Superconducting properties of SmO1-xFxFeAs wires with Tc = 52 K prepared by

the powder-in-tube method

Zhaoshun Gao, Lei Wang, Yanpeng Qi, Dongliang Wang, Xianping Zhang, Yanwei Ma*

Key Laboratory of Applied Superconductivity, Institute of Electrical Engineering,

Chinese Academy of Sciences, P. O. Box 2703, Beijing 100190, China

Huan Yang, Haihu Wen

Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

Abstract:

We demonstrate that Ta sheathed SmO1-xFxFeAs wires were successfully

fabricated by the powder-in-tube (PIT) method for the first time. Structural analysis

by mean of x-ray diffraction shows that the main phase of SmO1-xFxFeAs was

obtained by this synthesis method. The transition temperature of the SmO0.65F0.35FeAs

wires was confirmed to be as high as 52 K. Based on magnetization measurements, it

is found that a globe current can flow on macroscopic sample dimensions with Jc of

~3.9×103 A/cm2 at 5 K and self field, while a high Jc about 2×105 A/cm2 is observed

within the grains, suggesting that a significant improvement in the globle Jc is

possible. It should be noted that the Jc exhibits a very weak field dependence behavior.

Furthermore, the upper critical fields (Hc2) determined according to the

Werthamer-Helfand-Hohenberg formula are (T= 0 K) ≈ 120 T, indicating a very

encouraging application of the new superconductors.

* Author to whom correspondence should be addressed; E-mail: ywma@mail.iee.ac.cn

1

The recent discovery of the iron based superconductor La(O1-xFx)FeAs with

critical temperature 26 K has triggered great interest as a new family of high

temperature superconductors [1]. Immediately, the Tc has been enhanced up to above

50 K by the substitution of La using other smaller rare earth elements [2-7]. The

present research indicates that the layered structure with the conducting Fe2As2 layers

should be responsible for high temperature superconductivity，and the Re2O2 ( Re=

rare earth ) layers behave as the charge reservoirs, all these look very similar to the

case of cuprates. On the other hand, several groups have studied the Hc2, the

irreversible magnetization and grain connectivity [8-15]. The resistive transition in

fields suggests that Hc2 is remarkably high, which indicates encouraging potential

applications. One crucial issue relevant for practical applications is the wire and tape

fabrication. In particular, iron-based layered superconductors are mechanically hard

and brittle and therefore not easy to drawing into the desired wire geometry. Recently,

we have reported that Fe sheathed LaO0.9F0.1FeAs wires with Ti as a buffer layer were

successfully fabricated by the powder-in-tube (PIT) method [16]. In this work, we

have synthesized tantalum-clad SmO01-xFxFeAs wires and investigated the

superconducting properties of SmO1-xFxFeAs wires.

The SmO1-xFxFeAs composite wires were prepared by the in situ powder-in-tube

(PIT) method using Sm, As, SmF3, Fe and Fe2O3 as starting materials. The raw

materials were thoroughly grounded by hand with a mortar and pestle. The mixed

powder was filled into a Ta tube of 8 mm outside diameter and 1 mm wall thickness.

It is note that the grinding and packing processes were carried out in glove box in

which high pure argon atmosphere is filled. After packing, the tube was rotary swaged

and then drawn to wires of 2.25 mm in diameter. The wires were cut into 4~6 cm and

sealed in a Fe tube. They are then annealed at 1160~1180 oC for 45 hours. The high

purity argon gas was allowed to flow into the furnace during the heat-treatment

process to reduce the oxidation of the samples.

The phase identification and crystal structure investigation were carried out

using x-ray diffraction (XRD). Standard four probe resistance and magnetic

measurements were carried out using a physical property measurement system

2

(PPMS). Microstructure was studied using a scanning electron microscopy

(SEM/EDX) after peeling away the Ta sheath.

