Doping dependence of magnetic and transport properties in single crystalline Co-doped BaFe2As2

Article abstract of DOI:10.1016/j.physc.2009.10.074

We report the doping dependence of transport and magnetic properties in Co-doped BaFe₂As₂. With increasing Co concentration x, structural and magnetic transitions are suppressed and superconductivity emerges in the range of 0.3c at low temperatures and low fields obtained from bulk magnetization is reasonably large and the doping dependence shows a maximum at x～0.07 similar to Tc. The values of J_c at low temperatures reach about 1×106A/cm2 around the optimally doped region, which is potentially attractive for technological applications.

Full text of DOI:10.1016/j.physc.2009.10.074

Physica C 470 (2010) S408–S410
Contents lists available at ScienceDirect
Physica C
journal homepage: www.elsevier.com/locate/physc
Doping dependence of magnetic and transport properties in single crystalline
Co-doped BaFe As
2
2
a,b,*
a
a,b
Yasuyuki Nakajima
, Toshihiro Taen , Tsuyoshi Tamegai
a
Department of Applied Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
JST, Transformative Research-Project on Iron Pnictides (TRIP), 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Accepted 24 October 2009
Available online 30 October 2009
We report the doping dependence of transport and magnetic properties in Co-doped BaFe As . With
2
2
increasing Co concentration x, structural and magnetic transitions are suppressed and superconductivity
emerges in the range of 0:3 < x < 0:15. T-linear resistivity is observed at the optimally doped composi-
tion x ¼ 0:075 and the temperature exponent of the resistivity increases with x. Critical current density
Keywords:
Iron pnictides
Transport properties
Magnetic properties
J at low temperatures and low fields obtained from bulk magnetization is reasonably large and the dop-
c
ing dependence shows a maximum at x ꢀ 0:07 similar to T
c
. The values of Jc at low temperatures reach
about 1 ꢁ 10 A=cm2 around the optimally doped region, which is potentially attractive for technological
6
applications.
Ó 2009 Published by Elsevier B.V.
1
. Introduction
2. Experiments
Since the discovery of the high-T
c
iron-based oxypnictide
Single-crystalline samples of BaðFe1ꢂxCo
x
Þ As
2
were grown by the
2
superconductor LaFeAsO1ꢂx
superconductors have attracted great interest. Following this dis-
covery, oxygen-free iron-arsenide AFe As
F
x
with T
c
ꢀ 26 K [1], other iron-based
FeAs/CoAs self-flux method. FeAs and CoAs were prepared by placing
mixtures of As pieces and Fe/Co powder in a silica tube and reacting
them at 1065 °C for 10 h after heating at 700 °C for 6 h. A mixture
with a ratio of Ba:FeAs/CoAs = 1:5 was placed in an alumina crucible.
The whole assembly was sealed in a large silica tube, and heated up
to 1150 °C for 10 h followed by slow cooling down to 800 °C at a rate
of 5 °C/h, which is slightly different from the synthesis reported be-
fore [5]. After cleaving, we can obtain shiny samples. The typical
2
2
ðA ¼ Ba; Sr; CaÞ was dis-
covered. These materials show superconductivity with the
substitution of alkali metals, such as Na, K, and Cs, for A resulting
ꢂ
in the introduction of holes in the ðFeAsÞ layers [2], or Co for Fe
resulting in the introduction of electrons [3]. Among them, Co-
doped BaFe
conductivity induced by the substitution in the conducting layers
strongly compares well with a drastic suppression of T in cuprates
4] and may suggest a possibility of novel superconducting state.
We report here the doping dependence of transport and magnetic
properties in Co-doped BaFe As in order to investigate normal and
2 2
As has attracted much attention because the super-
3
dimensions of the resulting crystals is 4 ꢁ 4 ꢁ 0:1 mm . Co concen-
c
trations were determined by energy dispersive X-ray spectroscopy
measurements. Magnetization was measured by a commercial
SQUID magnetometer (Quantum Design MPMS–XL5) and resistivity
measurement was performed by a four-contact method.
[
2
2
superconducting properties. With increasing Co concentration x,
structural and magnetic transitions are suppressed and supercon-
ductivity occurs in the range of 0:03 < x < 0:15. T-linear resistivity
is observed at the optimally doped composition near x ¼ 0:075 and
3. Results and discussion
2
temperature exponent of resistivity crosses over to T in the over-
Fig. 1 shows the temperature dependence of in-plane resistivity
doped region. Critical current density Jc at low temperatures and
low field obtained from bulk magnetization is relatively large
qab for BaðFe Co Þ As . For BaFe As ; x ¼ 0, the sharp decrease of
1ꢂx
x
2
2
2
2
qab associated with the structural and magnetic transition [2,6] is
observed at 134 K. With increasing x, temperature of the resistive
anomaly is suppressed monotonically. For x P 0:038, supercon-
c
c
and shows a doping dependence similar to T . The values of J reach
6
2
about 1 ꢁ 10 A=cm at 2 K around the optimally doped region.
ductivity is observed and disappear at x J 0:15. At x ¼ 0:075; T
shows a maximum of ꢀ24 K and very sharp transition within
< 1 K is observed in the resistivity and magnetization as
c
*
Corresponding author. Address: Department of Applied Physics, The University
DT
c
of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Tel.: +81 3 5841 6848;
fax: +81 3 5841 6848.
shown in the right inset of Fig. 1. We find T-linear resistivity at
the optimally doped composition x ¼ 0:075 and the temperature
E-mail address: yasuyuki@ap.t.u-tokyo.ac.jp (Y. Nakajima).
0
921-4534/$ - see front matter Ó 2009 Published by Elsevier B.V.
doi:10.1016/j.physc.2009.10.074

