J Supercond Nov MagnDOI 10.1007/s10948-014-2579-z

ORIGINAL PAPER

Influence on the Tc of Co and F Co-doping into SmFeAsOat Fe and O Site

Zhongtian Yang · Liya Zhu

Received: 19 March 2014 / Accepted: 13 May 2014© Springer Science+Business Media New York 2014

Abstract Series of superconductive samples in which Coand F co-doping into SmFeAsO were prepared by mechan-ical alloying (MA) and sequent sintering method to inves-tigate the influence on Tc of the charge carrier introducedinto SmO layers and FeAs layers simultaneously. For theSmFe1− xCoxAsO with optimum Co doping content ofx = 0.1, F doping can still enhance the Tc until up to 23 Kwhen doping content is 15 % (from our previous works,the highest Tc of SmFe1− xCoxAsO is 12.5 K). The chargecarrier density of SmFe0.9Co0.1AsO0.85F0.15 is larger thanthat of SmFe0.9Co0.1AsO, characterized by Hall coefficientmeasurement. On the other hand, for optimum F-dopingSmFeAsO0.8F0.2, simultaneous Co doping decreases the Tcdramatically. For the under-doping SmFeAsO0.9F0.1, the Tcrises with the increasing Co doping content to 10 % and thenfalls down with more Co doping into the compounds. Theresults can offer some experimental evidence of the compe-tition and restriction among the factors influencing the Tcof the SmFeAsO by doping Co and F simultaneously.

Keywords Co-doping · Influence on Tc · Competition andRestriction

Z. Yang (�) · L. ZhuCollege of Science, Collaborative Innovation Centerfor Magnetoelectric Industry, China Three Gorges University,Yichang, 443002, Chinae-mail: zhtyang@163.com

1 Introduction

Element substitution and carrier doping were the mainmeans on enhancement of Tc and increasing the numberof superconductor members in the study of high-Tc super-conductors. After FeAs-based layered compounds were dis-covered as a new high-Tc superconductor system in 2008[1], F doping into SmFeAsO yields the first FeAs-basedsuperconductor whose Tc is higher than 40 K [2] and thensubsequently attracts wider attention and study, and the Tcof SmFeAsO1 − xFx system can get as high as 55 K [3]. Itis agreed that F doping may bring electron-like carriers inthe conducting layer or bring the shrinkage of the c-axis lat-tice distance, can suppress the phase transitions of the parentcompound SmFeAsO, and causes superconductivity [4, 6].

In spite of F doping into O site, Co doping into Fe site inthe LnFeAsO (Ln = rare earth element) compounds attractspeople’s interest too [7, 11], because it clearly displays thedifference between the new FeAs-based superconductor andthe traditional cuprate superconductor, since superconduc-tivity is always damaged by doping in the CuO2 planes incuprate superconductors.

Usually, the transition temperature of LnFeAsO with Codoping into FeAs layers is significantly lower than that ofLnFeAsO with F doping into LnO conductive layers, thoughthey are all electron-like charge carrier-inducing. The reasonis difficult to investigate and not clear to date yet. Severaleffects such as change of charge carrier density, shrinkageof lattice parameters, and disorder of Fe-As layers coex-ist and may influence the superconductive properties aloneor together by Co doping into LnFeAsO compounds [7, 8].This makes the mechanism of this system considerablyconfused. But, an interesting question will exist when the

J Supercond Nov Magn

charge carrier was induced simultaneously into FeAs layersand LnO layers.

In this paper, two doping methods were selected to inves-tigate the influence on the Tc of the co-doping. One is F dop-ing into SmFe0.90Co0.10AsO with the optimum Co concen-tration (x = 0.1) and highest Tc among SmFe1− xCoxAsOfrom our previous works. The other is Co doping intoSmFeAsO1 − xFx with the optimum F content x = 0.2 andthe under-doping content x = 0.1. The results may give usmore in-depth understanding on the mechanism of the newFe-As-based superconductor.