Figure 1 shows the optical images for a typical transverse and longitudinal

cross-section of SmO1-xFxFeAs wires after heat treatment. A reaction layer with a

thickness 10~30 μm was formed between the superconducting core and Ta tube due to

the very high annealing temperature.

Figure 2 presents the XRD patterns of SmO1-xFxFeAs wires after peeling off the

sheath materials. As can be seen, the sample consists of a main superconducting phase,

but some impurity phases are also detected. Such impurity phases might be reduced

by optimizing the heating process and stoichiometry ratio of start materials. The

lattice parameters are a = 3.9310(5) Å, c = 8.522(1) Å for the sample with x=0.35, and

a = 3.9275(5) Å, c = 8.498(2) Å for the sample with x=0.3, which indicates a clear

shrinkage of lattice parameters by increasing substitution of F− for O2−.

Figure 3 shows the temperature dependence of resistivity at zero magnetic field

for the samples SmO1−xFxFeAs. With decreasing temperature, the electrical resistivity

of the sample with x=0.35 decreases monotonously and a rapid drop was observed

starting at about 52 K, indicating the onset of superconductivity. The sample with

x=0.30 shows the same behavior as that of the sample with x=0.35 except for a

slightly low onset Tc. The residual resistivity ratio RRR = ρ(300K)/ ρ(55 K) were 2.8

and 2.3 for sample x=0.35 and x=0.3, respectively, indicating stronger impurity

scattering than LaO1−xFxFeAs [17].

Figure 4 shows the SEM images illustrating the typical microstructure of the

fractured core layers for SmO0.7F0.3FeAs wires. It can be seen that the SmO0.7F0.3FeAs

has a density structure with few voids in Fig. 4(a). EDX analysis also revealed that the

sample possibly contains impurity phases such as SmAs and unreacted Fe or Fe2O3.

The crystal structure of this kind of materials is based on a stack of alternating SmO

and FeAs layers. A layer-by-layer growth pattern can be clearly seen as shown in Fig.

4(b), very similar to what has been observed in Bi-based cuprates.

The magnetization loops with a strong ferromagnetic background were observed

in our samples, which are believed to be contributed by the unreacted Fe or Fe2O3

3

impurity phases [10]. We calculated the global Jc for our samples on the basis of Jc =

20ΔM/a(1-a/3b), where ΔM is the height of the magnetization loop and a and b are

the dimensions of the sample perpendicular to the magnetic field, a < b. Intragrain Jc

was also evaluated on the basis of Jc =30ΔM/<R> through magnetic measurements

after grinding the superconducting core into powder. <R> is the average grain size,

which is about 10 μm determined by SEM. The Jc field dependence is shown in Fig.5.

It can be seen that the Jc based on the sample size is much lower than that what should

exist in individual grains. The Jc value at 5 K for the SmO0.65F0.35FeAs powder is

2×105 A/cm2 with a very weak dependence on field, which shows that the

SmO0.65F0.35FeAs has a fairly large pinning force in the grain. However, the global Jc

values of 3.9×103 A/cm2 at 5 K obtained for the Sm samples is significantly lower

than that seen in random bulks of MgB2 which generally attained 106 A/cm2 at 4.2 K

[18]. The main reason may be ascribed to the second impurity phases or the presence

of weak links between the grains. As supported by XRD and SEM/EDX results, major

impurity phases such as SmAs, SmOF, and unreacted Fe or Fe2O3 were observed in

our present samples. These macroscale impurity phases can significantly reduce

percolating current path and limit the globe Jc. Although the whole-sample current

densities are significantly lower than that in random bulks of MgB2, the grain

connectivity seems better than random polycrystalline cuprates [12]. It is believed that

significant improvement in the globe Jc is expected upon either optimization of

processing parameters or achieving high grain alignment in analogy to high Tc

Bi-based cuprates.

