. It needs to be mentioned that the slope (dh 2 /dt) T determined with different riteria is a linear funtion of the urve shown in Fig. 7. At low temperatures, well below T, where the flux flow dominates resistivity, an empirial relation ρ/ρ n = H/H has been shown. Using the onventional single-band Werthamer-Helfand-Hohenberg (WHH) theory: WHH 2 2 H (0) T (d H d T) T,

4 4 RARE METALS, Vol. 30, No. 5, Ot 2011 One an estimate H 2 (0) from the slope of the H 2 (T) urve at T = T. For the 90% ρ n riterion (T = 56 K), H 2 (0) is about 256 T. H 2 (0) an be estimated from the Ginzburg- Landau (G-L) mean-field theory. Aording to this theory, 2 H2 0 2 and 2 (1 t 2 ) (1 t 2 ), where 0 is the flux quantum, is the oherene length, and t = T/T is the redued temperature. From the above relation, one obtains 2 2 H2( T) H2(0)(1 t ) (1 t ). The value of H 2 (0) obtained from the G-L equation is about 203 T for the 90% ρ n riterion. Apparently, the estimated values of the upper ritial fields obtained from the WHH theory (256 T) and the G-L theory (203 T) are muh higher BCS p than the BCS paramagneti limit H = 1.84T in the weak-oupling regime. However, the BCS model underestimates H p in the presene of strong e-ph oupling. In suh a ase, the atual Pauli paramagneti limit is H p (1 BCS ) H P, where λ is the e-ph oupling onstant. Using the value of λ~1.5 as estimated from the ρ(t) urve of a PrO 0.6 F 0.12 FeAs sample [19], the paramagneti limit H p inreases to ~240 T. Suh a high value of zero-temperature upper ritial field is omparable with that observed in uprate superondutors. Fig. 6. Temperature dependene of resistivity for the SmO 0.7 F 0.2 FeAs sample in the low-temperature region under different fields. Fig. 7. Upper ritial fields H 2 versus temperature for the SmO 0.7 F 0.2 FeAs sample as determined from the 90%, 50%, and 10% riteria of the normal-state resistivity, respetively. 4. Conlusion The F-doped SmO 0.7 F 0.2 FeAs was prepared by the two-step solid-state reation method. The optimal nominal SmO 0.7 F 0.2 FeAs is found to have the highest T at 56.5 K (onset) in the Sm-system. The XRD results indiate a lear shrinkage of lattie parameters in the F-doping sample ompared with that of the undoped one. The eletrial resistivity and the DC-suseptibility (measured under a magneti field of A/m) of the SmO 0.7 F 0.2 FeAs show that a lear superonduting onset transition (T (onset)) ourred at 56.5 K and a zero resistivity transition (T (zero)) at 53 K for the nominal SmO 0.7 F 0.2 FeAs, whih is the optimal T in this Sm-system. The transition width ΔT is about 3 K, meaning that our sample has a high quality. The typial M-H hysteresis loops at 10 K show that the rapid derease in the wide range of the hysteresis loop is not a typial behavior of granular superondutors sine the weak superondutivity in the grain boundaries is extremely sensitive to the magneti field. The global J was alulated on the basis of J = 20ΔM/[a(1 a/(3b))]. At 10 K, the J of SmO 0.7 F 0.2 FeAs is more than A/m 2, whih has a very weak dependene on the field, showing that SmO 0.7 F 0.2 FeAs has a fairly large pinning fore in the grain. Aording to the onventional single-band Werthamer-Helfand-Hohenberg (WHH) theory, H 2 (0) is about 256 T. Aknowledgments The authors are very grateful to NI Baorong, Professor from Fukuoka Institute of Tehnology, Japan, for his guidane and assistane. This work is finanially supported by the National Basi Researh Program of China (No. 2006CB601005), the National High Tehnology Researh