2 Experimental and Analysis Methods

We prepared the samples by a mechanical millingand subsequent sintering method. Firstly, the nominalSmFe0.9Co0.1AsO was synthesized. The stoichiometricamounts of SmAs, Co2O3, Fe, and Fe2O3 were well groundin an agate mortar and then loaded into a cylindrical vialof 10 cm in diameter together with a batch of activationballs of 5, 8, and 11 mm in diameters (ratio 5:7:15). Theweight ratio between balls and powder was 8:1. The vialwas then filled with purified argon gas and sealed, to avoidatmospheric contamination. Mechanical activation was car-ried out using a high-energy shaker mill (GN-2, ShenyangXinke Instrument & Equipment Co., Ltd., China) operatedat 480 rpm for 4 h. Secondly, repeating the above pro-cess, nominal SmFe0.9Co0.1AsO0.5F0.5 was prepared withthe stoichiometric SmOF, SmAs, Fe2As, and Co2As byhigh-energy mechanical milling too. SmOF was obtainedby the reaction of Sm2O3 and NH4F at 1,273 K for 1.5 h.SmAs, Fe2As, and Co2As powder was obtained by slowlyreacting Sm, Fe, Co powder, and As pieces at 900 Cfor 10 h, respectively. Thirdly, the SmFe0.9Co0.1AsO andSmFe0.9Co0.1AsO0.5F0.5 were mixed at different propor-tions to get series samples of SmFe0.9Co0.1AsO1− xFx (x =0 ˜0.25), and then, the resulting compositions were pressedinto pellets of 10 mm in diameter and 2 mm in thicknessunder a uniaxial pressure of 10 MPa, sealed in evacuatedsilica tubes, and then annealed at a temperature of 1,160 Cfor 3 h.

A series of SmFe1− xCoxAsO0.8F0.2 (x = 0 ∼ 0.25)was prepared by mixed SmFeAsO0.8F0.2 and SmFe0.67

Co0.33AsO0.8F0.2 at different proportions. The latter twocompounds were synthesized by mechanical milling in asimilar process described above. It is noted that all the pro-cess was performed in a glove box under high-purity argonexcept milling and annealing.

Phase identifications and crystal structure investigationswere carried out on a powder X-ray diffractometer (RigakuD/Max 2200/PC) at 40 kV and 40 mA (Cu-Kα radiation).The electrical resistivity and Hall effect were measured by

the standard four-probe and five-probe technique, respec-tively, with silver paint contacts down to 4 K in a PhysicalProperty Measurement System (PPMS; Quantum DesignCompany).

3 Results and Discussions

Figure 1 shows the X-ray diffraction (XRD) patterns of theseries of SmFe0.9Co0.1AsO1− xFx and SmFe1− xCoxAsO0.8F0.2 samples resulting from MA and subsequent sin-tering processes. All main peaks can be well indexed basedon the ZrCuSiAs tetragonal structure with P4/nmm spacegroup. Peaks connected with impurity phase SmOF werealso observed in the XRD patterns, as marked by an arrow.In the series of SmFe0.9Co0.1AsO1− xFx , it is clear that theintensity of the impurity increases with increasing x, but inseries of SmFe1− xCoxAsO0.8F0.2, the intensity of impuritykeeps abroad unchanged. This indicated that the amount ofSmOF mainly depends on the F doping content.

Figure 2 shows the temperature dependence of resistiv-ity for SmFe0.9Co0.1AsO1− xFx samples. From the figure,the Tc of SmFe0.9Co0.1AsO1− xFx increases with increas-ing F doping content. For x = 0.15, the Tc is 23 K. This isthe highest Tc in this SmFe0.9Co0.1AsO1− xFx system. Forx = 0.20, the Tc does not obviously enhance longer. Thetransition range gets wider. This indicated that more impu-rities were in the sample which agreed with the XRDresult.

Figure 3 shows the temperature dependence of resis-tivity for SmFe1− xCoxAsO0.8F0.2 samples. The 20 % Fdoping content is the optimum in F-doping SmFeAsO sys-tem. It is easy to find that the Tc decreases monotonouslywith the increase of Co doping content. This variationof Tc is much different from the series samples ofSmFe0.9Co0.1AsO1− xFx . As we know, after the chargecarrier density of the SmFeAsO1− xFx reaches a specificvalue, the Tc cannot rise continuously with the increaseof the charge carrier density. In this case, the monotonousdecrease of Tc indicated that the other effects induced byCo doping on enhancement of Tc are negative.

We also prepared a series of Co doping intoSmFeAsO0.90F0.10. The 10 %F doping is in the under-doping region. The Tc of these SmFe1− xCoxAsO0.90F0.10

samples is showed in Fig. 4. For x = 0.05 and x = 0.10,the Tc was enhanced from 15 to 19 K. For x = 0.15, the Tcdecreased. For x = 0.20, the superconductivity of the sam-ple disappears. This result is similar to single site-dopingSmFe1− xCoxAsO. The difference between these two sys-tems is that the highest Tc of SmFe1− xCoxAsO0.90F0.10

(19 K) is higher than that of SmFe1− xCoxAsO.The Hall coefficients of SmFeAsO, SmFe0.9Co0.1AsO,

SmFeAsO0.80F0.20, and SmFe0.9Co0.1AsO0.85F0.15 were

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Fig. 1 Powder XRD patterns ofSmFe0.9Co0.1AsO1− xFx andSmFe1− xCoxAsO0.8F0.2samples

10 20 30 40 50 60 70 80

2 (degree)

Inte

nsit

y (

a.u

.)

SmFe0.9Co0.1AsO1-xFx

x=0.20

x=0.15

x=0.10

x=0.05

x=0

10 20 30 40 50 60 70 80

2 (degree)

Inte

nsit

y (

a.u

.)

SmFe1-xCoxAsO0.8F0.2

x=0.20

x=0.15

x=0.10

x=0.05

x=0

measured, and the results are shown in Fig. 5. The leftinset illustrates the comparison of SmFe0.9Co0.1AsO andSmFe0.9Co0.1AsO0.85F0.15. The underside insets show theHall resistivity of SmFeAsO and SmFe0.9Co0.1AsO asrepresentatives of all samples. The Hall coefficients RH

were obtained from the slopes of curves of Hall resisti-vity from the equation RH = ρxy /H . Both Hall coeffi-cients of these samples are negative, indicating the electron-like charge carrier in them. The Hall coefficient of theparent compound SmFeAsO showing a pronounced riseat 150 K, which may coincide with the SDW anomalypeak, has been observed too and discussed in the workof Liu et al. [13]. But, this pronounced rise disappearsin the other three curves of Hall coefficients since theSDW transition has been suppressed by doping. In thewhole temperature range, the RH values have only slightly

fluctuated. So, we can use the single-band model to esti-mate the charge carrier density. From the figure, the Hallcoefficient value of SmFe0.9Co0.1AsO0.85F0.15 is small thanthat of SmFe0.9Co0.1AsO. Using the single-band equationn = 1/RHe, we can judge that the charge carrier den-sity of SmFe0.9Co0.1AsO0.85F0.15 is larger than that ofSmFe0.9Co0.1AsO.

In the Co-doping SmFeAsO series, the highest Tc canonly reach 17.2 K, reported by just one group [7]. Thegeneral results are just about 12–15 K from most groups.The highest Tc is much lower than the case of F dop-ing into SmFeAsO, since these two dopings are all elec-tron doping. The reason is not clear yet; someone esti-mated that it was the disorder induced by Co doping intoFe-As layers, but this conclusion lacks direct evidence.From our experiments, when the Co doping content was

Fig. 2 Temperature dependenceof resistivity forSmFe0.9Co0.1AsO1− xFx

samples

0 10 20 30 40 50 60 70 80

-0.001

0.000

0.001

0.002

0.003

0.004

0.005

0.006

SmFe0.90Co0.10AsO1-xFx

Temperature(K)

Re

sis

tiv

ity

(O

hm

-m)

x=0.00

x=0.05

x=0.10

x=0.15

x=0.20

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Fig. 3 Temperature dependenceof resistivity forSmFe1− xCoxAsO0.80F0.20samples

0 20 40 60 80

0.000

0.001

0.002

0.003

SmFe1-xCoxAsO0.80F0.20

Re

sis

tiv

ity

(O

hm

-m)