Figure 6 shows the resistive superconducting transitions for the

SmO0.65F0.35FeAs sample under magnetic fields. The large magnetoresistance at

normal state may be caused by magnetic impurity. It is found that the onset transition

temperature is not sensitive to magnetic field, but the zero resistance point shifts more

quickly to lower temperatures due to the weak links or flux flow. We tried to estimate

the upper critical field (Hc2) and irreversibility field (Hirr), using the 90% and 10%

points on the resistive transition curves. The inset of Fig. 6 shows the temperature

dependence of Hc2 and Hirr with magnetic fields up to 9 T for the SmO0.65F0.35FeAs

4

wires. It is clear that the curve of Hc2 (T) is very steep with a slope of – dHc2/dT|Tc =

2.74 T / K. For the type II superconductor in the dirty limit, Hc2 (0) is given by the

Werthamer-Helfand-Hohenberg formula [19], Hc2(0) = 0.693×(dHc2/dT)×Tc. Taking

Tc= 51 K, we get Hc2(0) ≈ 96.8 T. If adopting a criterion of 99 %ρn(T) instead of

90%ρn(T), the Hc2(0) value of this sample obtained by this equation is higher than 120

T. We can also see that the irreversibility field in the inset of Fig. 5 is rather high

comparing to that in MgBB2. These high values of Hc2 and Hirr indicate that this new

superconductor has an encouraging application in very high fields.

We have successfully prepared Ta clad SmO1-xFxFeAs wires by the PIT process.

The Jc of composite wires has a very weak dependence on magnetic field with a much

high Hc2(0) above 120 T. Although some problems such as phase impurity remain

unsolved, our preliminary results explicitly demonstrate that the feasibility of

fabricating iron based superconducting wires.

The authors thank Xiaohang Li, Chenggang Zhuang and Liangzhen Lin for their

help and useful discussion. We also thank Profs. Zizhao Gan, and Liye Xiao for the

great encouragement. This work is partially supported by the Beijing Municipal

Science and Technology Commission under Grant No. Z07000300700703, National

‘973’ Program (Grant No. 2006CB601004) and National ‘863’ Project (Grant No.

2006AA03Z203).

5

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7

Captions

Figure 1 Optical images for a typical transverse (a) and longitudinal (b)

cross-section of the wires after heat treatment.

Figure 2 XRD patterns of SmO1-xFxFeAs wires after peeling away the Ta sheath.

The impurity phases of SmAs and SmOF are marked by * and #,

respectively.

Figure 3 Temperature dependences of resistivity for SmO1-xFxFeAs wires after

peeling away the Ta sheath.

Figure 4 (a) Low magnification and (b) high magnification SEM micrographs for

the SmO0.7F0.3FeAs samples.

Figure 5 Magnetic field dependence of Jc at 5 K for the bar and powder of

SmO1-xFxFeAs wires.

Figure 6 Temperature dependence of resistivity under magnetic fields up to 9 T for

the SmO0.65F0.35FeAs wire. The inset shows the temperature dependence of

Hc2 and Hirr determined from 90% and 10% points on the resistive

transition curves.

8

Fig.1 Gao et al.

9

20 30 40 50 60 70

21211

4

104

200

112

111

110

102

101

SmO1-xFxFeAs

x=0.30

# SmOF* SmAs

Inte

nsity

(a.u

.)

2-Theta

# **

* x=0.35

Fig.2 Gao et al.

10

50 100 150 200 250 3000.0

0.5

1.0

1.5

2.0

2.5

T (K)

ρ (m

Ω c

m)

SmO1-xFxFeAs wires x=0.35 x=0.3

Fig.3 Gao et al.

11

Fig.4 Gao et al.

12

0 1 2 3102

103

104

105

106

4

J c (A

/cm

2 )

μ0H (T)

Powder x=0.35 Powder x=0.3 Bar x=0.35 Bar x=0.3

T= 5 K

Fig.5 Gao et al.

13

30 35 40 45 50 550.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

40 45 50 550

2

4

6

8

10

μ0 H

(T)

Hc2

Hirr

T (K)ρ (m

Ω c

m)

T (K)

Fig.6 Gao et al.

14