5 Liu Z.Y. et al., Fabriation and haraterization of SmO 0.7 F 0.2 FeAs bulk with a transition temperature of 56.5 K 5 and Development Program of China (No. 2009AA032401), the National Natural Siene Foundation of China (Nos and ), the Beijing Muniipal Natural Siene Foundation (No ), and the Program for New Century Exellent Talents in University of Ministry of Eduation of China (No ). Referenes [1] Bednorz J.G. and Mueller K.A., Possible high T C superondutivity in the Ba-La-Cu-O system, Z. Phys.,1986, B64 (2): 189. [2] Chu C.W., Hor P.H., Meng R.L., Gao L., Huang Z.J., and Wang Y.Q., Evidene for superondutivity above 40 K in the La-Ba-Cu-O ompound system, Phys. Rev. Lett., 1987, 58: 405. [3] Nagamatsu J., Nakagawa N., Muranaka T., Zenitani Y., and Akimitsu J., Superondutivity at 39 K in magnesium diboride, Nature, 2001, 410: 63. [4] Flükiger R., Suo H.L., Musolino N., Benedue C., and Lezza P., Superonduting properties of MgB 2 tapes and wires, Phys. C, 2003, 385: 286. [5] Kamihara Y., Hiramatsu H., Hirano M., and Hosono H., Abstrats in physial inorgani hemistry iron-based layered superondutor: LaOFeP, J. Am. Chem. So., : [6] Watanabe T., Yanagi H., Kamiya T., Hirano M., and Hosono H., Nikel-based oxyphosphide superondutor with a layered rystal struture, LaNiOP, Inorg. Chem., 2007, 46: [7] Kamihara Y., Watanabe T., Hirano M., and Hosono H., Iron-based layered superondutor La[O 1 x F x ]FeAs (x = ) with T = 26 K, J. Am. Chem. So., 2008, 130: [8] Chen X. H., Wu T., Wu G., Chen H., and Fang D. F., Superondutivity at 43 K in Samarium-arsenide oxides SmFeAsO 1 x F x, Nature, 2008, 453: 761. [9] Chen G.F., Li Z., Wu D., Li G., and Wang N.L., Superonduting properties of the Fe-based layered superondutor LaFeAsO 0.9 F 0.1, Phys. Rev. Lett., 2008, 101: [10] Ren Z.A., Yang J., Lu W., Yi W., and Zhao Z.X., Superondutivity at 52 K in iron-based F-doped layered quaternary ompound Pr[O 1 x F x ]FeAs, Mater. Res. Innov., 2008, 12 (3): 105. [11] Ren Z.A., Yang J., Lu W., Yi W., Zhou F., and Zhao Z.X., Superondutivity in iron-based F-doped layered quaternary ompound Nd[O 1 x F x ]FeAs, Europhys. Lett., 2008, 82: [12] Ren Z.A., Lu W., Yang J., and Zhao Z.X., Superondutivity at 55K in iron-based F-doped layered quaternary ompound Sm[O 1 x F x ]FeAs, Chin. Phys. Lett. 2008, 25: [13] Chen G.F., Li Z., Wu D., Li G., Luo J.L., and Wang N.L., Superondutivity at 41K and its ompetition with spin-density- wave instability in layered CeO 1 x F x FeAs, Phys Rev Lett. 2008, 100: [14] Yang J., Li Z.C., Lu W., Yi W., and Zhao Z.X., Superondutivity at 53.5 K in GdFeAsO 1 δ, Superond. Si. Tehnol., 2008, 21: [15] Gao Z.S., Wang L., Ma Y.W., and Wen H.H., Superonduting properties of granular SmFeAsO 1 x F x wires with T = 52 K prepared by the powder-in-tube method, Superond. Si. Tehnol, 2008, 21: [16] Wei Z., L H.O., and Ruan K.Q., Superondutivity at 57.3 K in La-doped iron-based layered ompound Sm 0.95 La 0.05 O F 0.15 FeAs, J. Superond. Nov. Magn., 2008, 21: 213. [17] Martinelli A., Ferretti M., Putti M., and Siri A.S., Synthesis, rystal struture, mirostruture, transport and magneti properties of SmFeAsO and SmFeAs(O 0.93 F 0.07 ), Superond. Si. Tehnol., 2008, 21: [18] Wang L., Gao Z.S., Qi Y.P., and Ma Y.W., Strutural and ritial urrent properties in polyrystalline SmFeAsO 1 x F x, Superond. Si. Tehnol., 2009, 22: [19] Bhoi D., Manda P., and Choudhury P., Resistivity saturation in PrFeAsO 1 x F y superondutor: evidene of strong eletron-phonon oupling, Superond. Si. Tehnol., 2008, 21: