Possible superconductivity above 25K in single-crystalline Co-doped BaFe2As2

Article abstract of DOI:10.1143/JPSJ.78.023702

We present the superconducting properties of single-crystalline Ba(Fe _0:9Co_0:1)₂As₂ by measuring its magnetization, resistivity, upper critical field, Hall coefficient, and magneto-optical images. The magnetizat

Full text of DOI:10.1143/JPSJ.78.023702

Journal of the Physical Society of Japan
Vol. 78, No. 2, February, 2009, 023702
#2009 The Physical Society of Japan
LETTERS
Possible Superconductivity above 25 K in Single-Crystalline Co-Doped BaFe2As2
ꢀ
1;2
1
1;2
Yasuyuki NAKAJIMA , Toshihiro TAEN , and Tsuyoshi TAMEGAI
1Department of Applied Physiscs, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
2JST, Transformative Research-Project on Iron Pnictides (TRIP), 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
(Received November 17, 2008; accepted November 26, 2008; published January 26, 2009)
We present the superconducting properties of single-crystalline Ba(Fe0:9Co0:1)2As2 by measuring
its magnetization, resistivity, upper critical field, Hall coefficient, and magneto-optical images. The
magnetization measurements reveal a fish-tail hysteresis loop at high temperatures and a relatively high
critical current density above Jc ¼ 105 A/cm2 at low temperatures. The upper critical field determined by
resistive transition is anisotropic with an anisotropic parameter of ꢁ3:5. Hall effect measurements
indicate that Ba(Fe0:9Co0:1)2As2 is a multiband system and that the mobility of electrons is dominant.
Magneto-optical imaging reveals a prominent Bean-like penetration of vortices, although there is a slight
inhomogeneity in a sample. Moreover, we observe distinct superconductivity above 25 K, which leads us
to speculate that a higher transition temperature can be realized by fine-tuning the Co-doping level.
KEYWORDS: iron arsenide, magneto-optical imaging, critical current density, Hall coefficient, upper critical field
DOI: 10.1143/JPSJ.78.023702
Since the discovery of the high-Tc iron-based oxypnictide of As pieces and Fe/Co powder in a silica tube and reacting
superconductor LaFeAsO1ꢂxFx with Tc ꢁ 26 K,1) other iron- them at 1065 ꢃC for 10 h after heating at 700 ꢃC for 6 h. A
based superconductors have been sought for to obtain a mixture with a ratio of Ba : FeAs : CoAs ¼ 1 : 4:5 : 0:5 was
higher transition temperature. In rare-earth-substituted iron placed in an alumina crucible with quartz fiber as a cup. The
oxypnictides RFeAsO1ꢂxFx (R = rare earth), transition whole assembly was sealed in a large silica tube, and heated
temperature has been increased up to 55 K.2) In these iron up to 1150 ꢃC for 10 h followed by slow cooling down to
oxypnictides, superconductivity is induced by introducing 1090 ꢃC at a rate of ꢁ1:3 ꢃC/h, after which the silica tube
electrons in the (FeAs)ꢂ layers by substituting F for O. was put in a centrifuge to separate crystals from flux. The
Following these discoveries, oxygen-free iron-arsenide typical size of the resulting crystals is ꢁ2 ꢄ 2 ꢄ 0:05 mm3.
AFe2As2 (A ¼ Ba, Sr, Ca) was discovered. These materials Magnetization was measured by a commercial SQUID
show superconductivity with the substitution of alkali magnetometer (Quantum Design MPMS-XL5). Resistivity
metals, such as Na, K, and Cs, for A resulting in the measurement was performed by a four-contact method and
introduction of holes in the (FeAs)ꢂ layers.3–6) In hole-doped Hall coefficient measurement was conducted using the
oxygen-free iron arsenides, transition temperature is increas- standard six-wire configuration. Hall voltage was obtained
ed to ꢁ38 K.3,4) On the other hand, very recent studies from the antisymmetric part of transverse voltage by
revealed that the electron doping by the substitution of Co subtracting the positive and negative magnetic field data.
or Ni, which have one or two excess d electrons compared Magneto-optical images were obtained using the local-field-
with Fe in conducting layers, induces superconductivity dependent Faraday effect in an in-plane magnetized garnet
in oxygen-free iron arsenide.7,8) In fact, nuclear magnetic indicator film and employing a differential method.12,13)
resonance measurements have revealed that Co atoms
Figure 1 shows the temperature dependence of zero-field-
donate electrons without generating localized moments.9) cooled (ZFC) and field-cooled (FC) magnetizations at 5 Oe
Although the highest transition temperature in electron- along c-axis. A very sharp transition starting at Tc ꢁ 24 K is
doped BaFe2As2 is reported to be ꢁ23 K,10) which is lower observed. It should be noted that Tc is the highest among
than that in hole-doped BaFe2As2, the fact that the those that have been reported to date.7,9,10,14–17)
substitution of transition metals leads to the induction of
Figure 2(a) depicts the magnetization at several temper-
superconductivity contrasts strongly with a drastic suppres- atures as a function of field. At high temperatures above
sion of Tc in cuprates.11) However, detailed study of 15 K, fish-tail magnetization is observed, which is very
transition temperature as a function of Co-doping level is similar to that in YBa2Cu3O7ꢂꢀ single crystals.18) From the
limited.10) It is an open question whether the highest Tc in magnetization hysteresis loop, we can obtain the critical
electron-doped BaFe2As2 can be increased further. current density Jc using the Bean model with the assumption
In this study, we prepare a single-crystalline sample of of field-independent Jc. According to the Bean model, Jc is
Ba(Fe0:9Co0:1)2As2 and present its superconducting proper- given by
ties by studying its magnetization, resistivity, upper critical
ꢀM
Jc ¼ 20
;
ð1Þ
field, Hall coefficient, and magneto-optical images. We
address the possibility of further enhancing transition
temperature by fine-tuning Co-doping level.
að1 ꢂ a=3bÞ
where ꢀM is Mdown ꢂ Mup, Mup and Mdown are the magnet-
Single-crystalline samples with a nominal composition of ization when sweeping fields up and down, respectively, and
Ba(Fe0:9Co0:1)2As2 were grown by the FeAs/CoAs self-flux a and b are the sample widths with a < b. Figure 2(b) shows
method. FeAs and CoAs were prepared by placing mixtures the field dependence of Jc obtained from the data shown in
Fig. 2(a) using eq. (1) and the effective sample dimensions
ꢀE-mail: yasuyuki@ap.t.u-tokyo.ac.jp
with a ꢁ 0:55 mm and b ꢁ 0:63 mm. At low temperatures,
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J. Phys. Soc. Jpn., Vol. 78, No. 2
LETTERS
Y. NAKAJIMA et al.
400
300
200
100
0
(a)BaFe1.8Co0.2As2
0
I || ab
FC
-20
I || ab
H || c
150
100
50
-40
π
ρμ
ρμ
-60
5 T
0 T
Ba(Fe0.9Co0.1)2As2
H = 5 Oe
H || c
0
15
20
25
30
-80
ZFC
T (K)
0
50
100
150
200
250
300
0
5
10
15
20
25
30
35
T (K)
T (K)
70
60
50
40
30
20
10
0
Fig. 1. (Color online) Temperature dependence of the zero-field-cooled
(ZFC) and field-cooled (FC) magnetizations at H ¼ 5 Oe along c-axis in
Ba(Fe0:9Co0:1)2As2.
5.0
(b)
BaFe1.8Co0.2As2
4.0
3.0
2.0
γ
c
γ
= Hc2ab/Hc2
23.0
23.5
T (K)
(a)
5
BaFe1.8Co0.2As2
H || ab
H || c
0
21
22
23
24
25
π
T (K)
5 K
7.5 K
10 K
-5
Fig. 3. (Color online) (a) Temperature dependence of zero-field resistiv-
ity in Ba(Fe0:9Co0:1)2As2. Inset: low-temperature in-plane resistivity data
at H ¼ 0, 10, 20, 30, 40, and 50 kOe along c-axis. (b) Temperature
12.5 K
15 K
17.5 K
Ba(Fe0.9Co0.1)2As2
20 K
dependence of upper critical field along ab-plane ( ) and c-axis (
)
-50
-25
0
25
50
obtained using the midpoint of resistive transition in Ba(Fe0:9Co0:1)2As2.
Dashed lines are linear fits to the data. Inset: temperature dependence of
anisotropy of upper critical field along ab- and c-directions ꢂ ꢅ Hca2b=Hcc2
H (kOe)
.
106
(b)
transition is suppressed by Co doping. The residual
resistivity ratio ꢁð300 KÞ=ꢁðTcÞ is ꢁ2:6, which is comparable
to that reported before.7,10,15,17) The inset of Fig. 3(a) depicts
low-temperature in-plane resistivity data at H ¼ 0, 10, 20,
30, 40, and 50 kOe along the c-axis. With increasing field, Tc
decreases and transition width is broadened only slightly.
We plot the upper critical field Hc2 along the ab- and
c-directions determined by the midpoint of resistive tran-
sition as a function of temperature in Fig. 3(b). The slopes
of Hc2 along the ab- and c-directions at Tc are ꢂ52:1
and ꢂ16:8 kOe/K, respectively. From the Werthamer–
Helfand–Hohenberg theory,21) which describes orbital
depairing field of conventional dirty type-II superconduc-
tors, we can obtain the values of Hc2ð0Þ ¼ 0:69TcjdHc2=
105
104
103
5 K
7.5 K
10 K
12.5 K
15 K
17.5 K
20 K
Ba(Fe0.9Co0.1)2As2
10 20
H (kOe)
0
30
40
50
Fig. 2. (Color online) (a) Field dependence of magnetization in Ba(Fe0:9
-
Co0:1)2As2 at 5, 7.5, 10, 12.5, 15, 17.5, and 20 K. (b) Field dependence of
critical current density obtained from the data shown in Fig. 2(a) in
Ba(Fe0:9Co0:1)2As2 at 5, 7.5, 10, 12.5, 15, 17.5, and 20 K.
dTjT¼T ꢁ 850 and 280 kOe along the ab- and c-directions,
c
respectively. These values are very high and comparable to
that of hole-doped iron-arsenide superconductor Ba1ꢂxKx-
Jc is larger than 105 A/cm2. This value is much larger than Fe2As2.22) The inset of Fig. 3(b) shows the anisotropy of the
that reported in ref. 7 and about twice as large as those upper critical field ꢂ ꢅ H =Hcc2. ꢂ is ꢁ3{4 and slightly
ab
c2
reported in refs. 14 and 15. Although the present Jc is about increases with decreasing temperature. This value is similar
one order of magnitude smaller than the typical value for to that of hole-doped iron arsenide.23)
YBa2Cu3O7ꢂꢀ single crystals,19) it is well within the range
for practical applications.
Hall effect measurements strongly support that electrons
are introduced by Co doping and that Ba(Fe0:9Co0:1)2As2 is a
Figure 3(a) shows the temperature dependence of in-plane multiband system. Figure 4(a) shows the Hall resistivity ꢁxy
zero-field resistivity in Ba(Fe0:9Co0:1)2As2. With decreasing at several temperatures as a function of field. In the present
temperature from 300 K, resistivity decreases monotonically temperature region, ꢁxy is negative and shows an H-linear
and then drops suddenly at Tc. There is no anomaly dependence. We plot the Hall coefficient RH in Ba(Fe0:9
-
accompanied by a spin-density-wave transition reported in Co0:1)2As2 obtained from ꢁxy as a function of temperature in
the parent material BaFe2As2,20) which indicates that the Fig. 4(b). The sign of RH is negative in the whole temper-
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J. Phys. Soc. Jpn., Vol. 78, No. 2
LETTERS
Y. NAKAJIMA et al.
0.0
200μm
(a) 7.8 K
(c) 17.5 K
(e) 22.5 K
(b) 15 K
(d) 20 K
(f) 25 K
(a)
-0.5
26 K
55 K
85 K
130 K
-1.0
170 K
210 K
240 K
ρμ
300 K
Ba(Fe0.9Co0.1)2As2
I || ab
-1.5
-2.0
H || c
0
10
20
30
40
50
H (kOe)
0
-1
-2
-3
-4
-5
(b)
Ba(Fe0.9Co0.1)2As2
I || ab
H || c
Fig. 5. Magneto-optical images of remanent state after applying H ¼
690 Oe in Ba(Fe0:9Co0:1)2As2 at (a) 7.8, (b) 15, (c) 17.5, (d) 20, (e) 22.5,
and (f) 25 K.
0
50 100 150 200 250 300
T (K)
Fig. 4. (Color online) (a) Hall resistivity ꢁxy as a function of field at
several temperatures. (b) Temperature dependence of the Hall coefficient
in Ba(Fe0:9Co0:1)2As2.
plane domains of the garnet film and have nothing to do with
the flux distribution. At lower temperatures, vortices that
penetrated from the edges of the crystal cannot reach the
ature range, while that in the hole-doped BaFe2As2 is center owing to the large shielding current. However,
positive,24) which is consistent with the introduction of prominent penetrations of vortices are observed near the
electrons by Co substitution for Fe. RH decreases with defects close to the right edge of the sample. The penetration
decreasing temperature from 300 to ꢁ60 K and slightly of vortices develops with increasing temperature. It should
increases after showing a broad minimum at ꢁ50 K. The be noted that the image at 25 K apparently shows existence
absolute value of RH just above Tc is three times larger than of superconductivity in the lower part of the sample. In fact,
that at 300 K. In a simple single-band system without strong zero-field resistivity starts to drop at ꢁ25 K, as shown in the
spin fluctuations, the Hall coefficient is written as, RH ¼ inset of Fig. 3(a). The highest transition temperature in
1=nq, where q is the charge of a carrier and n is the carrier Ba(Fe1ꢂxCox)2As2 reported so far is ꢁ23 K.10) The differ-
density. RH is almost T-independent. By contrast, the Hall ence can originate from the difference in Co concentration
coefficient in a multiband system, for instance, consisting and/or its distribution. It may be possible to obtain an even
of electron and hole bands, is given by RH ¼ ðnhꢃ2h ꢂ higher transition temperature in this system by fine-tuning
2
neꢃ2eÞ=½eðnhꢃh þ neꢃeÞ ꢆ, where nh (ne) is the density of the Co doping level.
holes (electrons) and ꢃh (ꢃe) is the mobility of holes
In summary, we have presented a systematic study of
(electrons). The Hall coefficient in a multiband system can magnetization, resistivity, upper critical field, Hall coeffi-
be temperature-dependent. The obtained results indicate that cient, and magneto-optical imaging in single-crystalline
Ba(Fe0:9Co0:1)2As2 is a multiband system similar to the Ba(Fe0:9Co0:1)2As2. The magnetization measurements reveal
25)
parent material BaFe2As2 and mobility of the electrons a fish-tail hysteresis loop at high temperatures and a rel-
produced by Co doping becomes dominant with decreasing atively high critical current density larger than 105 A/cm2.
temperature.
Figure 5 shows magneto-optical images of Ba(Fe0:9
The upper critical field obtained by resistive transition is
anisotropic and its anisotropy is about 3.5. Hall effect
-
Co0:1)2As2 in the remanent state at several temperatures measurements indicate that Ba(Fe0:9Co0:1)2As2 is a multi-
after cycling the field up to 690 Oe for 5 s. Although the band system and the mobility of electrons is dominant. The
sample for the magneto-optical experiment is different magneto-optical imaging reveals prominent Bean-like pen-
(0:58 ꢄ 0:45 ꢄ 0:03 mm3) from that for the magnetization etrations of vortices although there is slight inhomogeneity
measurement, both crystals show similar properties. The in a sample. We also find distinct superconductivity above
bright region in the figures corresponds to the area trapping 25 K, which leads us to expect that a higher transition
the vortices. Dark faint zig-zag features originate from in- temperature can be realized by fine-tuning Co-doping level.
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Y. NAKAJIMA et al.
15) R. Prozorov, N. Ni, M. A. Tanatar, V. G. Kogan, R. T. Gordon, C.
Martin, E. C. Blomberg, P. Prommapan, J. Q. Yan, S. L. Bud’ko, and
P. C. Canfield: arXiv:0810.1338.
1) Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono: J. Am. Chem.
Soc. 130 (2008) 3296.
2) Z. A. Ren, W. Lu, J. Yang, W. Yi, X. L. Shen, Z. C. Li, G. C. Che,
X. L. Dong, L. L. Sun, F. Zhou, and Z. X. Zhao: Chin. Phys. Lett. 25
(2008) 2215.
16) R. T. Gordon, N. Ni, C. Martin, M. A. Tanatar, M. D. Vannette,
H. Kim, G. Samolyuk, J. Schmalian, S. Nandi, A. Kreyssig, A. I.
Goldman, J. Q. Yan, S. L. Bud’ko, P. C. Canfield, and R. Prozorov:
arXiv:0810.2295.
3) M. Rotter, M. Tegel, and D. Johrendt: Phys. Rev. Lett. 101 (2008)
107006.