Temperature (K)

x=0.00

x=0.05

x=0.10

x=0.15

x=0.20

10 %, the SmFe0.9Co0.1AsO sample gets the highest Tc,12.5 K [14], similar to previous results. In this compounddoped by Co, when some O atoms were substituted byF, the Tc was enhanced obviously. From the Hall coef-ficient measure, it can be seen that the charge carrierdensity of SmFe0.9Co0.1AsO0.85F0.15 is larger than that ofSmFe0.9Co0.1AsO. That is to say, increasing the charge car-rier density of the SmFe0.9Co0.1AsO by doping F into Osite can enhance its Tc. But in the previous experiments ofSmFe1− xCoxAsO, when Co doping content was more than10 %, the charge carrier density of the SmFe1− xCoxAsO

can be increased too, but the Tc cannot be higher. So, wecan judge that there must be other negative effects on Tcby more Co substitution. They may be a disorder of Fe-Aslayers or a magnetic property of Co. It is noted that whenx >0.20, the Tc did not increase no longer, which was prob-ably the result of the absence of F doping into compounds asa matter of fact, since the impurity of SmOF increases withthe increase of nominal F doping content, from the result ofXRD patterns.

The result displayed in Fig. 4 is not difficult to under-stand. For SmFeAsO0.90F0.10, the charge carrier density

Fig. 4 Temperature dependenceof resistivity forSmFe1− xCoxAsO0.90F0.10samples

0 20 40 60

0.0

1.0x10-5

2.0x10-5

3.0x10-5

4.0x10-5

5.0x10-5

SmFe1-xCoxAsO0.90F0.10

Temperature (K)

Res

isti

vity

(oh

m-m

)

x=0.05

x=0.10

x=0.15

x=0.20

x=0.25

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0 50 100 150 200 250 300

-50

-40

-30

-20

-10

0

0 50 100 150 200 250 300

-8

-6

-4

-2

0

2

Temperature (K)

RH( 1

0-2

cm3 /C

)

0 2 4 6 8 10 12

-40

-30

-20

-10

0

xy(

cm

)

Magnetic field (T)

275

250

225

200

175

150

125

100

75

50

25

0 2 4 6 8 10 12

-450

-400

-350

-300

-250

-200

-150

-100

-50

0

50

xy(

cm

)

Magnetic field (T)

300

275

250

225

200

175

150

125

100

75

50

25

SmFeAsO

Temperature (K)

RH( 1

0-2cm

3 /C) SmFe0.90Co0.10AsO

SmFe0.90Co0.10AsO0.85F0.15

SmFeAsO0.80F0.20

SmFeAsO

Fig. 5 Temperature dependence of the Hall coefficients forSmFeAsO, SmFe0.9Co0.1AsO, SmFeAsO0.80F0.20, and SmFe0.9Co0.1AsO0.85F0.15. The underside insets show the Hall resistivity ofSmFeAsO and SmFe0.9Co0.1AsO

does not reach saturation value. So, the augmentation ofcharge carrier density can also enhance the Tc by low con-tent of Co doping, but more content of Co doping may bringlarger negative effects which may decrease the value of Tc.It is also to say that under the 15 % Co doping content,two methods such as substitution of Fe by Co or substitu-tion of O by F for increasing the charge carrier density mayenhance the Tc. When the Co doping concentration is largerthan 15 %, the influence of charge carrier density on the Tcthen is not the main reason.

4 Conclusions

From above results and discussions, we can draw someconclusions as follows:

1. For the optimum content of Co doping, further sub-stitution of O by F can enhance the Tc; the reason isthe increase of the charge carrier density in the com-pounds. But when the Co doping content exceeds 15 %,its negative effect is dominant.

2. When the charge carrier density reaches a certain valueby means of F doping, slight Co doping may lead theTc decrease dramatically. This result offers a direct evi-dence of negative effect on Tc induced by Co doping,excluding the effect of introducing charge carriers.

3. When the charge carrier density does not reach satu-ration value in SmFeAsO0.90F0.10, slight Co doping can

also enhance the Tc, but the highest Tc of SmFe1− x

CoxAsO0.90F0.10 is lower than that of SmFe1− xCoxAsO0.80F0.20 (the optimum F doping concentration).

Acknowledgments This work is supported by the NationalNatural Science Foundation of China (grant no. 11247020), NaturalScience Foundation of Hubei Province (no. 2012FFB03704), Scienceand Technology Research and Development Project of Yichang (no.A2012-302-27), and Science Foundation of China Three GorgesUniversity (no. KJ2009B011).

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