Y. Nakajima et al. / Physica C 470 (2010) S408–S410
S409
1
1
50
00
150
100
50
by Refs. [7,9]. The Left inset of Fig. 1 shows the phase diagram of
7
6
5
4
3
2
1
00
00
00
00
00
00
00
0
0
FC
Ts
BaðFe1ꢂxCo
Þ As
obtained from the transport and magnetization
and
x
2
2
-
20
measurements, where structural transition temperature T
s
-40
-60
TSDW
spin-density wave transition temperature TSDW were determined
by the temperature derivative of resistive anomaly and T is deter-
5
0
Tc
ZFC
c
-
80
0
1
0
20
T (K)
30
0
mined by the onset of diamagnetism, which is consistent with the
previous study reported in Ref. [8].
0.00
0.05
0.10
0.15
x = 0
x
0.023
The inset of Fig. 2 shows the field dependence of magnetization
for x ¼ 0:045; 0:06; 0:075; and 0:113 at several temperatures. For
x ¼ 0:045 and 0.113, corresponding to under- and over-doped re-
gion, respectively, a maximum at zero field is very sharp. By con-
trast, for x ¼ 0:06 and 0.075 near the optimally doped region, a
peak at zero field is broadened and the irreversible magnetization
is much larger than those for x ¼ 0:045 and 0.113. For x ¼ 0:075,
prominent fish-tail magnetization is observed at 15 K [5] while
for x ¼ 0:045; 0:06, and 0.113, the fish-tail effect is not observed
in the present temperature and field range.
0
.038
.06
0
0.075
0.113
0.15
Ba(Fe1-xCo ) As2
x 2
0
50
100
150
200
250
300
T (K)
Fig. 2 shows the field dependence of critical current density Jc
Fig. 1. Temperature dependence of the in-plane resistivity
qab
for
; x ¼ 0; 0:023; 0:038; 0:06; 0:075; 0:113, and 0.15. The right
and zero-field-cooled (ZFC) and
for x ¼ 0:045; 0:06; 0:075, and 0.113. J is obtained from hysteresis
c
BaðFe1ꢂxCo
x
Þ As
2
2
loop in the magnetization shown in each inset using the Bean
model,
inset shows the temperature dependence of
q
ab
field-cooled (FC) magnetization for x ¼ 0:075. The left inset shows the phase
diagram for BaðFe1ꢂxCo
x
Þ As
2 s
. T and TSDW were by the temperature derivative of
is determined by the onset of diamagnetism. Dashed lines
2
D
M
resistive anomaly and T
c
Jc ¼ 20
;
ð1Þ
are guides for the eye.
að1 ꢂ a=3bÞ
where
D
M is Mdown ꢂ Mup; Mup and Mdown are the magnetization
2
exponent of the resistivity increases with x. At x ¼ 0:15; T -behav-
ior is observed below 40 K, which indicates the non-Fermi-liquid to
Fermi-liquid crossover with increasing x. The doping dependence
when sweeping fields up and down, respectively, and a and b are
the sample widths ða < bÞ. For x ¼ 0:075, non-monotonious field
dependence of J at high temperatures above 10 K reflecting the
c
of resistivity for BaðFe1ꢂxCo
x
Þ2As
2
are very similar to those reported
fish-tail magnetization is observed. On the other hand, for
(
a)
(b) 106
x = 0.045
2
K
x = 0.06
1
05
10 K
2
K
1
05
15 K
5
K
17.5 K
20 K
04
104
1
7.5 K
1.0
.0
2 K
2
K
5
0
15 K
0
1
03
02
-5
-
1.0
5 K
1
03 10 K
-50
0
50
-50
0
50
H (kOe)
H (kOe)
1
0
10
20
30
40
0
10
20
30
40
H (kOe)
H (kOe)
(
c)
(d)
1
1
1
1
1
06
x = 0.075
x = 0.113
2
K
1
05
5
K
2 K
05
10 K
5
K
1
5 K
104
7.5 K
04
20 K
2
0
1
0 K
1
0
2 K
2
K
0
03
5 K
-10
15 K
-2
1
03
-50
0
50
-50
0
50
H (kOe)
H (kOe)
02
0
10
20
30
40
10
20
30
40
50
H (kOe)
H (kOe)
3
3
3
Fig. 2. Field dependence of critical current density for BaðFe1ꢂxCo
x
Þ As
2
, (a) x ¼ 0:045ð330 ꢁ 415 ꢁ 30
l
m Þ, (b) 0.06 ð410 ꢁ 520 ꢁ 35
l
m Þ, (c) 0.075 ð600 ꢁ 1200 ꢁ 20
l
m Þ
2
3
and (d) 0.113 ð410 ꢁ 540 ꢁ 10
l
m Þ obtained from magnetization using the Bean model. Inset shows the field dependence of magnetization at several temperatures.

S410
Y. Nakajima et al. / Physica C 470 (2010) S408–S410
x ¼ 0:045; 0:06, and 0.113, J decreases monotonically with increas-
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2
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6
2
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