17) F. Ning, K. Ahilan, T. Imai, A. S. Sefat, R. Jin, M. A. McGuire, B. C.
Sales, and D. Mandrus: J. Phys. Soc. Jpn. 77 (2008) 103705.
4) G. F. Chen, Z. Li, G. Li, W. Z. Hu, J. Dong, X. D. Zhang, P. Zheng,
N. L. Wang, and J. L. Luo: Chin. Phys. Lett. 25 (2008) 3403.
5) K. Sasmal, B. Lv, B. Lorenz, A. Guloy, F. Chen, Y. Xue, and C. W.
Chu: Phys. Rev. Lett. 101 (2008) 107007.
18) M. Daumling, J. M. Seuntjens, and D. C. Larbalestier: Nature 346
¨
(1990) 332.
19) T. Tamegai, L. Krusin-Elbaum, L. Civale, P. Santhanam, M. J. Brady,
W. T. Masselink, F. Holtzberg, and C. Feild: Phys. Rev. B 45 (1992)
8201.
6) G. Wu, H. Chen, T. Wu, Y. L. Xie, Y. J. Yan, R. H. Liu, X. F. Wang,
J. J. Ying, and X. H. Chen: J. Phys.: Condens. Matter 20 (2008)
422201.
20) M. Rotter, M. Tegel, D. Johrendt, I. Schellenberg, W. Hermes, and
R. Pottgen: Phys. Rev. B 78 (2008) 020503R.
7) A. S. Sefat, R. Jin, M. A. McGuire, B. C. Sales, D. J. Singh, and D.
Mandrus: Phys. Rev. Lett. 101 (2008) 117004.
¨
21) N. R. Werthamer, E. Helfand, and P. C. Hohenberg: Phys. Rev. 147
(1966) 295.
8) L. J. Li, Q. B. Wang, Y. K. Luo, H. Chen, Q. Tao, Y. K. Li, X. Lin, M.
He, Z. W. Zhu, G. H. Cao, and Z. A. Xu: arXiv:0809.2009.
9) F. L. Ning, K. Ahilan, T. Imai, A. S. Sefat, R. Jin, M. A. McGuire,
B. C. Sales, and D. Mandrus: J. Phys. Soc. Jpn. 77 (2008) 103705.
10) N. Ni, M. E. Tillman, J.-Q. Yan, A. Kracher, S. T. Hannahs, S. L.
Bud’ko, and P. C. Canfield: arXiv:0811.1767.
22) M. Altarawneh, K. Collar, C. H. Mielke, N. Ni, S. L. Bud’ko, and P. C.
Canfield: arXiv:0807.4488.
23) N. Ni, S. L. Bud’ko, A. Kreyssig, S. Nandi, G. E. Rustan, A. I.
Goldman, S. Gupta, J. D. Corbett, A. Kracher, and P. C. Canfield:
Phys. Rev. B 78 (2008) 014507.
11) H. Fujishita and M. Sato: Solid State Commun. 72 (1989) 529.
12) A. Soibel, E. Zeldov, M. Rappaport, Y. Myasoedov, T. Tamegai, S.
Ooi, M. Konczykowski, and V. B. Geshkenbein: Nature 406 (2000)
282.
24) G. Wu, R. H. Liu, H. Chen, Y. J. Yan, T. Wu, Y. L. Xie, J. J. Ying,
X. F. Wang, D. F. Fang, and X. H. Chen: Europhys. Lett. 84 (2008)
27010.
25) C. Liu, G. D. Samolyuk, Y. Lee, N. Ni, T. Kondo, A. F. Santander-
Syro, S. L. Bud’ko, J. L. McChesney, E. Rotenberg, T. Valla, A. V.
Fedorov, P. C. Canfield, B. N. Harmon, and A. Kaminski: Phys. Rev.
Lett. 101 (2008) 177005.
13) M. Yasugaki, K. Itaka, M. Tokunaga, M. Kameda, and T. Tamegai:
Phys. Rev. B 65 (2002) 212502.
14) A. Yamamoto, J. Jaroszynski, C. Tarantini, L. Balicas, J. Jiang, A.
Gurevich, D. C. Larbalestier, R. Jin, A. S. Sefat, M. A. McGuire, B. C.
Sales, D. K. Christen, and D. Mandrus: arXiv:0810.0699.
023702-4